中国岩溶

Research on recharge paths and recharge ratios near the four major spring groups in Jinan

LI Changsuo, GAO Shuai, YIN Yanwei, PANG Wei, SUN Bin, LIU Haoran, CHEN Huanliang, GANG Shenting, XING Liting, GENG Fuqiang

2023, 42(5): 875-886. doi: 10.11932/karst20230501

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Due to its dynamic stability and good water quality, karst water in the northern region of China has become an important water source for urban water supply, and industrial and agricultural activities, providing an important support for economic and social development. Karst springs in Jinan City are also important natural resources integrated with water supply, tourism and ecology. However, since the 1970s, the karst groundwater mining has increased greatly with the rapid development of urban construction; consequently, four major karst spring groups in Jinan gradually dried up and even incurred the cutoff. Until the early 21^st century, the four major karst spring groups began to gush again, thanks to the implementation of a large number of ecological control measures. But the water quality of these spring groups is not as good as it was before and the flow is also not large as before. Moreover, there exist ecological and environmental issues such as water quality deterioration, water flow decline, etc. Scholars from around the world have conducted much hydrogeological research in Baotu Spring basin, including the source of recharge of karst groundwater, the cycle of evolution, the relationship between karst cold water and northern geothermal energy, the spatial distribution of karst water-bearing media and flow field characteristics, the conversion relationship of karst water and surface water, etc. However, it is difficult to accurately grasp preponderant runoff paths in karst aquifers at a scale of the hydrogeological unit, which is one of most difficult challenges in the study of karst groundwater. oundwater. The authors are convinced that the analysis of preponderant runoff paths of karst groundwater is prone to be not precise, due to the limitation of spatial accuracy at a scale of the hydrogeological unit. Therefore, in order to clarify the main preponderant runoff paths near the four major karst spring groups in Jinan, and to protect the spring water resources in a more scientific and reasonable way, this study chooses the local area near the outlet of the karst springs as the object, analyzes the preponderant runoff paths of the four major karst spring groups in Jinan, and calculates the contribution ratio of the preponderant runoff paths for each karst spring group by using quantitative analysis of flow velocity and flow direction, groundwater flow field analysis, hydrochemical isotope analysis, cluster analysis, and calculation of the mixing ratio of the three-terminal elements. The study shows that the preponderant runoff paths of the four karst spring groups can be divided into the western, southern and southeastern preponderant runoff paths. Among these paths, the southeastern preponderant runoff path mainly comes from the karst groundwater recharge in the runoff area on the east side of the Qianfo mountain fault; the southern preponderant runoff path mainly comes from the karst groundwater recharge in the runoff area on the west side of the Qianfo mountain fault; the western preponderant runoff path mainly comes from the karst groundwater recharge along the contact zone of gray rock and igneous body in the western suburb of Jinan. In general, water chemical components, and hydrogen and oxygen isotope fractions of these four karst spring groups can be devided into two areas in terms of spatial distribution, namely, the east-west strip distribution area of Zhenzhu spring group and Wulong spring group, and the east-west strip distribution area of Baotu Spring group and Heihu spring group, indicating that the four karst spring groups are subjected to a mixture of different preponderant runoff paths. But even so, the main recharge path of every spring group is different. For example, both Baotu Spring group and Zhenzhu spring group are recharged by the south recharge path with the respective recharge flow ratio of 40.21% and 51.04%, Heihu spring group by the east-south path with the recharge flow ratio of 47.42%, and Wulong spring by the west path, with the recharge flow ratio of 47.13%. The research findings can provide references for the formation mechanism and ecological protection of karst springs in North China.

Study on the intensity of boreal karstification under different geological conditions: A case study at the recharge area of Baotu Spring drainage area, Jinan, Northern China

LIU Wen, XU Congcong, YU Lingqin, LI Haixiang, LU Qianqian, ZHU Qinfeng, LIU Haoran

2023, 42(5): 887-897. doi: 10.11932/karst20230502

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It is an indisputable fact that the process of karstification can form carbon sink, and the research on this process is in the ascendant. However, the factors affecting karstification process are very complicated, and the geological background is one of the most important aspects. The effect mechanism of different geological backgrounds on karstification and the data of dissolution rate under different geological backgrounds can provide a basis for understanding the karst process and its mechanism under sub-humid climate and improving the estimation accuracy of karst carbon sink. The recharge area of Baotu Spring area in Jinan, a typical boreal karst spring area, is selected as the research area, and the standard dissolution tablet test of carbonate rock is used for field measurement.At the northern edge of the mountainous region in central Shandong, the research area is located in the Baotu Spring area of Jinan, bordering Mount Tai in the south and the Yellow River in the north (116°40'30"-117°14'10"E; 36°14'50"-36°46'10"N). The spring area borders the Mashan fault and Dongwu fault to the west and east, respectively. These two faults are permeable in the north and weakly permeable in the south. The southern boundary is the ridgeline of Mount Tai, and the igneous body in the north is called "Jinan Rock". Constrained by the special topography and geological structure, precipitation infiltration in the southern mountainous area recharges the Cambrian-Ordovician karst aquifer which presents strong karst development, and then moves from south to north. According to the statistics of Jinan Meteorological Bureau, Jinan City is characterized by a warm temperate continental climate with four distinct seasons at an annual average temperature of 14.7 ℃. Its average precipitation per year is 647.9 mm, more than 70% of which is concentrated in July, August and September with less from December to March. During the four-year test, pH values of precipitation in Jinan (Guishan) National Basic Meteorological Station averaged between 6.96 and 7.61, which means there was no acid rain in this period.The standard dissolution tablet method is widely used in the study of karstification process because of its advantages such as short-term monitoring, simple preparation of sampling, mature embedding method and the condition closer to the natural state than that in a laboratory. The study area is located in the low mountainous but hilly area of the north wing of Mount Tai and the southern Jinan far away from the urban area, which is also the recharge area of Baotu Spring area. The area is forest and shrub land with less interference from human activities. We buried standard dissolution carbonate tablets around the end of November 2017, and retrieved them around the end of November 2021, with a total test period of four years. When burying these tablets, we selected test sites with different basement lithology in the same small watershed where the meteorological conditions are not significantly different, to ensure the relative consistency of test backgrounds such as precipitation and temperature. Meanwhile, we collected layered soil samples for testing physical and chemical indexes, atmospheric and soil CO₂, soil water content, etc. Results show that the average dissolution rates of granite, carbonate, loess and shale are 3.49 mg·cm⁻²·a⁻¹, 0.26 mg·cm⁻²·a⁻¹, 0.11 mg·cm⁻²·a⁻¹ and 0.09 mg·cm⁻²·a⁻¹, respectively. In other words, the influence of basement lithology on dissolution rate is significant. The dissolution rate of granite area is much larger than that of the other ones. The dissolution rate of carbonate area is larger than that of loess and shale, and the lowest in shale area. From a vertical profile perspective, the dissolution rate of carbonate rock area, loess area and shale area is atmospheric>surface>soil, and the rate in soil decreases with the increase of depth, while the dissolution rate in granite area shows a completely opposite trend.It is found that there is no obvious correlation between soil CO₂ and dissolution rate. The variation trend of soil organic matter content in the profiles of carbonate rock, loess and shale is consistent with that of dissolution rate, indicating that the former may have a direct effect on the latter, but the consistency does not show in the granite area. There is an exponential correlation between soil pH and dissolution rate at all sites, and the dissolution rate decreased significantly with the increase of pH. Soil water content has an obvious linear relationship with the dissolution rate, and is the limiting factor of karstification process in boreal karst area compared with soil CO₂.This study is a useful exploration in the boreal karst area, the results of which help to deepen the understanding of the karstification process under the semi-humid and semi-arid climate conditions, and provide a comparative example for other karst areas in the north and even the whole karst area of the same type.

Analysis of dynamic characteristics and driving factors of Jinan spring water on a long-time scale

SUN Bin, LI Changsuo, XU Qingyu, GAO Shuai, LIU Chunwei, XING Liting, YU Lingqin

2023, 42(5): 898-906, 916. doi: 10.11932/karst20230503

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Springs of Jinan City in Shandong Province are typically large karst ones in Northern China. With 136 mouths of springs, there are four major spring groups, namely, Baotu Spring, Heihu spring, Wulongtan spring and Pearl spring, distributed around the moat in the urban area. Springs are the lifeblood of Jinan City and play an important role in the development of this city. However, since the dry season in 1972, the springs had witnessed seasonal cutoffs. Baotu Spring even experienced complete cutoffs throughout the respective year of 1982, 1988 and 1989. From 1999 to 2002, the cutoff of this spring spanned 926 days, the longest cutoff time in history. Until September 2003, thanks to a series of measures taken by the government, the four major springs began to gush all year-round, but the spring flow is much less than before. Currently, studies usually focus on short-time-span spring dynamics in Jinan instead of long-term comparative studies on spring dynamics in different periods. Especially, the strong human influence on spring dynamics after springs gushed again needs further research. Few analyses of the influence factors of complex human activities, such as the impact of urban construction are relatively few. In these studies, the whole built-up area has been taken as the object, which ignores the fact that the runoff discharge area itself does not have the condition of infiltration, resulting in the overestimation of hardening area in statistics. In order to comprehensively explore the dynamic evolution of the four major spring groups and the main control factors affecting spring gushing, we have conducted a study on the dynamic characteristics and causes of springs over the past 60 years for the development and utilization of karst groundwater resources and environmental protection in the Jinan region. To identify the dynamic change pattern of the four spring groups in Jinan, we have studied the dynamic change characteristics of the springs and their influencing factors such as natural-human activities, etc. through correlation analysis, partial correlation analysis, regression analysis, and water balance analysis, based on a series of monitoring data for a long term. The results indicate that the dynamics of the four springs in Jinan City have experienced four stages: stage 1 from 1959 to 1967, at which the springs gushed spectacularly with a large amount of flow at a high water level; stage 2 from 1968 to 1980, at which the landscape of springs attenuated with a small amount of flow at a low water level; stage 3 from 1981 to 2002, at which the springs experienced long-term intermittent cutoff with a very small amount of flow at a rather low water level; stage 4 from 2003 up till now, at which the springs gushed again because of the regulated restoration. At stage 4, the average flow rate of springs decreased from 366.6 thousand m³·d⁻¹ to 142.0 thousand m³·d⁻¹, then to 46.9 thousand m³·d⁻¹, but subsequently recovered to 170.6 thousand m³·d⁻¹; the average water level of springs decreased from 30.48 m to 27.91 m, then to 26.25 m, but afterward, recovered to 28.24 m. In addition, this study quantitatively identifies the influencing factors and the degree of impact in each stage. The main control factors at stage 1 are precipitation and extraction, with an average impact degree of 581.7 thousand m³·d⁻¹and −95.8 thousand m³·d⁻¹; the main control factors at stage two are precipitation, extraction and water engineering, with an average impact degree of 520.1 thousand m³·d⁻¹, −426.1 thousand m³·d⁻¹ and −59.8 thousand m³·d⁻¹; the main control factors at stage 3 are extraction, precipitation, water engineering, and urban construction, with an average impact degree of −518.2 thousand m³·d⁻¹, 514.2 thousand m³·d⁻¹, −69.2 thousand m³·d⁻¹ and −24.0 thousand m³·d⁻¹; the main control factors at stage 4 are precipitation, extraction, ecological replenishment sources, urban construction, and water engineering, with an average impact degree of 553.8 thousand m³·d⁻¹, −254.4 thousand m³·d⁻¹, 103.6 thousand m³·d⁻¹, −89.9 thousand m³·d⁻¹ and −65.6 thousand m³·d⁻¹. In general, precipitation and extraction control the spring flow, but other driving factors gradually increased with the development at the four stages. Before the springs gushed again, the impact of precipitation gradually weakened, but the influence of extraction, urban construction and water conservancy projects gradually increased by degrees. After the springs gushed again, the impact of the government's regulation and control played a dominant role. To keep a continuous gush of spring, first of all, we should ensure that Baotu Spring area 3-type water conversion can be effectively carried out. Besides, we should strengthen dynamic monitoring and regulation, rationally coordinate major factors such as groundwater mining, ecological replenishment and water conservancy projects, etc., and formulate an optimal groundwater development program in a scientific and elaborate way, relying on the sharing mechanism among different departments. Finally, in order to reduce the impact of urbanization on springs, we should strengthen the protection and restoration of ecological environmental in the recharge area of southern springs so as to improve the capacity of water resources conservation. The research results can provide basis for spring runoff protection.

Study on the influence of external recharge of Yufu river on karst groundwater

DING Guantao, LI Changsuo, WEI Shanming, WANG Shaojuan, LI Yan, LU Qianqian, SUN Bin, Liu Haoran

2023, 42(5): 907-916. doi: 10.11932/karst20230504

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Jinan is famous for its abundant spring water resources, but with the advancement of urbanization, the ground continues to harden, resulting in a significant reduction of the area effectively supplying karst water, and hence a sudden decrease in water supply. The decrease has seriously threatened the continuous gushing of spring water in Jinan. At the same time, the urban population is constantly rising, so is the demand for water supply. The contradiction between spring water protection and urban water supply is becoming increasingly serious. In order to alleviate the contradiction in Yufu river, the projects in which groundwater was recharged by surface water were completed in different phases. In these projects such as the water discharge project of the Wohushan reservoir, the project of converting surface water into groundwater, and the Jinan City project of the East Line of the South-to-North Water Diversion Project, the surface water from the Wohushan reservoir, the Yuqing lake reservoir, and the Yangtze River has been respectively used for the recharging. They play an important role in maintaining the continuous gushing of spring water for the normal use of groundwater in cities. In the South-to-North Water Diversion Project, water from the Yangtze River is significantly different from local karst water in quality. Therefore, long term recharging will have a certain impact on karst water. In response to this issue, taking the recharging source of the Yufu river from 2021 to 2022 as an example, we studied the impact of external water supply on karst water in the Yufu river basin, and analyzed the hydrochemical characteristics and the change of hydrochemical composition of groundwater in the affected area. In order to fulfill the research purpose, we used isotope testing and comprehensive hydrochemical analysis. The results show that the recharging of the Yufu river has a certain replenishment effect on karst water. According to its degree of replenishment, the research area is divided into three zones. After two months of replenishment, Zone I is distributed on the east bank of the Yufu river, with the highest mixing ratio of river water in karst water, ranging from 50% to 70%, and is most affected by river water replenishment. Within a certain range of the periphery of Zone I, the proportion of river water in Zone II is 10% to 50%, and the degree of influence of infiltration river water on karst water isotopes and hydrochemistry decreases. Zone III is far away from the Yufu river, with river water accounting for less than 10% and less affected by the river water. After 11 months of replenishment, the proportion of river water in the karst water monitoring points in Zone I has decreased, but the proportion is still around 50%, indicating that Zone 1 is still the main affected area for river water replenishment. The proportion of river water in monitoring points in Zone II has significantly increased, with J108 reaching 37%, indicating that the impact range of river water has expanded to Zone II. The proportion of river water in monitoring points in Zone III has also increased, but it is not significant with the percentage of around 10%, indicating the limited impact from river water. The hydrochemical characteristics of river water from the external source are significantly different from those of karst water and spring water. The hydrochemical type of river water is SO₄·Cl-Na·Mg type, while the karst water is mainly HCO₃-Ca·Mg type and HCO₃-Ca type. Spring water is influenced by the replenishment of Xingji river with HCO₃·SO₄-Ca·Mg type. Two months later, the hydrochemical type of groundwater has gradually transitioned from SO₄·Cl-Na·Mg type to HCO₃-Ca·Mg type in the water source area of the western suburb along Yufu river. Due to the water diversion and replenishment of the Xingji river in the direction of the spring group, the groundwater type first changed from SO₄·Cl-Na·Mg type to HCO₃-Ca type, and then transitioned to HCO₃·SO₄-Ca·Mg. The recharging of water from the external source has a certain impact on the quality of the local karst water. The changes in the ion content of karst water in the affected area are mainly caused by the physical mixing of river water and karst water, accompanied by changes in water-rock balance. After a hydrological year, each ion gradually approaches its original content level after supplementing the source. The research results provide a basis for the scientific implementation of recharging and replenishment of the Yufu river, and have certain significance in promoting the organic unity of spring protection and water supply. This article does not consider the impact of seasonal atmospheric precipitation on groundwater hydrochemistry and isotopes. The atmospheric waterline used for isotope analysis is not the local atmospheric waterline, and these issues may have a certain impact on the results obtained, which is a topic that we need to further study.

Study on the hydraulic connection between downtown and eastern and western suburbs in Jinan

YU Lingqin, LIN Guangqi, LIU Yuanyuan, QI Huan, MENG Qingzhai, MA Hekuan, JIANG Lulu, LIU Chunwei

2023, 42(5): 917-930, 955. doi: 10.11932/karst20230505

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In order to identify the hydraulic connection between the spring water in Jinan city and the karst water in the western and eastern suburbs, we conducted a study to analyze the hydraulic connection between the three from the perspective of stratigraphic structure and hydrodynamic field, combining with the restults of pumping tests and tracer tests. Meanwhile, we also performed hydrochemical and correlation analyses.The results show that from the perspective of stratigraphic structure, there is no obvious water-blocking structure in the area of Huaiyuan Square-NO.9 Middle School-NO.14 Middle School near the rock contact zone on the north side of Liuchang mountain between downtown and western suburbs, and a continuous karst aquifer group is developed below the rock body at a burial depth of about 350 m. The Chaomidian fracture on the west side has good water conductivity and the urban spring water and karst water in western suburbs can be hydraulically connected through the aquifer of Sanshanzi Formation below the rock contact zone. The tongue-shaped intrusive rock body exists in the area of Yanchi mountain-the Qili river between downtown and the eastern suburbs, which plays a certain role in blocking the karst water of downtown springs and eastern suburbs, but the Wenhuaqiao fracture does not form a water blocking structure, and the aquifers on both sides are less misshapen, and the distribution of karst aquifers to the east of the Wenhuaqiao fracture and south of Yaojia is continuous and stable. The karst aquifers are exposed at a depth of 485 m north of the Honglou Campus of Shanda University. The urban spring water and karst water in eastern suburbs have a unified water system, which can be hydraulically connected through the aquifer of Sanshanzi Formation below the contact zone of rock mass.The western borehole pumping test shows that the top plate of the aquifer in the areas of Huaiyuan Square-NO.9 Middle School-NO.14 Middle School is gradually deepened from south to north. Both the horizontal and vertical fissures in the intrusive rock above are evenly developed. The fissured water can be recharged by the top-supply of the underlying karst water group. Due to the overall water-richness and smooth groundwater runoffs, the urban spring and the karst water in the western suburbs have hydraulic connection. The eastern borehole pumping test shows that the rock body in the area of Yanch mountain-the Qili river is gradually thickening from south to north with the depth increase of the top plate of the aquifer. The fissures in the rock body are developed with overall water-richness of the eastern Jinan strata and smooth groundwater runoffs.The water level map shows that there is no sudden change of water level on both sides of Liuchang mountain-Langmao mountain-Wanling mountain between urban area and western suburbs, and the water levels of the two areas are roughly the same with continuous and steady water level, indicating a connection between the two in the hydrodynamic field. Groundwater on the west side of the area of Yanchi mountain-the Qili river flows in the northwest direction, while groundwater on the east side of the same area flows in the northeast direction. However, both the urban area and the eastern suburbs are recharged by the same source. Therefore, if karst water in the eastern suburbs is exploited excessively, part of karst water flowing to the urban area from the southern mountainous area will be taken away, which will affect, to a certain extent, spring water in the urban area.The water chemistry analysis based on Piper's trilinear diagram and hydrogeological conditions shows that the chemical types of urban spring water are the same as those of karst water in the eastern and western suburbs, and the three areas have the same evolutionary environment. According to the correlation analysis between the urban spring water and the karst water in the western suburbs, the main chemical indexes of the fissure hole JZ and SSZ boreholes are correlated with those of the urban and western suburban boreholes, with all coefficients larger than 0.84, which indicates that the fissure water in JZ and SSZ boreholes is closely correlated with karst water. There is a vertical runoff channel in the area of Huaiyuan Square-NO.9 Middle School-NO.14 Middle School, and the intrusion above fracture water in the rock body can be recharged by the top-supply of the lower karst water rock group. The urban spring water and the karst water in the western suburbs are of the same source. According to the correlation analysis of urban spring water and karst water in eastern suburb, the correlation of the main chemical indexes between karst water collected from eastern suburban boreholes and urban spring water is larger than 0.95, an significant positive correlation, indicating the same source of karst water in both eastern suburban and urban area. According to the content distribution of the constant ion components in karst water in the study area, water chemical components of the urban spring and the karst water of the western and eastern suburbs are formed mainly by the dissolution of carbonate rocks. Karst water of the three areas is in a similar groundwater environment, and there is a certain hydraulic connection.

Research on the response mechanism of groundwater level to rainfall in the western suburb of Jinan based on wavelet analysis

ZHAO Zhenhua, LUO Zhenjiang, HUANG Linxian, XING Liting, LI Hongtao, SUN Hongjie

2023, 42(5): 931-939. doi: 10.11932/karst20230506

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With the rapid development of social economy, karst groundwater in Jinan City keeps facing various problems such as water resource shortage and water pollution, which seriously threatens the protection of springs and the utilization of groundwater resources. The western suburb of Jinan City is located at the northeast edge of Jinan spring catchment; the Yufu river and Shahe river abundant in groundwater are important recharge sources for Jinan spring catchment. Therefore, it is very important to accurately reveal the response mechanism of groundwater to precipitation in the western suburb, which can help to improve the potential of groundwater exploitation and spring protection. In this study, the monthly precipitation data from 2010 to 2019 in the western suburb of Jinan City and the monthly water level data of Quaternary and karst groundwater are used to carry out seasonal statistical characteristics and wavelet analysis. Results show that, (1) According to the seasonal statistical characteristics of precipitation and groundwater, the precipitation in the western suburb of Jinan City mainly concentrates in summer, accounting for about 67.5% of the annual precipitation. The groundwater level is the highest in autumn, with the average water levels of the Quaternary and karst being 30.28 m and 29.74 m, respectively. The groundwater level is the lowest in spring, with the average water levels of the Quaternary and karst being 29.44 m and 28.78 m, respectively. Seasonal fluctuation of groundwater dynamics in the study area is the coupling effect of human and natural factors. Since the beginning of spring in March, precipitation has decreased and the amount of evaporation has increased along with the rise of temperature. At the same time, a large amount of groundwater has been extracted for agricultural irrigation, resulting in a drop in the groundwater level. Although the precipitation is relatively abundant after the beginning of summer, the infiltration of precipitation into the aquifer has a certain lag (generally about 2–5 months), resulting in the fact that the highest groundwater level occurs in autumn. (2) Through continuous wavelet transform analysis, it can be concluded that the main oscillation period of precipitation is 0.98–1.17 a and it passed the 95% red noise test from April 2011 to October 2018, indicating that it has significant periodic characteristics. The main oscillation period of Quaternary groundwater is 0.73–1.16 a and 1.17–1.74 a and it passed the 95% red noise test from April 2011 to January 2015. The main oscillation period of karst groundwater is 0.87–1.09 a and 0.46–1.23 a, but it passed the 95% red noise test only from March 2013 to October 2014 and from June 2016 to October 2018. The main oscillations periods are not completely consistent with groundwater level and precipitation, reflecting that the dynamics of groundwater in the study area is not only affected by precipitation, but also by other factors such as human activities. (3) Cross wavelet transform analysis shows that the main resonance period of precipitation and groundwater level is about 1 year. The groundwater level dynamics of Quaternary and karst aquifers lag behind precipitation by 144.14 d and 172.62 d, respectively, with the difference of only 28.48 d, which may reveal that the hydraulic connection between Quaternary and karst aquifer is very close. The dynamic curves of the Quaternary and karst groundwater level show that the dynamic changes of the two are relatively consistent, although the dynamics of the karst groundwater level lags behind, but with relative short lagging time, which further indicates that these two aquifers have a good complementary relationship. In addition, the drilling data of monitoring wells in the study area show that there is no obvious water-resisting layer between the Quaternary and the karst aquifer, and the karst aquifer has well-developed fissures, strong water-abundance, and is cut by faults, with strong water-conducting capacity. All the above results show that the response of groundwater level to precipitation is significant and stable.

Dynamic characteristics of groundwater level and exploitable amount of groundwater source in Jiuxian county, Tai'an

WEI Kai, WANG Yanling, ZHAO Zhiwei, WU Yanan, ZHAI Daiting, YAN Baizhong

2023, 42(5): 940-955. doi: 10.11932/karst20230507

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In this study, the effect of human activities on the evolution characteristics of groundwater levels in the water source area of Jiuxian county and its influencing factors have been discussed, based on the data of groundwater levels, annual precipitation, groundwater mining output and karst collapse in the study area from 1980 to 2021. The critical groundwater level to prevent karst collapse has also been determined. In addition, based on Modflow-GWM software, a coupled simulation model for the flow and management of karst water system in Tai'an City and Jiuxian county has been constructed, thereby exploring the allowable extraction of water resources in terms of the karst collapse prevention. The research results indicate that, (1) The annual dynamics of interstitial water are significantly affected by precipitation, with the lowest annual water level in late June and the annual highest from late August to late September during the rainy season, presenting the characteristics of "precipitation-replenishment" type. The dynamics of karst groundwater within the year are affected by the obstruction of pore water discharge in the Quaternary system. The highest water level within the year generally lags slightly behind the concentration from September to October, while the lowest water level occurs from May to June. The groundwater level changes by about 5 meters within the year. The water level changes exhibit a characteristic of "a low water level in the dry season and a high water level in the wet season", and the dynamic changes of water levels are characterized with the type of "precipitation infiltration-mining". (2) The groundwater level in the water source area of Jiuxian county showed a significant downward trend from 1980 to 1990, and the karst groundwater level showed a basic fluctuation and decline from 1990 to 2003. The groundwater level in the water source area has increased significantly since 2004. (3) Since 1988, the water source area in Jiuxian county has entered a period of high incidence of karst collapse. From then on to 1994, the water level had rebounded, fluctuating between 96 m and 108 m and was located near the bedrock surface. This period had also experienced a high incidence of karst collapse in this area, with the collapse area expanding northward from Jiuxian county to the Yuanzhuang-Yanglou area. After 1995, the intensity of karst collapse in the area decreased, but there still occurred collapse from time to time. By 2001 the development period of karst collapse in the area, the water level showed an overall downward trend and fluctuated above and below the bedrock surface, with an elevation of 95–110 m. In the first half of 2003, high-intensity mining continued, with water levels ranging from 86 m to 98 m. In 2013, the high-intensity mining in the rainy season and continuous low water levels caused geological disasters of high-intensity karst collapse at a large-scale. Since 2005, water levels have shown an overall upward trend, fluctuating between 106 m and 116 m above the bedrock surface. During this period, one time of collapse occurred in 2006, and no new collapse was observed. Based on the relationship between water level dynamics and karst collapse, the critical water level of the water source area to prevent karst collapse is 108 m, located 2 m above the roof of the karst aquifer. (4) According to the groundwater management model, the mining output of karst water in the simulation area at the critical water level is determined to be 82,000–85,000 m³·d⁻¹, of which the exploitable volume of centralized exploitation in the water source area of Jiuxian county is 32,000−35,000 m³·d⁻¹. This study is of great significance for the sustainable development and utilization of karst groundwater resources and the tackling of environmental geological problems.

Study on the zoning of karst development in the Jiaodong Peninsula: Take the Zhongqiao area of Yantai City as an example

YANG Ning, SHI Meng, YIN Tao, YU Linhong, WANG Yuanfeng, ZHANG Jie, FENG Peipei

2023, 42(5): 956-968. doi: 10.11932/karst20230508

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Karst is developed in all provinces of China. In Shandong Province, karst is mainly developed in its southwest part, and is sporadically distributed in the Jiaodong area. Particularly, the typical development area is located in the Zhongqiao area of Yantai City—the eastern part of the Jiaodong Peninsula in Zangjiazhuang basin whose overall terrain is high at peripheral edge and low in the middle. There are four distinct seasons in the Zhongqiao area with the annual precipitation mainly concentrating in June and July. Three seasonal rivers—the Baiyang river, the Yandi river and the Shandong river—run through the whole area. With large areas of Quaternary, Cretaceous and Sinian strata exposed as well as fault structures developed, this area presents a simple geological condition, typically distributed with limestone. According to the different burial conditions, the karst area can be divided into three types: exposed karst area, covered karst area and buried karst area. There is a wide distribution of soluble rocks in this area, where karst is developed in three forms, namely, dissolved pores, dissolved cracks and karst caves. However, the irregular development of underground karst fractures leads to the relatively large difference in the abundance of karst underground water. Consequently, there have been many geological disasters of karst collapse usually occurring at a small scale after heavy rainfall in the history of the Zhongqiao area. These collapses caused certain losses to the local economy, though with no casualties.In order to find out the distribution, degree and law of karst development in the Zhongqiao area, we have carried out a comprehensive study on the karst collapse according to geological environment survey, geophysical exploration, geological drilling, pumping testing, etc. Results of geological survey and geological drilling show that soluble rock strata in the study area are mainly made up of two limestone sections of Xiangkuang Formation of Penglai Group. The distribution of three burial types and the characteristics of karst development have also been identified. Results of geophysical exploration and geological drilling demonstrate the distribution of karst strata in the study area. Meanwhile, the distribution range, development characteristics and changes of developmental depths of dissolved pores, caves and dissolved cracks have been investigated. It is found that this area is mainly composed of the north-east fault structure. Results of geological drilling and pumping testing indicate the groundwater flow direction and the distribution characteristics of karst underground water abundance, with the maximum water inflow per well greater than 3,000 m³·d⁻¹. The degree of karst development is positively correlated with the variables such as the purity of karst strata, the water abundance of water bearing rock group in karst fissure, the distribution of anomalous points in geophysical exploration, and the level of dissolved crack ratio and cave ratio. From five evaluation factors including karst formation, water abundance, apparent resistivity, the ratio of borehole to karst and the ratio of borehole to cave, we used analytic hierarchy process to divide the karst degree in the 60 m zone and divided the degree into five grades, karst development zone, the zone with karst development at a relatively high level, the zone with karst development at a medium level, the zone with karst development at a low level and the zone with undeveloped karst. With the decline of these five grades of karst degree, both the purity of karst strata and the water abundance decrease successively, whereas the apparent resistivity, the ratio of borehole to karst and the ratio of borehole to cave all increase gradually. The distribution range of each karst degree is successively distributed without overlap. The probability of karst collapses is as follows, karst development area>the zone with karst development at a relatively high level>the zone with karst development at a medium level>the zone with karst development at a low level>the zone with undeveloped karst. Collapses mainly concentrate in the karst development area. The degree of karst development is the basic condition of karst collapse; therefore, finding out the distribution, degree and law of karst development in the Zhongqiao area may provide a geological basis for the prevention and control of geological disasters of karst collapse, and guarantee the normal life of local residents and effective operation of enterprises. The study result may also provide a basis for further research on the distribution, degree and law of karst development in the Jiaodong area.

Comparative analysis of the genesis models of different geothermal reservoirs in Chengning uplift area in northwest Shandong based on hydrochemical isotope technology

CUI Rui, WANG Xuepeng, FENG Bo, LIU Xiyao, FENG Shoutao, LIU Shuai

2023, 42(5): 969-981, 994. doi: 10.11932/karst20230513

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The Chengning Uplift Area is one of the important areas rich in extremely abundant geothermal resource in China. At present, geothermal resources in the study area are mostly used in the fields such as bathing, medical treatment, and heating. According to the existing geothermal geological data and analysis results, the total geothermal resources in this area are 1.70×10²⁰ J, equivalent to 57.86×10⁸ t of standard coal. Therefore, elucidating the genesis model of geothermal fields in this area is of great significance for the sustainable development and utilization of geothermal resources. This study uses hydrochemical isotope technology to compare and analyze the genesis of the sandstone thermal reservoir of Guantao Formation and the Cambrian-Ordovician karst thermal reservoir in the Chengning Uplift Area. The research results indicate that the cations in the geothermal water from sandstone and karst thermal reservoirs in this area are mainly Na⁺, and the anions are mainly Cl^-. The mineralization degree of geothermal water in sandstone thermal reservoir is 4.19–5.96 g·L⁻¹, with a pH value of 7.35–9.43, indicating neutral to weakly alkaline water. The mineralization degree of geothermal water in karst thermal reservoir is 5.91–11.10 g·L⁻¹, and the pH value is 6.50–7.29; therefore, it is classified as neutral water according to its acidity and alkalinity. The geothermal water from sandstone thermal reservoir in this area is a product of atmospheric precipitation infiltration and replenishment during geological history, supplying water by lateral runoff. However, the geothermal water in karst thermal reservoir does not directly come from the nearest infiltration recharge of atmospheric precipitation, but through a longer distance runoff process, which has an obvious phenomenon of hydrogen and oxygen drift. The two sets of elevation of geothermal water supply, temperature of thermal reservior and depth of hot water circulation are respectively 459 m and 557 m, 66 ℃ and 72 ℃, and 1,420 m and 1,795 m. From this, it can be seen that geothermal water in the study area is in a well-sealed geological environment, without shallow water mixing, and is deep circulating geothermal water. In addition, the research results also reveal that the supply area of geothermal water of Chengning Uplift is located in the Mount Tai area, and the heat source of its geothermal system is the heat flow conducted from the deep crust and a small part from the upper mantle. The deep fault in the study area generated a certain amount of frictional heat during its active period, and also served as a good channel for underground heat flow, connecting and conducting upwards the heat generated by magma in the deep crust and upper mantle. In addition, the area is a sedimentary basin with deep depression, which generates gravity compression heat under the pressure of the thick Meso-Cenozoic sedimentary layer. The heat generated by these sources is stored in the pores and cracks of the thermal reservoir under the thermal insulation effect of the cover layer with strong thermal resistance and poor thermal conductivity, and is the main heat source for the formation of hot water in the area.

Characterization of seawater intrusion based on multivariate statistical analysis and water chemistry characteristics: A case study of Laoshan district, Qingdao City

GANG Shenting, LYU Minghui, LU Qianqian, GAO Shuai, ZHAO Zhiqiang, CHEN Huanliang, PENG Tongqiang, WANG Xi, XING Liting, LI Lixia

2023, 42(5): 982-994. doi: 10.11932/karst20230509

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Groundwater is an important source of freshwater in coastal areas. With the rapid development of industrialization and urbanization, the water demand for production and living in coastal areas has been rising year by year. Therefore, the increasing exploitation of groundwater has triggered seawater intrusion and increasingly prominent environmental problems of groundwater in many places. Researching the hydro-chemical characteristics and evolutionary patterns of groundwater enables effective monitoring and protection of the groundwater environment. Taking the groundwater in Laoshan district, Qingdao City as the research object, we mainly focused on the issues of groundwater chemical characteristics, groundwater chemical processes, the degree of seawater intrusion and its impact on groundwater. In addition, under the theoretical guidance of hydrogeology, we analyzed the characteristics of seawater intrusion and evolution of groundwater hydrochemistry in the study area by means of data collection, theoretical analysis, field investigation and sample collection and testing. The research findings can provide a scientific basis for the rational development and utilization of groundwater in the area.The results show that the groundwater in the study area has Na⁺, Ca²⁺, Cl⁻, and ${\rm{SO}}_4^{2-}$ as the main dominant ions, and most of the groundwater chemistry types are Cl·SO₄-Na and SO₄·Cl-Ca·Mg types. The Cl⁻ concentration in the groundwater varied considerably and its mean value exceeded the cut-off value for the presence or absence of seawater intrusion (250 mg·L⁻¹), indicating that some degree of seawater intrusion may have occurred in groundwater. Groundwater in the Laoshan district of Qingdao City is neutral to weakly alkaline (mean pH=7.0–8.0), which is an effect of long-term hydrogeochemical processes in the coastal area. The results obtained by the PCA model show that changes in groundwater chemistry are mainly controlled by natural factors (rock-water interaction) or anthropogenic factors (agricultural and domestic activities). The five chemical characteristics of Cl⁻, mineralization, ${\rm{SO}}_4^{2-}$, γCl⁻/γHCO$_3^- $ and SAR were selected as evaluation factors. Based on the inverse distance weighting (IDW) method and geographic information systems (GIS), we achieved the spatial mapping of seawater intrusion locations, showing that the seawater intrusion, in Laoshan district was mainly distributed in the intrusion sections such as the east of Jiangjia Tuzhai—the north of Puli community, the area of Wanggezhuang-Gangxi-Gangdong, Yangkou bay and the area of Danying village-Quanzhou island. The study results are of great significance for the use of groundwater resources and the prevention and control of seawater intrusion in Laoshan district. In addition, the research ideas and methods provide a reference for the study of groundwater genesis in other coastal areas in the world.

Study on the influencing factors of spring water in Jinan City based on wavelet transform method

LIU Chunwei, QI Huan, LIN Guangqi, LIU Haoran, PANG Wei, YU Lingqin, MA Hekuan

2023, 42(5): 995-1004. doi: 10.11932/karst20230510

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In order to find out the influence of two main factors—precipitation and groundwater exploitation—on the gushing of spring in Jinan urban area, we selected the exploitation data of the western suburb in Jinan (the water source area of Emei Mountain, Dayangzhuang and Lashan), Jinan urban area (the water source area of Quancheng road, Jiefangqiao, Yinhuchi, and Linan) and the eastern suburb in Jinan (the water source area of Huaneng road). We also selected dynamic observation data of urban spring water level and precipitation during the same period. Based on cross wavelet change, wavelet coherence spectrum and multiple wavelet coherence spectrum, we analyzed the time-lag relationship, comprehensive impact and impact degree between atmospheric precipitation, groundwater exploitation and spring water in Jinan City, in order to provide a basis for the optimal allocation of groundwater development and utilization and the promotion of spring protection in this city.The following conclusions are drawn in this study. (1) By cross wavelet transform, we analyzed the resonance period, significant period and phase relationship between the spring level and the influencing factors in the time-frequency domain. It is found that during the research period, there was a frequency domain period of about 1 year between spring water level, precipitation and exploitation quantity of groundwater. The urban spring level and precipitation passed the 95% red noise test and there was a frequency domain period of 0.69-1.16 a outside COI. In a hydrological year, the urban spring level lagged behind the precipitation for 133.22 days. When the urban spring level and groundwater exploitation passed the 95% red noise test and there was a frequency domain period of 0.82-1.23 a outside COI, the response time lag between the urban spring level and the western suburb and the eastern suburb of Jinan was 125.43 days and 83.85 days, respectively. There existed a hydraulic relationship between spring water in Jinan urban area and groundwater in the western suburb and the eastern suburb of this city. The cross wavelet transforms between urban spring level and the exploitation quantity in the western suburb of Jinan, and between the urban spring level and the exploitation quantity in the urban area also showed a dispersed low energy region of 0.39-0.58 a, and the 95% red noise test reflected the seasonal characteristics of the exploitation of Jinan water source. (2) We carried out the wavelet coherence analysis of urban spring water level, precipitation and exploitation quantity of groundwater to explore the change characteristics of urban spring water level and influencing factors in the whole period. The wavelet coherence average value (AWC) between the urban spring level and precipitation was 0.58. The AWC value between the urban spring level and the respective exploitation quantity in the western suburb, in the city area and in the eastern suburb is 0.47, 0.40 and 0.32. PASC was 13.90%, 16.81% and 10.09%, respectively. During the research period, both precipitation and karst groundwater exploitation exerted obvious effects on urban spring levels. In terms of groundwater exploitation in different locations, the urban exploitation exerted the greatest influence on the urban spring level, followed by the western suburb. The eastern suburb of Jinan was influenced least. (3) According to exploitation conditions during the research period, the combination of precipitation and exploitation quantity in the western suburb of Jinan, and its combination in the urban area were regarded as the biggest influencing factors for the change of spring water level. Under the influence of the three factors, MWC produced by the combination of precipitation and exploitation quantities in the western suburb and the urban area reached 0.91, and PASC increased by more than 5%. Under the combined action of the four influencing factors, MWC reached the maximum value of 0.94, while PASC decreased. One reason was that the PASC threshold with statistical significance increased after additional factors were added, and the other reason was that the overlapping effect caused by collinearity among influencing factors reduced the variance contribution of some factors. Therefore, the multivariate wavelet coherence spectrum showed that the combination of precipitation and the exploitation quantities in the western suburb and the urban area is the most influential factor for the change of urban spring water level under the exploitation conditions during the research period.

Hydrogeochemical characteristics and formation mechanism of the karst thermal reservoir at the northern edge of the Luzhong Uplift

JIANG Lulu, SUI Haibo, KANG Fengxin, LI Changsuo, WEI Shanming, YU Lingqin, LI Yue

2023, 42(5): 1005-1026, 1036. doi: 10.11932/karst20230514

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The geothermal area in the northern edge of the Luzhong Uplift is located between Mount Tai and the northwest plain of Shandong. In terms of the geotectonic division, the study area is located in the Luzhong Uplift of the Luxi Uplift in the North China Plate, which is fan-shaped with the arc facing north in the plane, and it is a monoclinal structure with the stratum tilting gently to the north. There is a very thick layer of Cambrian-Ordovician carbonate strata in the northern edge of the Luzhong Uplift, which has the geothermal geological conditions for the formation of a large karst geothermal field. Under the influence of the collision of the Pacific Plate and Eurasian Plate, especially the tectonic activity on the rim of the Pacific Plate since the Mesozoic, NW and NE oriented fracture structures have been widely developed in the study area. This combination of tectonic structures with different ages and properties provides the preconditions for the formation of karst geothermal resources characterized by the convection-conduction between low and medium temperatures in this area. The main aquifer in the study area is the Cambrian and Ordovician carbonate aquifer, which is directly or indirectly replenished by atmospheric precipitation in the exposed karst area in the upper and middle reaches. Some of the precipitation is concentrated and flows out in a small area, and the remaining water continues to flow to form a karst water enrichment zone in the downstream piedmont, intermountain basins and hidden limestone distribution areas in the valleys.The karst thermal reservoirs in the northern edge of the Luzhong Uplift in Shandong Province are rich in low-and medium-temperature geothermal resources, which are characterized by a large water yield and easy recharge. Thus, we can study the hydrogeochemical characteristics of the geothermal water to analyze the formation mechanism of the geothermal resources, which is of great significance in promoting the effective development and utilization of geothermal resources. In this study, 32 geothermal wells in the geothermal area in the northern edge of the Luzhong Uplift are studied, and the Piper diagram, Schoeller diagram, ion component ratio characteristics, isotopic characteristics, Na-K-Mg ternary diagram, mineral saturation index and SiO₂ geothermal temperature scale are used to analyze the recharge source, water-rock interactions, transport pathway, and cyclic evolution characteristics of the geothermal water.The results show that in the evolution of karst groundwater into geothermal water, the hydrogeochemical types gradually change from HCO₃·SO₄-Ca and HCO₃-Ca to SO₄·Cl-Na·Ca, SO₄·Cl-Ca·Na, SO₄·HCO₃-Ca, HCO₃·SO₄-Ca and Cl·SO₄-Na·Ca, and the TDS content increases successively. The contents of ${\rm{SO}}_4^{2-}$, Ca²⁺, Na⁺, F⁻, Li, Sr and H₂SiO₃ increase continuously, while the percentage of ${\rm{HCO}}_3^{-}$ content decreases. The temperatures, chemical compositions, and ion contents and characteristics of the karst water formed in different sections of the geothermal field are different, which proves that the recharge sources, circulation depths, and circulation paths of the karst water at different depths and in different sections of the geothermal field are different. The karst water with a lower temperature comes from the shallow and middle circulation flow system, while the geothermal water with a higher temperature in the deep part comes from the deep circulation with a larger circulation depth and a longer path of the cycle.The aquifers of the carbonate reservoirs in the study area are composed of limestone and dolomite deposited in a marine environment. The metamorphic coefficient (γNa/γCl), Cl/Br ratio, and desulfurization coefficient indicate that the aquifers of the carbonate reservoirs in the study area are poorly sealed, and the atmospheric precipitation gradually leaches the rock salt-bearing strata, so the geothermal water shows its characteristics of dissolved water. According to the Na-K-Mg equilibrium diagram, the hydrothermal water in the carbonate reservoir in the geothermal area at the northern edge of the Luzhong Uplift is in a non-equilibrium area and is not saturated. That is, the water-rock interaction has not reached the equilibrium state, and the dissolution is still in progress.The temperature of karst thermal reservoir in the geothermal area is 39–70 ℃, and the circulation depth of geothermal water is 856–1,877 m. By calculating the thermal storage temperature and the circulation depth of the geothermal water and combining with the analysis of the characteristics of the temperature measurement curve, it was determined that there is not only a conduction-type geothermal system but also a convection-conduction-type system in the study area.The geothermal water samples from the study area are distributed near the regional precipitation line, and the stable hydrogen and oxygen isotope compositions are similar to those of modern atmospheric precipitation, indicating direct or indirect recharge via the infiltration of atmospheric precipitation. According to the ¹⁴C ages of the geothermal water at the northern edge of the Ludong Uplift and the significantly negative hydrogen and oxygen isotope compositions of the geothermal water, the main source of the geothermal water supply is paleo-atmospheric precipitation in the southern mountainous area under the cold climate conditions during the Late Pleistocene, and after infiltration, the subsurface runoff is heated by the Earth's heat flow through deep circulation.

Hydrochemical characteristics and delimitation of sensitive protection area in the exposed area of Baimai spring group in Zhangqiu

LIU Yuxiang, LU Qianqian, LIU Yuanyuan, WANG Shaojuan, DING Guantao, HAN Yu

2023, 42(5): 1027-1036. doi: 10.11932/karst20230511

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Zhangqiu is rich in spring resources, known as the "Little Spring City", and Baimai spring group is among the 72 famous springs in Jinan. In recent years, the flow of Baimai spring group has been cut off for many times, and the competent departments have implemented a series of spring protection measures to basically maintain the continuous gushing of spring water. Against the backdrop of the planned development and construction of the core exposed area of the spring group, it is urgent to investigate the shallow micro-stratigraphic structure of the exposed area and its relationship with the spring water, in order to avoid the negative effect of engineering construction on the gushing of spring water. Based on the data collected from years of exploration for water supply in the exposed area of spring, this study has been conducted to identify the shallow stratigraphic structure and structural development characteristics of the exposed area of spring, with the methods of geophysical exploration, drilling, and water level observation. Meanwhile, the main influencing factors of spring water gushing in the exposed area have been analyzed, and the sensitive protection area has been delimited. The research findings may provide a scientific basis for the development and construction of the core exposed area of the spring group, and also for the spring water protection. The results show that the fault structures in the exposed area are highly developed, with a total of 5 faults and 1 fracture zone, becoming a channel for karst water upwelling. The distribution of spring water is closely related to structural development. The West Mawan spring group is mainly controlled by the Yanchi fault, with spring water distributed along both sides of the Yanchi fault, while the East Mawan spring group is mainly distributed along the secondary fault F3 of the Mingshui fault. The gravel layer is widely developed on both sides of the fault, showing a characteristic of thinning and gradually pinching out from the middle to both sides. The gravel layer is in direct contact with the underlying Permian Carboniferous bedrock, and the structure cuts the Permian Carboniferous aquifer, forming a connection between karst water and the pore water of the gravel layer. Pressurized karst water flows up along the fault structure and enters the gravel layer for storage. The hydraulic connection between pore water and karst water is very close, and the gravel layer has become the "skylight" of karst water. According to the data on groundwater level measurement of the exposed area in 2018, the elevation of pore water level is 55.68–55.71 m, and the elevation of spring and karst water level is 55.78–55.88 m. The karst water level is very close to the pore water level, slightly higher than the pore water level by about 10 cm, indicating a close hydraulic connection between the two, and the karst water supports the supply of pore water. The gravel layer provides storage space for karst water after upwelling, and springs emerge at shallower burial depths. The main factors affecting the gushing of spring water in the exposed area are the control of fault structures and the connectivity and permeability of gravel layers. Based on the comprehensive development thickness of fault structures and gravel layers, sensitive and relatively sensitive areas are delimited, with the area of 0.78 km² and 0.67 km², respectively. It is generally not suitable for excavation of foundation pits in sensitive protected areas; When conducting engineering construction in these areas, the excavation depth of the foundation pit should not be too large and the gravel layer should not be damaged. In addition to taking protective measures during the development and construction of underground engineering, measures of protecting water quantity and quality should also be taken above the surface of the core exposed area. For example, constructing buildings (structures) that may affect the flow of spring water or damage the appearance of famous springs should be prohibited. The discharge of sewage and wastewater into springs is not allowed. The projects such as construction of new karst water wells, excavation of springs, and interception and diversion of water should be banned. In addition, the protection in the recharge area of spring water is equally important. It is necessary to accelerate tree planting and afforestation, seal off mountains for afforestation and grass cultivation, protect vegetation, and conserve water sources. It is also necessary to restrict the construction of buildings (structures) or development projects unrelated to spring water protection, and avoid hardening the ground in the recharge area to reduce the recharge amount. It should be forbidden to dump, pile up or fill urban domestic waste, construction waste, industrial solid waste and hazardous waste. The research findings will provide reference for the protection of similar exposure areas of spring water.

A study on the groundwater recharge range of Baotu Spring based on wavelet analysis

QI Huan, DONG Mengyu

2023, 42(5): 1037-1046. doi: 10.11932/karst20230512

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As a famous "spring city", karst groundwater used to be the main source of water supply in Jinan City. However, with the acceleration of urbanization and the rapid increase of population, the pressure on urban water supply has increased, and hence the contradiction between supply and demand has become increasingly prominent. This situation will aggravate the over-exploitation of karst groundwater, resulting in a substantial decline in the karst groundwater level and outflow of Baotu Spring. How to deal with the contradiction between the continuous gushing of spring water and the citizens' demand for drinking high-quality underground water has always been an important issue of general concern to the Jinan Municipal Government and the general public. In recent years, Jinan City has successively adopted measures such as groundwater recharge and reducing groundwater exploitation to maintain the gushing of Baotu Spring, but the situation of keeping the springs gushing in the dry season is still severe.Scholars at home and abroad have conducted many studies on the formation mechanism, supply channels, dynamic characteristics, and influencing factors of Baotu Spring. However, there is still a lack of research on the groundwater recharge range of Baotu Spring, and on the proportion of urban and western suburbs to Baotu Spring's recharge, which has affected Jinan's scientific conservation of springs and rational development and utilization of groundwater resources. In order to find out the groundwater recharge range of Baotu Spring in Jinan City and the proportion of the recharge range of Baotu Spring in urban and western suburbs, this study selects the data on karst water levels from 20 long-term groundwater monitoring points in Baotu Spring area from 2010 to 2020 as well as the precipitation data of the same period, and analyzes the time lag of the groundwater level to the precipitation by using the method of cross wavelet transform. According to the hydrogeological conditions and the functional division of spring water, the groundwater recharge range of Baotu Spring is discussed, and the proportion of the groundwater recharge range of Baotu Spring in the urban and western suburbs is calculated.The results show that, (1) From the direct recharge area to the exposed area of confluence in the Baotu Spring area, with the rise of the length of groundwater runoff, the time lag of groundwater level to precipitation shows an increasing trend from 78.58 days to 129.22 days. The time lag gradient in the western suburb of Jinan City is larger than that in the urban area, with the time lag gradient decreasing gradually, and then tending to be stable. (2) A large area of groundwater recharge is distributed in the lower reaches of the Yufu river, with the characteristics of relatively flat terrain, slow surface runoff, strong rainfall infiltration capacity, slow groundwater flow velocity, and long runoff path, so the time lag of groundwater level to precipitation along the river is greater than that on both sides. The area of Liuchang mountain-Langmao mountain-Wanling mountain is a local surface watershed, with fast surface runoff, weak rainfall infiltration capacity, and relatively fast groundwater flow. Therefore, the time lag of the groundwater level to the precipitation in this section is smaller than that on both sides, and there is a hydraulic connection between the urban area and the western suburb of Jinan City. (3) The time lag contour of Baotu Spring water level to precipitation (107.30 days) is taken as the northern boundary of the Baotu Spring recharge range; the eastern boundary is the Dongwu fault; the western boundary is the Mashan fault; the southern boundary is the surface watershed. The groundwater recharge range of Baotu Spring is 1,390.54 km². (4) The area of Liuchang mountain-Langmao mountain-Wanling mountain is taken as the dividing line between the urban area and the western suburb of Jinan City, and the indirect recharge area of the spring is divided according to the surface water catchment. It is calculated that the recharge range of Baotu Spring in the western suburb is 1,133.09 km², and the recharge range of Baotu Spring in the urban area is 257.45 km², with a ratio of 4.4∶1 between the two recharge ranges. The supply of groundwater in the western suburb of Jinan has played a strong role in supporting the continuous gushing of Baotu Spring. Since the karst water system is a complex dynamic system, the response time lag of groundwater level to precipitation is affected by factors such as precipitation seasonality and human activities. Later on, the recharge range of Baotu Spring can be refined and verified by a smaller time scale, or from the perspective of water chemistry.

Groundwater hydrochemical characteristics and evolution of the karst water system in the Feicheng fault block in Shandong Province

ZHANG Wenqiang, TENG Yue, TANG Fei, WANG Jinxiao, XU Qingyu, ZHANG Hailin

2023, 42(5): 1047-1060, 1084. doi: 10.11932/karst20230515

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The Feicheng area is one of the important industrial bases in Shandong Province with the early development of coal mine, power generation and chemical industry. Besides, vegetable farming is widely distributed. Karst groundwater is the main source of water supply and the only source of drinking water in this area. In recent years, due to human activities such as industrial and agricultural exploitation of groundwater, and reduction of water emissions from coal mine closures, the groundwater dynamic and chemical fields in this area have undergone changes. The current status of groundwater environmental quality needs to be determined. On the basis of hydro-geological survey, this study collected 59 groundwater samples and 6 river water samples in the dry season of 2022, and comprehensively used mathematical statistical methods, hydrochemical methods (Piper three-line diagram, Gibbs model, mineral saturation index, ion proportion analysis) to explore the hydrochemical characteristics and evolution rules of groundwater in the Feicheng fault block.The results show that, (1) The average pH of groundwater in the study area is 7.34–7.47, all of which are weakly alkaline, and Ca²⁺, Mg²⁺, ${\rm{HCO}}_3^{-}$ and ${\rm{SO}}_4^{2-}$ are the main ions in the water. The hydrochemical composition is mainly controlled by water rock interaction, while the influence of atmospheric precipitation and evaporation concentration is relatively small. The water-rock interaction of calcite and dolomite plays a major role in controlling the chemical composition of karst water and fissure water, and the dissolution of evaporite plays an important role in the chemical composition of pore water. The mineral saturation index shows that most of calcite and dolomite is in a saturated state, while gypsum and halite minerals are in a dissolved but unsaturated state.(2) Karst hydrochemical types in the area are mainly HCO₃-Ca (Mg) type, accounting for 47.9%, followed by HCO₃·SO₄-Ca type and HCO₃·Cl-Ca type, 16.7% and 20.8%, respectively. Pore water is mainly of SO₄·HCO₃-Ca type and HCO₃·Cl·SO₄-Ca type, with SO₄·NO₃-Ca type appearing locally in the area of Wangguadian town. The chemical types of river water are relatively complex, including SO₄·HCO₃-Ca·Na type, Cl·SO₄-Ca·Na type, etc.(3) From the perspective of component content, the average TDS content of pore water, karst water, and fissure water is 1,207.13 mg·L⁻¹, 670.96 mg·L⁻¹, 514.5 mg·L⁻¹, respectively. The coefficient of variation of Ca²⁺ and Mg²⁺ in groundwater is relatively small, indicating that they are relatively stable ions. The average values of ${\rm{SO}}_4^{2-}$, Cl⁻ and ${\rm{NO}}_3^{-}$ in pore water are 288.29 mg·L⁻¹, 155.86 mg·L⁻¹, 195.41 mg·L⁻¹, respectively, with high content and small coefficient of variation, indicating that they are mainly influenced by external inputs from human activities. The content of ${\rm{SO}}_4^{2-}$, Cl⁻ and ${\rm{NO}}_3^{-}$ in karst water is relatively low, but the coefficient of variation is large, with uneven concentration distribution, and occurrence of enrichment in local areas. The average ${\rm{SO}}_4^{2-}$ in the Huihe river reaches 345.83 mg·L⁻¹, which is much higher than that of the surrounding groundwater and has a small coefficient of variation, indicating that its source is mainly from external inputs and may be related to the drainage of surrounding coal mines.(4) Some sources of Ca²⁺ are related to the dissolution of gypsum minerals and the infiltration of pollutants containing Cl⁻ and ${\rm{NO}}_3^{-}$, and the nitrate pollutants mixed into aquifers may promote the dissolution of carbonate rocks. The cation exchange is weak in groundwater, but strong in river water.(5) From a time-scale perspective, the groundwater quality in the study area has shown a decreasing trend compared to the quality in 1999 and 2013, with significant increases in the content of Cl⁻, ${\rm{SO}}_4^{2-}$, and ${\rm{NO}}_3^{-}$ in groundwater over the years. From the perspective of groundwater types and spatial distribution, the chemical characteristics of groundwater in the Feicheng area are significantly influenced by human activities. Both the concentrations of nitrogen and chlorine are generally low in the fissure water of magmatic rocks, and hence the water quality is relatively the best. The overall quality of karst water is good, and the rise of the water level in the closed pit mine in Feicheng has not caused cross-layer pollution to the surrounding karst water. However, there are local occurrences of "high nitrogen" or "high chlorine" in Wangzhuang, Taoyuan, Shiheng, Dayang, and other places, which may be affected by pollution sources such as agricultural fertilization, livestock breeding, and domestic sewageinfiltration. Both the concentrations of nitrogen and chlorine are high in most of the pore water, and hence the water quality is generally poor, with ${\rm{NO}}_3^{-}$, as the main ion exceeding the permitted level, which may be related to the application of chemical fertilizer for large-scale agricultural vegetable farming in the study area, and the infiltration of surface nitrate and other pollutants into the groundwater with rainwater. The nitrogen concentration in both Kangwang river and the Huihe river is generally low, while those of chlorine and sulfur are high, reflecting the significant impact of urban sewage discharge and coal mine drainage on these two rivers.

Hydrogeochemical evolution characteristics of Baotu Spring in Jinan City, based on long-term monitoring

LIU Haoran, ZHANG Wenqiang, LIU Wen, MA Xueying, GUAN Qin, ZHANG Hailin

2023, 42(5): 1061-1073. doi: 10.11932/karst20230516

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Baotu Spring in Jinan City is of great significance in the fields of ecological regulation, history and culture, tourism economy, etc.; therefore, it is urgent to study its hydrogeochemical evolution characteristics and genesis mechanism, so as to provide a scientific basis for the high-quality protection of Baotu Spring. Based on the long-term hydrogeochemical monitoring data of Baotu Spring, its water chemical dynamic characteristics as well as its hydrogeochemical evolution over past 60 years have been analyzed in this study.The results show that the chemical composition of Baotu Spring presented a significant regularity from 1958 to 2022. The average contents of main ions in Baotu spring were ${\rm{HCO}}_3^{-}$, Ca²⁺, ${\rm{SO}}_4^{2-}$, Cl⁻, ${\rm{NO}}_3^{-}$, Na⁺, Mg²⁺ and K⁺ in a descending order, among which ${\rm{HCO}}_3^{-}$ and Ca²⁺ were the dominant ions of anion and cation in spring water. The variation coefficients of ${\rm{SO}}_4^{2-}$ and Na⁺ contents were relatively large, indicating the varieties of groundwater environment in different periods. The variation coefficients of ${\rm{HCO}}_3^{-}$, Ca²⁺ and Mg²⁺ contents were relatively small, showing that the source of related ions was stable. The specific gravity of ${\rm{SO}}_4^{2-}$, Cl⁻, and Na⁺+K⁺ gradually increased, and the hydrochemical type evolved from HCO₃-Ca to HCO₃·SO₄-Ca. In recent years, the hydrochemical type has shown the HCO₃·SO₄-Ca·Na type in several periods, indicating that the water chemical components of Baotu Spring are increasingly complex and diversified. The water chemical components of Baotu Spring in the Gibbs chart showed a trend of upward shift to the right, indicating that this spring was mainly controlled by water-rock interaction, and was increasingly influenced by other factors such as human activities. The ion ratio method indicates that the mineral weathering dissolution of Baotu Spring shifts from carbonate rock to silicate rock, and the ratio of (K⁺+Na⁺)/Cl⁻ is generally below and gradually away from the line of 1∶1, indicating that there are other sources of Cl⁻ different from those of Na⁺, and the sources have increased year by year. The contents of Ca²⁺ and ${\rm{HCO}}_3^{-}$ ions in Baotu Spring show a gradual increasing trend, which indicates that the dissolution of carbonate rock has increased gradually. In addition, the Chlor-alkali index (CAI) shows that the water-rock interaction in Baotu Spring is dominated by the dissolution of carbonate rock, while the dissolution of gypsum and silicate rock plays a secondary role, but has gradually increased, indicating that the spring recharge area in the southern part of the gypsum and silicate rock stratum has a stronger recharge effect on groundwater runoff in Baotu Spring. The cation exchange in Baotu Spring is weak on the whole, but it has gradually increased over the years, and the cation exchange is stronger in the dry season than the wet season. The mineral saturation index of Baotu Spring is in a supersaturation state as a whole, and its dispersion becomes larger with the passage of time. Over the years, the mineral saturation index of Baotu Spring during the wet season has been generally higher than that during the dry season and normal season, but the performance is various in different periods. From 1950s to 1980s, the mineral saturation index of Baotu Spring during the wet season was lower than that during the dry season, and the mineral saturation index of Baotu Spring during the wet season was higher than that during the dry season after 1990s, indicating that with the increase of human activities in different historical periods, the source of groundwater and the water-rock interaction changed in dry and wet seasons. The contents of ${\rm{NO}}_3^{-}$/Cl⁻ and Cl⁻ increased first and then decreased under the influence of agricultural activities and domestic sewage discharge. In 1950s–1960s, the contents were greatly influenced by agricultural activities. Since 2000s, with the continuous improvement of spring protection and ecological environment, agricultural activities have gradually weakened the effect, while the influence caused by emission of domestic sewage and other human activities have been strengthened by degrees. Due to the influence of other human activities other than gypsum dissolution, the contents of ${\rm{SO}}_4^{2-}$ have increased gradually since 2010s.In summary, in the quasi-natural state from 1950s to 1960s, the contents of chemical components of Baotu Spring were relatively low. From 1970s to 1980s, the contents were increasingly influenced by agricultural activities, industrial and mining activities and domestic sewage discharge. From 1990s to 2000s, the contents of ${\rm{NO}}_3^{-}$, Cl⁻ and ${\rm{SO}}_4^{2-}$ gradually increased under the comprehensive influence of human activities such as agricultural pollution in the recharge area, industrial and mining activities, domestic waste landfill, etc. Since 2010, the pollution of industrial and mining enterprises and domestic life has alleviated, and the protection measures of Baotu Spring such as ecological groundwater recharge have become normal. The contents of Ca²⁺ and Mg²⁺ in non-carbonate karst hydrolysis increased gradually, and the contents of Cl⁻ and ${\rm{SO}}_4^{2-}$ affected by human activities increased significantly. The research shows that the water chemical components of Baotu Spring have mainly come from the water-rock interaction since 1958, and the influence of human activities has been increasing continuously. In different historical periods, different human activities (agricultural activities, industrial and mining activities, groundwater recharge, etc.) have different effects on the hydrogeochemical evolution of Baotu Spring.

Source-tracing of sulfate in groundwater of Xizhang Well Group in the Liuzheng water source area of Zibo City

LIU Zhizheng, LI Shuang, LIU Baihan, ZHANG Xuesong, LIU Huafeng, PENG Junfeng, LIU Yunde

2023, 42(5): 1074-1084. doi: 10.11932/karst20230517

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Affiliated to both Linzi district of Zibo City and Qingzhou City of Weifang City, the Liuzheng water source area belongs to the run-off and drainage area of the Dawu hydrogeological unit. This area plays a significant role in the water supply for residents and enterprises, and thus enjoys an important strategic position. There are four pumping well groups in the water source area, among which the Xizhang Well Group is located in the west of the water source area, including four wells from LK05 to LK08. The water source area is located in the intermountain depression and river valley area with the Quaternary system of 20-meter to 70-meter thickness, underlain with Jiulong Group (Є₃-O₁J) and Majiagou Group (O_2-3M). Zihe fault zone, Bianhe Fault, Wangzhai fault and other fault structures are developed in this area, which control the stratum distribution and groundwater movement. At present, the water-bearing rock groups with water-supply significance in the water source area are mainly those of carbonate fissure karst, which are representatives in areas of karst water source in the north of China. The aquifer lithology, highly rich in water, is mainly composed of limestone, dolomitic limestone and dolomite of the Majiagou Group and Jiulong Group of Sanshanzi Formation. The overall runoff of karst groundwater is northeast oriented along the fault zone of the Zihe river, and the groundwater dynamics are influenced by human mining and atmospheric precipitation. The hydrochemical type is mainly HCO₃-Ca (Ca·Mg), and the water quality is generally good, but the indexes of sulfate, total hardness and ammonia nitrogen in the Xizhang Well Group exceed the permitted standard of Class III groundwater. Based on the test analysis of karst groundwater in Xizhang Well Group and its surrounding areas, it is found that the area where sulfate exceeds the permitted level is mainly distributed in the area of Yanghuya-Shuangyangyouzhi, and the sulfate content decreases from the center of this area to its surrounding parts. At present, the research on karst water system in the Liuzheng water source area mainly focuses on the sequential dynamic change of water quality and quantity and the spacial distribution characteristics. However, few studies have been conducted on the directional sources of chemical components in karst groundwater. In order to ensure the safety of water supply, it is urgent to carry out the source-tracing analysis of groundwater sulfate under the influence of human activities. The sources of chemical components in groundwater are generally studied systematically by statistical analysis, hydrogeochemical analysis, fuzzy mathematics, numerical simulation and other methods based on geological and hydrogeological conditions. All kinds of methods have specific applicable conditions and advantages, but most of them are difficult to trace the source quickly and accurately, and identify the specific migration process. In this study, hydrogeochemical analysis, tracer test, isotope indication analysis and other methods are used to carry out a comprehensive source-tracing analysis of sulfate in the Xizhang Well Group. The results show that the method used in this study is effective in source-tracing. Based on the analysis of the dynamic characteristics of groundwater and the mechanism of recharge, runoff and discharge in the Liuzheng water source area, the power source of sulfate source was analyzed by tracer test. Besides, the main path of sulfate migration was described, and the target area of sulfate source—Wangzhai basin, a special recharge area of water source—was determined. On the basis of power source analysis, isotope samples were collected in the target area and its surrounding areas, and the contribution of different end-member sources to groundwater sulfate was calculated by using the ternary mixing model of sulfate content and sulfur and oxygen isotope composition, and then the material sources of the pollution in the Xizhang Well Group were quantified. The calculation results show that the proportion of sulfate from sulfide oxidation in groundwater is large, and there are 17 water samples exceeding 50%. Because there is A Pollution Source instead of coal-bearing strata in the area, the A end-member represents that the sulfate of this area comes from A Pollution Source. There were no water samples occupying more than 50% in the B end-member, indicating that the contribution of atmospheric precipitation to sulfate in water was small. There were 3 water samples accounting for more than 50% in the C end-member, indicating that other human factors also contributed to a certain proportion. The comprehensive analysis confirms that the main source of sulfate in the karst groundwater of the Xizhang Well Group is the A Pollution Source in Wangzhai basin, followed by the atmospheric precipitation and other unknown factors.

Study on the early warning model based on the occurrence index of karst collapse in Linyi City

ZOU Lianqing, CHU Fujian, WANG Zhentao, YAO Chunmei, CHEN Yanan, LIAN Bo, WEI Zhongai, MENG Yonghui

2023, 42(5): 1085-1097. doi: 10.11932/karst20230518

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The urban area of Linyi City is endowed with the basic conditions for the development of karst collapse. In this area, Ordovician limestone with karst development is widely distributed, and the limestone is covered by a thin Quaternary overburden with a binary phase structure. From the 1980s to the early 21^st century, the rapid development of Linyi City, the increase of groundwater exploitation and the decline of the overall regional water level led to the frequent occurrence of geological disasters of karst collapses, causing serious economic losses and social impact.In this study, the karst collapse in Linyi City is taken as the research object. The distribution law, geological conditions, and inducing factors of karst collapses are studied through statistical analysis. According to the analysis of the genesis mechanism of karst collapse in Linyi City, an early warning model based on the occurrence index of karst collapse is proposed. Taking the water level as the main monitoring factor, this model is constructed to comprehensively judge the early warning by the combination of multi-factors, and is verified by historical data.During the high incidence period from 1993 to 2012, a total of 17 karst collapses occurred in the study area. These karst collapses were caused by geological conditions such as topography, geological structure, and formation lithology, as well as inducing factors such as groundwater exploitation, human engineering activities, and precipitation. The distribution law of karst collapse reflects the consistency with its related influencing factors. Karst collapses are mainly distributed in the caprock area of double-layer structure with shallow karst development and small overburden thickness. They are also distributed near the surface water body and fault zone. Generally, karst collapses take place in the funnel areas and in the seasons with a large variation of water levels, They also occur in the influence range of human engineering activities. With a comprehensive index method, an early warning model based on the judgment made through multi-factors is established. The model comprehensively considers geological conditions and inducing factors and selects 10 influencing factors, including karst development degree, caprock thickness, caprock structure, distance from structure, distance from surface water, distance between water level and limestone roof, water level amplitude, distance from the center of depression cone, precipitation, and human engineering activities. Different weights of influencing factors and the range of influencing factors of each sub-condition are given and the early warning level is divided into four grades according to the occurrence index value.The model is verified by using historical data of the collapse when it occurred. For example, the data of 2003 was used for simulation and early warning validation. The karst collapse occurred in the east of the No.32 building in Lanshan community on May 8 and June 22 in 2003, and all the collapse points were located in the area at a high early warning level, and also fell into the time period of high early warning level. The results show that the model has high reliability in the verification of early warning in key monitoring areas of Linyi City, which can provide a reference for monitoring and early warning of karst collapse in other areas.

Response of soil microbial biomass carbon and nitrogen to vegetation succession in different soil depths of karst fault basin

YAN Jiahui, QIU Jiangmei, LI Qiang

2023, 42(5): 1098-1105. doi: 10.11932/karst20230519

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The karst fault basin in Yunnan Province is a typical ecological fragile area in Southwest China, and it is characterized by dramatic topographic changes, coexistence of faulted basins, basins and mountains, and seasonal drought and water shortage, so surface water erosion likely occurs. Soil microorganism promotes the circulation of nutrient elements in soil, among which microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN)—the indicators of soil quality—are of great research significance. In this study, primitive forests, grasslands, artificial forests, shrubs and corn fields in the karst fault basin of Yunnan were selected as the study objects. The changes of MBC and MBN in soil were measured, and then their response to vegetation types was analyzed. The results showed that the contents of soil MBC and MBN in shrubs were the highest, indicating that the shrubs in this study area are more conducive to the accumulation of soil MBC and MBN. In contrast, the contents of soil MBC and MBN in primary forests were lower than those in shrubs, which was considered to be the result of the the decrease of effective nutrients available for vegetation growth due to the degradation of primary forests, or the intense competition between plant growth and microorganisms for nutrients in primary forests and the fact that the demand capacity of vegetation growth was greater than the conversion capacity of soil microorganisms. The study also found that the contents of MBC and MBN increased at first and then decreased with vegetation succession, which may be related to the organic matter content and oxygen availability as well as the surface layer that conforms to microbial growth conditions. The content of soil MBC decreased with the increase of soil depth. Moreover, the soil depth and vegetation type are the significant influencing factors of the content of soil MBC (P<0.05), and the soil depth was the most significant factor (P<0.01). These findings provide a theoretical basis for the control of soil erosion and rocky desertification in karst fault basin of Yunnan Province.

Stoichiometric characteristics of C, N and P in soil and litter of shrublands in karst areas of Guangxi

YUE Xiangfei, LI Yanqing, LIU Peng

2023, 42(5): 1106-1116. doi: 10.11932/karst2023y032

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This study was conducted at seven typical karst areas (Longlin, Lingyun, Donglan, Hechi, Luocheng, Rongshui and Guilin) of Guangxi, China. These areas present subtropical monsoon climate, with annual temperatures averaging from 19.27 ℃ to 20.73 ℃, and the annual rainfall averaging from 1,149.2 mm to 1,931.3 mm. This study focused on the stoichiometric characteristics and the influencing factors of organic carbon (C), nitrogen (N) and phosphorus (P) in soil and litter of typical karst areas. By field sampling and laboratory experiment, the stoichiometric characteristics of soil organic C, N, and P, and litter C, N and P of 63 shrub quadrats in the seven typical karst areas were investigated. Besides, the correlation between the variation in stoichiometric characteristics of soil and environmental factors were analyzed.The research findings show that: (1) the mean content of C, N and P in 0–40 cm soil was 31.90 g·kg⁻¹, 3.95 g·kg⁻¹ and 2.65 g·kg⁻¹, and the coefficient of variation was 40.13%, 41.46% and 65.24%, respectively. The mean value of C/N, C/P and N/P was 8.10, 14.82 and 7.80, and the coefficient of variation was 14.88%, 48.78% and 41.33%, respectively. The values of C/N, C/P and N/P in 0–20 cm soil were slightly higher than those in 20–40 cm. There were significant differences in soil C/N, C/P and N/P between 0–20 cm and 20–40 cm (P >0.05). (2) There were significant correlations among the contents of soil C, N and P (P <0.01). The stoichiometric characteristics of soil nutrients elements were significantly affected by litter N and P (P<0.05) which was negatively correlated with the C/N ratio of litter. There was a moderately negative correlation between mean annual precipitation and soil C and P. However, the mean annual temperature was negatively correlated with soil C, C/N and C/P, which was positively correlated with soil N.The soil C, N and P of typical karst shrubland in Guangxi presents high spatial heterogeneity, with the characteristics of low C and high P, and the litter is characterized by low C, high P, low C/P and low N/P. Litter N and P is the important factor affecting the stoichiometric characteristics of soil nutrients in shrubland of the karst area. The increase of mean annual temperature will promote the increase of soil N and decrease soil C/N, and the increase of mean annual precipitation will reduce soil C and P. Our research results have great significance for the theoretical improvement of the stoichiometric study on soil ecology, vegetation restoration in degraded karst ecosystem and the response of karst ecosystem to climate change in Southwest China.

Multi-scaled analysis of spatial-temporal evolution of vegetation ecological quality in the karst area of Guangxi

MO Jianfei, CHEN Yanli, MO Weihua

2023, 42(5): 1117-1130. doi: 10.11932/karst2023y037

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The karst area in Guangxi is characterized by its extensive and typical landform development, covering a total area of 8.334 million hectares, 18.9% of the total karst area in Southwest China and 35.1% of the total land area in Guangxi. This area is marked by poor and shallow soil, dominated by shrubs, shrub-grass, and grasslands as its vegetation. Therefore, it is subject to climate changes and meteorological disasters with a weak disaster-bearing capacity. The area is a typical ecologically vulnerable region in the southwest of China, and it is also the focus area for the Guangxi government to carry out ecological protection and restoration and rural revitalization. The scientific and rational assessment of vegetation ecological quality and its spatial-temporal evolution in the karst area is crucial for ecological restoration and governance, and the achievement of the "dual-carbon" goal. In order to monitor and assess the status of vegetation ecological quality in karst areas more objectively, and to clarify the spatial-temporal heterogeneity of vegetation ecological quality at different time scales, this study took the vegetation in the karst area of Guangxi as object. Based on the principle of "similar habitat" for vegetation ecological restoration, the "3S" technology was used to monitor and assess the vegetation ecological quality in the study area at different spatial and temporal scales. Firstly, the climate data was used to calculate the multi-year moisture index (MI) of the study area and delineate the climate gradient. Secondly, the maximum net primary productivity (NPP_m) of the vegetation in the corresponding period was calculated. The correlation between MI and NPP_m of the vegetation was analyzed, and the NPP_m edge function of different vegetation types was constructed to determine the natural "baseline" of the potential productivity of vegetation within the climate gradient. Then a comprehensive vegetation quality model (MQI) for the karst area of Guangxi was built. Finally, the ecological quality index of vegetation was respectively calculated at monthly, quarterly, annual, and interannual scales to conduct a multi-scaled analyze of spatial-temporal evolution of vegetation ecological quality in the study area from 2000 to 2019. The results showed that: (1) There were obvious differences in the potential of vegetation ecological restoration under different climatic conditions in the karst area of Guangxi. The NPP_m was a dynamic value that changes with climatic conditions, and different vegetation types responded differently to climatic conditions. Among these types, farmland vegetation was the most sensitive, followed by shrub-grass, and forest was the least affected. (2) There were significant temporal and spatial differences in the vegetation ecological quality at monthly, quarterly, annual, and interannual scales in the karst area of Guangxi. Temporally, the ecological quality indexes of vegetation at monthly and quarterly scales followed a parabolic pattern, and the annual index showed an increasing trend with fluctuation. The interannual evolution of vegetation ecological quality experienced four stages: slow growth, gradual growth, rapid growth, and significant growth. Spatially, vegetation ecological quality gradually increased from the northeastern to the southwestern part and from the northern to the southern part of the study area, with an overall high ecological quality. (3) There was a significant improvement in vegetation ecological quality in the karst area of Guangxi. From 2000 to 2019, the index of vegetation ecological improvement was 0.71/20a in the study area, with 98.83% of the regional vegetation ecological quality showing an upward trend over the 20-year period. Most of the improvement of regional vegetation ecology was favorable, primarily attributed to the national policies of returning farmland to forests, projects of rocky desertification control, and the favorable climate conditions in Guangxi. (4) The model for vegetation ecological quality of the study area exhibits high regional suitability and is able to finely and accurately reflect the spatial-temporal evolution characteristics of vegetation ecological quality in the study area.

2023 Vol. 42, No. 5

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