Geochemical characteristics of spring water in Baishuitai of Yunnan and their indicative significance on climatic environment
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摘要: 为研究岩溶地下水的水文地球化学演化特征,对云南白水台地区雨水和泉水中氢氧稳定同位素(δD、δ18O)组成和微量元素含量进行了为期近5年(2018年1月至2022年10月)以月为频率的连续监测。结果表明:(1)白水台雨水中δD、δ18O具有明显的季节性变化特征,均表现为雨季偏轻,旱季偏重,主要受到水汽来源与蒸发条件的影响;(2)白水台泉水受当地大气降水补给,其δD、δ18O经历了石灰岩基层中不同通道、裂隙网络中的新、老水混合导致的同位素调蓄平滑作用以及与深部来源CO2的氧同位素交换作用,修正性继承了雨水中δD、δ18O的部分特征;(3)泉水中Ca、Mg、Sr、Ba、Si等元素主要来源于喀斯特水体侵蚀下围岩的溶解,其含量变化可能指示了降水量的变化;而Fe、Al、Mn等元素主要来源于大气降水对上覆土壤的淋滤作用,其浓度变化可能反映了降水强度的变化。Abstract:
Understanding the geochemical characteristics and dynamic changes of groundwater (e.g., springs, etc.) is an important scientific reference for accurately interpreting the paleoclimatic and environmental information on karst deposits (e.g., stalagmites, travertine, etc.). In this study, we continuously collected local rainwater samples and S3 spring water in Baishuitai area of Yunnan Province on a monthly frequency for nearly 5 years (January 2018 to October 2022). We analyzed the temporal variations of stable isotopes of hydrogen and oxygen (δD and δ18O) compositions and trace element contents in the two types of water bodies to identify the recharge source and hydrogeochemical evolution process of Baishuitai spring water, and to reveal the source differences of trace elements in spring water and their indicative significance on climate and environment. The results showed as follows. (1) Values of stable isotope composition of rainwater in Baishuitai region of Yunnan had obvious seasonal variation characteristics—high in the dry season and low in the rainy season. This result was mainly affected by water vapor source and evaporation conditions. During the dry season, water vapor mainly came from the stable continental air mass, and the evaporation effect was strong, which resulted in the enrichment of D and 18O in the remaining water vapor mass. During the rainy season, multiple fractionation and condensation processes occurred when water vapor migrated from the sea to the land, due to the influence of the southwest monsoon and the southeast monsoon. D and 18O were severely scoured, thus making δD and δ18O light in rain during the precipitation process. (2) The stable isotopic compositions of hydrogen and oxygen in Baishuitai spring water were on or near the atmospheric precipitation line, indicating that the spring water was mainly supplied by atmospheric precipitation. During the infiltration of atmospheric precipitation into the zone of shallow circulation runoff, new and old water from different channels and fissure networks was continuously mixed to regulate and store hydrogen and oxygen isotopes in groundwater, resulting a much smaller variation amplitude of δD and δ18O in spring water than in rainfall. During the rainy season in 2018, 2019 and 2022, δD and δ18O in spring water witnessed a similar trend to δD and δ18O in rainwater with a lag of about one month, indicating that δD and δ18O in water evolved to some extent in underground runoff, so that δD and δ18O in spring water correctly inherited some of the characteristics of δD and δ18O in rainwater. The δD-δ18O scatter points of spring water in Baishuitai region all shifted to the area near the left side of the atmospheric precipitation line. This means that δ18O in spring water became lighter under the exchange of lighter oxygen isotopes with CO2 gas from deep sources, while δD basically did not change, indicating that atmospheric precipitation was fully mixed with deep source CO2 during the infiltration process. The smoothing effect of isotope regulation and storage generated by the mixing of old water, as well as the exchange of oxygen isotope with CO2 from deep sources, indicated that the Baishuitai region contains a wide spring area, numerous underground passageways and fissures, and intricate karst features. (3) The trace elements in Baishuitai spring water show two types of dynamic change characteristics. Elements such as Ca, Mg, Sr, Ba, and Si constitute one category, mainly originating from the dissolution of surrounding rocks under karst water erosion. After entering the rainy season, the water–rock interaction was weakened by the dilution of rainfall. Concentrations of these elements decreased to varying degrees in the early rainy season, and increased in the late rainy season to the transition from the rainy season to the rainy and dry season, while these concentrations stayed stable or slightly fluctuated in the dry season. The decrease in precipitation promoted the precipitation of calcium carbonate caused by degassing, which increased the values of Mg/Ca, Sr/Ca and Ba/Ca. At the same time, water within the fractures between soil and bedrock was less disturbed by fresh water inputs. This reduction in the rate of water migration enhanced the interaction between water and rock, resulting in a preferential leaching of elements such as Sr and Ba. The consequent differential changes in the concentrations of these elements suggest that variations in their levels could be indicative of shifts in precipitation patterns. Elements like Fe, Al, and Mn constitute another category, primarily originating from the leaching effects of atmospheric precipitation on the overlying soil. The occurrence of peaks in these elements during the rainy season corresponds to periods of heavy annual precipitation, while peaks observed in the dry season may be associated with the pulse-like action triggered by a single instance of heavy precipitation. -
Key words:
- hydrogen and oxygen stable isotopes /
- trace element /
- spring water /
- travertine /
- Baishuitai of Yunnan
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0. 引 言
重建高分辨率的古气候环境变化历史对于深入理解全球气候变化及其驱动机制具有重要意义[1]。钙华又名石灰华,是在地表由岩溶泉、河、湖水沉积形成的大孔隙次生碳酸钙沉积。因其时间跨度长、沉积速率高、对气候响应敏感,钙华被认为是重建年际、年代际尺度古气候环境变化的良好载体,正逐渐成为第四纪研究的新热点[2−7]。然而,钙华沉积过程受到多种因素影响,如何准确利用钙华进行古气候环境重建仍需要进一步深入研究[5]。研究沉积钙华水体(如泉水等)的地球化学特征及动态变化规律有助于深入解译钙华中地球化学指标(如碳氧稳定同位素、微量元素等)的气候环境指示意义[8-9]。
在云南白水台地区,分布着我国规模最大的冷泉型钙华台地,近年来受到国内外学者的广泛关注[10−16]。当地若干富含高浓度碳酸氢钙的常流泉(图1中S1、S2和S3号泉)出露地表后,由于水中CO2的脱气作用,发生大量的碳酸钙沉积,从而发育了数十米厚的钙华台地。现代监测结果表明这些泉具有相近的化学组成,水化学类型均为HCO3-Ca型;泉水流量大且相对稳定,如S3号泉为60~70 L·s−1,水温常年波动小[17]。孙海龙等[18]分析了2006年5月至2007年5月期间白水台泉水(S1、S2和S3)和大气降水的氢氧稳定同位素特征,发现研究时间段内雨季大气降水的δD、δ18O偏负、旱季大气降水的δD、δ18O偏正,认为当地降水云团存在季节性变化;而泉水的δD、δ18O组成相对稳定,认为泉域范围具有相当的调蓄能力。吕保樱[19]则分析了2006年4月至12月期间白水台S1号泉泉水的微量元素特征,发现泉水中 Sr、Fe、Co、Ni、U等元素的含量随时间变化比较稳定,受单次降雨影响非常小。然而,由于对白水台泉水的监测时长相对较短(≤1个水文年),以往研究揭示的泉水中氢氧稳定同位素、微量元素等在季节和年际尺度上的动态变化规律仍需进一步验证;此外,白水台泉水中不同微量元素的来源差异及气候环境指示意义尚不清晰,有待进一步研究。
本文对云南白水台地区S3号泉水进行了为期近5年(2018年1月至2022年10月)以月为频率的连续采集,同时采集当地大气降水样品,通过分析两类水体中氢氧稳定同位素(δ D、δ18O)组成和微量元素含量的时序变化特征,识别白水台泉水的补给来源及水文地球化学演化过程,揭示泉水中微量元素的来源差异性及气候环境指示意义。
1. 研究区域概况
白水台位于云南省迪庆藏族自治州香格里拉县三坝乡白地村,与香格里拉市中心相距约 100 km(图1a)。该研究区位于横断山区西北部峡谷地带,海拔
2 380 ~2 600 m,区内有三叠系地层出露,岩性以石灰岩为主,还包括砂页岩、火山岩等,在构造运动与强烈岩溶作用下发育有断层、裂隙和溶洞等,为白水台地区降水入渗和地下径流提供良好条件[20]。研究区地处低纬度区域,属于亚热带季风气候,干湿季分异明显,雨季通常于5月开始10月结束,集中全年75%以上的降水量,最高温出现在7月,为20~25 ℃,最低温出现在1月,为2~5 ℃[21]。东西向大断裂的存在使得地下水以泉群的形式出露,泉水在地表流动过程中由于CO2脱气作用析出碳酸钙沉淀,形成面积达3 km2的钙华台地[9]。本文重点关注于S3号泉水,该泉水距离白水台景区2.5 km,海拔约
2 950 m,受到景区人为干扰因素小,能够更真实地反映白水台地区泉水地球化学特征。2. 研究方法
2.1 样品采集
2.1.1 泉水样品
为研究白水台泉水中氢氧稳定同位素和常微量元素的动态变化及内在联系,以月为频率连续监测当地S3号泉(图1),共采集泉水样品55件。采集的泉水用 0.45 µm 水相膜过滤,分别装入两个聚乙烯采样瓶中。向其中一瓶加入少量浓硝酸至溶液pH 小于1,用于常微量元素分析;另一瓶保持原液,用于氢氧同位素分析。采集好的样品冷藏备用。
2.1.2 雨水样品
为揭示云南白水台地区泉水的来源,本文采集了白水台2018年以来不同季节下的雨水样品。雨水的采样点位于白水台景区旁一宾馆的楼顶(图1b),共采集雨水样品21件。雨水样品的保存方法与泉水相同。
2.2 样品分析
2.2.1 氢氧稳定同位素
水样品的氢氧稳定同位素在华东师范大学地理科学学院液态水稳定同位素激光光谱分析仪(T-LWIA,型号:912-0050)上测试。采用国际原子能机构(IAEA)提供的标准样品(2C、3C、4E、5C)对测试结果进行标定。将样品冷却至室温,取1.0 mL注入进样瓶。每个样品设置三个平行样,每个平行样抽取6针进行测试(每针1.2 μL)。测试结果采用相对于维也纳标准平均海洋水(VSMOW)的千分差(δ)表示:
δ(‰)=(RSAMPLE/RV−SMOW−1)×1000 (1) δ为同位素组成,R为同位素比值测试值。测试精度δ D < 0.10‰,δ18O <0.02‰。为减小记忆效应造成的数据偏差 [22],计算过程中剔除每个样品的前两针数据,仅对后四针(每个样品共计12针)结果进行算术平均获得同位素组成。
2.2.2 常微量元素
取前期加酸保存好的水体样品,稀释至不同倍数。制备好的样品在华东师范大学地理科学学院电感耦合等离子体发射光谱仪(ICP-OES,Varian 710-ES)测定样品中Ca、Mg、Sr、Ba、Na、K、Fe、Al、Mn、Si等元素浓度。测试误差小于 5%。空白加标回收率结果显示:Ba、Fe、Mn、K、Na、Sr、Mg的回收率在 90%~110% 之间;Ca、Mn、Al、Si 的回收率在 80%~120% 之间。
3. 结果与分析
3.1 氢氧稳定同位素特征
3.1.1 组成和时序变化特征
D、18O能够敏感地响应环境变化,其丰度变化可以用于指示各类水体的水汽来源、反演大气过程[23-24]。白水台雨水的氢氧稳定同位素(δ D和δ18O)测试结果显示(图2(b)和(c)):δ D的变化范围为 −163.76‰~45.60‰,变化幅度为209.36‰,平均值为−67.53‰;δ18O的变化范围为 −21.77‰~6.83‰,变化幅度为28.60‰,平均值为−8.91‰。雨水中δ D、δ18O的变化范围宽、幅度大。其中,旱季δ D平均值为24.38‰,雨季δ D平均值为−84.76‰;δ18O在旱季平均值为2.32‰,雨季平均值为−11.01‰。雨水中δ D和δ18O均表现出雨季偏轻旱季偏重的季节变化特征。泉水的 δ D和δ18O测试结果显示:δ D的变化范围为 −110.56‰~−107.61‰,变化幅度为2.95‰,平均值为−109.45‰;δ18O的变化范围为−15.84‰~−14.76‰,变化幅度为1.08‰,平均值为−15.41‰;泉水中δ D、δ18O的变化范围窄,总体较为平稳,季节变化特征不明显。与雨水中的δ D、δ18O相比,泉水中δ D、δ18O的变化幅度明显小于雨水中δ D、δ18O的变化幅度。
图 2 2018—2022年白水台地区雨水与S3号泉水氢氧稳定同位素动态变化趋势图a. 丽江站站点月平均降水量 b. δD c. δ18O d. 氘盈余(d-excess)注:阴影部分代表雨季(5-10月),2020年1月数据缺失是由于该月未采样。Figure 2. Dynamic trend of hydrogen and oxygen stable isotopes of rainwater and S3 spring water in the Baishuitai region from 2018 to 2022a. average monthly precipitation of Lijiang station b. δD c. δ18O d. deuterium excess (d-excess). Note: Shaded portion indicates the rainy season from May to October; the date of January, 2020 is absent because samples were not collected.同时观察到,在2018年雨季内泉水和雨水中δ D、δ18O均呈现下降趋势;而在2019年雨季内泉水和雨水中δ D、δ18O均呈现先下降后上升的变化趋势,但泉水中δ D、δ18O的变化相对雨水中δ D、δ18O的变化存在约1 个月的滞后;而2022年旱季泉水中δ D、δ18O均呈现先降低后升高的趋势,随着进入雨季不断降低,雨水的变化先于泉水;这些结果表明泉水中δ D、δ18O的变化可能响应了雨水中δ D、δ18O的变化。
3.1.2 大气降水线
大气降水线(或雨水线)是指雨水中δD和δ18O之间的比率关系,通常可以用来指示一个地区的水汽来源与水循环过程[24]。其关系式为 δD = a ×δ18O + b,其中斜率(a)表示氢氧稳定同位素之间的分馏效应之比,而截距(b)表示氘对平衡状态的偏离程度。结果显示,白水台大气降水线方程为:δD=7.67δ18O+0.80(R2=0.97, n=19,图3(a))。与全球大气降水线方程(δD=8δ18 O + 10)[25]相比,白水台大气降水线方程的斜率和截距均偏低,这可能与研究区海拔较高,湿度低,旱季水汽的部分来源为局地再蒸发,降雨过程中重同位素优先凝结进入雨滴,使得降水中δ18 O偏高有关[24];与昆明大气降水线方程(δD=8.02δ18O+11.3)相比[26],白水台大气降水线方程的斜率接近,截距偏低,这可能与采样时段以及样本量有关。
图 3 云南白水台地区雨水、泉水δD-δ18O关系图a.白水台雨水线与全球、昆明雨水线对比 b. 图(a)中方框部分放大图,箭头为泉水δD、δ18O相对于白水台雨水线的偏移方向Figure 3. δD-δ18O relationship between rain water and spring water in Baishuitai, Yunnan Provincea. Comparison of Baishuitai rainwater line with global rainwater line and Kunming rainwater line; b is the enlarged picture of box portion in figure (a), and the arrow indicates the deviation direction of δD and δ18O of spring water relative to the rainwater line of Baishuitai.同时观察到,白水台泉水δD-δ18O数据点分布集中,均向白水台大气降水线的左侧附近区域偏移(图3(b))。
3.1.3 氘盈余
氘盈余(d-excess)代表D和18O的分馏速度差异,其表达式为d-excess=δD−8δ18O。由于受到水汽源地气象条件的影响(如温度、相对湿度、风速等),d-excess值通常可以用来反映水汽源地的气象条件和指示区域水岩作用的强度[27−28]。结果显示(图2(d)):白水台地区雨水d-excess值的变化范围为−15.88‰~17.31‰,平均值为 2.55‰, 低于全球降水线的d-excess值(10‰),可能与雨水样本量较小有关。进入雨季,雨水d-excess值呈现先减小后增加的变化趋势,这与前人研究结果相一致[18]。泉水d-excess值变化范围为9.44‰~17.62‰,平均值为13.81‰,略高于全球降水线的d-excess值。
3.2 微量元素浓度特征及时序变化
2018年至2022年期间白水台泉水中常微量元素的测试结果显示(表1):研究时段内泉水中Ca、Mg、Na、Sr、Si元素的浓度均大于1 mg·L−1,而K、Fe、Ba、Al、Mn元素的浓度均小于1 mg·L−1;与白水台雨水的微量元素浓度对比发现,泉水中的Ca、Mg、Sr、Ba、Na等元素浓度要比雨水中的高,而K、Fe、Mn、Al等元素浓度要比雨水中的低。同时发现,泉水中的Mn、Fe、Al等元素变异系数接近或大于1,表明这些元素的浓度变化波动大;而Mg、Na、Si、Sr、Ba、K、Na的变异系数接近或小于0.1,表明这些元素的浓度变化相对稳定。与白水台地区雨水微量元素的变异系数对比发现,泉水中Mn、Fe、Al等元素的变异系数比雨水中对应微量元素的变异系数更大,而Mg、Na、Sr、Ba、K、Na的变异系数远远小于雨水中对应微量元素的变异系数。
表 1 2018-2022年云南白水台雨水和泉水中常微量元素浓度分布Table 1. Concentration distribution of normal and trace elements in rainwater and spring water of Baishuitai, Yunnan, from 2018 to 2022元素
种类雨水(n=21) 泉水(n=55) 极小值 极大值 均值 变异系数 极小值 极大值 均值 变异系数 Ca/mg·L−1 0.00 6.65 2.71 0.77 147.56 219.07 190.20 0.07 Mg/mg·L−1 0.32 11.32 1.13 2.74 11.74 15.79 14.29 0.06 Na/mg·L−1 0.12 10.38 0.67 3.17 4.03 6.28 5.22 0.10 Si/mg·L−1 \ \ \ \ 1.32 2.54 1.80 0.15 Sr/mg·L−1 0.00 0.06 0.01 1.16 1.46 2.16 1.95 0.07 K/μg·L−1 35.56 7 361.82 1 055.81 1.74 500.21 984.13 603.85 0.12 Fe/μg·L−1 0.00 256.47 61.54 0.99 0.79 510.88 55.27 1.42 Ba/μg·L−1 0.12 200.98 33.27 1.84 43.67 69.21 62.11 0.07 Al/μg·L−1 5.32 184.24 52.59 0.89 0.00 126.57 19.39 0.96 Mn/μg·L−1 0.42 29.31 7.27 1.05 0.28 26.73 2.55 1.57 图4显示了各微量元素随时间的动态变化。由图可知,Ca、Mg、Sr、Ba、Si、Na这6种元素的浓度变化趋势相似,存在较为明显的季节变化特征:即雨季初期这些元素浓度呈现下降趋势,雨季后期有所回升,在旱季相对稳定或略有上升,总体表现为雨季偏低,旱季偏高。Fe、Al、Mn这3种元素的浓度变化趋势相似,而季节上表现出与Ca、Mg、Sr、Ba等元素相反的变化特征,即旱季偏低,雨季偏高。以Fe为例,研究时段内每年的雨季均会出现Fe元素浓度的峰值,但是出现时间略有不同,如2018年5月、2019年7月、2020年10月和2021年8月。研究时段内每年的旱季同样出现Fe元素浓度的峰值,并且均出现在每年12月附近。此外,除2019年12月外,K元素浓度的时序变化特征总体表现平稳。从年平均值来看,研究时段内元素浓度几乎平稳,2020年平均值较低,2021年偏高。
图 4 2018-2022年云南白水台S3号泉泉水微量元素动态变化趋势图a. 丽江站站点月平均降水量 b. Ca c. Mg d. Sr e. Ba f. Si g. Na h. K i. Fe j. Mn k. Al,注:阴影部分代表雨季(5—10月) ,2020年1月数据缺失是由于该月未采样)Figure 4. Dynamic trend of trace elements in spring water of S3 spring, Baishuitai, Yunnan, from 2018 to 2022a. average monthly precipitation of Lijiang station; b. Ca; c. Mg; d. Sr; e. Ba; f. Si; g. Na; h. K; i. Fe; j. Mn; k. Al. Note: Shaded portion indicates the rainy season from May to October; the date of January, 2020 is absent because samples were not collected4. 讨 论
4.1 云南白水台地区泉水的补给来源及其氢氧稳定同位素演化过程
白水台雨水的氢氧稳定同位素组成呈现明显的季节变化特征,这可能与该区域的水汽来源及蒸发条件有关[18]。在雨季,雨水的δD和δ18O偏轻,这主要是白水台在西南季风与东南季风的影响下,受到来自印度洋的阿拉伯海与南海海域的水汽补给,水汽传输过程中发生多次分馏和冷凝,D和18O冲刷严重,剩余气团水汽中富集1H、16O所导致。在旱季,雨水的δ18O最高达到6.83‰,明显偏重,这是因为西风环流携带的大陆性气团与海洋暖湿气流相比性质较为稳定,不易形成降水;加之相对湿度较低,蒸发作用强烈,使得剩余气团水汽中D、18O富集[23]。
白水台泉水的氢氧稳定同位素组成均落在大气降水线上或附近区域,表明泉水主要受到大气降水补给。分析发现,泉水中δD和δ18O的季节变化不明显,其变化幅度(2.95‰,1.08‰)远小于雨水的δD和δ18O变化幅度(209.36‰,28.60‰);但在2018年、2019和2022年的雨季期间泉水中δ D、δ18O在滞后约1个月的基础上与雨水中δ D、δ18O具有相似的变化趋势,这说明在地下径流过程中水体δD和δ18O发生了一定程度的演化,使得泉水中δ D、δ18O修正性继承了雨水中δ D、δ18O的部分特征。通常,地下水同位素演化主要包括蒸发、混合和交换[29]。白水台地区泉的类型为冷泉,常年水温在10 ℃左右[9],岩溶水体蒸发作用弱,由蒸发作用造成的水体氢氧稳定同位素变化小。由于白水台地区所在的白地盆地由断陷侵蚀作用而成,且区内石灰岩多具生物粒屑灰岩特征,粒屑间及其内部发育有溶蚀孔隙和裂隙,因而在持续的垂向溶蚀作用下形成厚大的岩溶发育带。地表洼地、漏斗与地下垂直和倾斜的洞管、溶隙相连形成径流空隙系统,局部可能发育有水平延展的多层溶穴系统[30]。由于山体表层土壤厚度较薄,大气降水极易通过裂隙等流动路径进入浅循环径流带,并汇集到溶穴中形成富水块段[31-32](图5)。这一过程,来自不同通道和裂隙网络中的新、老水不断混合,调蓄平滑地下水中的氢氧同位素,有可能形成氢氧同位素组成相对均一的泉水。前人研究也发现,大气降水作为渗流水流经上覆基岩时,均一化作用下雨水氧同位素的季节性信号被平滑,并认为这是水流通过石灰岩基岩时所经历的曲折通道造成的[33−35]。此外,白水台泉水中Ca2+、HCO−3的含量相对较高(分别达到4.8 mmol·L−1和11.5 mmol·L−1[9]),而研究发现生物成因CO2溶解产生的溶解无机碳浓度通常在2~5 mmol·L−1,说明生物成因CO2可能不足以作为侵蚀溶解白水台灰岩的主要动力。并且前人观测发现白水台泉水中CO2分压高达
14 400 Pa,远超过热带和亚热带其他地区观测到的泉水CO2最高分压(<10 000 Pa);同时根据碳稳定同位素组成计算发现深部来源CO2占据泉水中CO2的80%以上,表明深部来源CO2在该地区的灰岩侵蚀溶解中起了主要作用[36]。通常地下水与CO2之间发生如下同位素交换反应:C16O2+2H182O=C18O2+2H162O 由于深部来源CO2的氧同位素偏轻,泉水与深部来源的CO2发生氧同位素交换时,泉水中氧同位素会变轻,而这一交换并不会造成氢同位素的变化。分析发现,白水台地区泉水δD-δ18O散点均向白水台大气降水线的左侧附近区域偏移,即相对于大气降水,泉水中δ18O变轻而δD基本没有变化,这表明大气降水在下渗过程与深部来源CO2进行了充分混合。
上述分析结果表明白水台泉水主要接受当地大气降水补给;白水台泉水中δD、δ18O受到石灰岩基层中不同通道、裂隙网络中的新、老水混合导致的同位素调蓄平滑作用以及与深部来源CO2的氧同位素交换作用,修正性继承了雨水中δD、δ18O的部分特征,暗示了白水台地区泉域范围大,地下通道、裂隙多,岩溶构造较为复杂。
4.2 云南白水台地区泉水微量元素的来源及其指示意义
岩溶地下水化学成分的形成与分布是在地质条件及物理化学平衡等因素的综合作用下元素迁移的结果,因此岩溶水的化学成分主要取决于补给水的化学成分和含水介质的物质成分[37]。本次研究揭示云南白水台地区的泉水主要接受当地大气降水补给。由于受到强烈的构造运动和风化作用,该山体岩石破碎,断层和裂隙发育,为大气降水的入渗和地下水的径流提供了极好的条件;大气降水在入渗过程中通过淋滤上覆土壤层,携带残积黏土沿山顶和斜坡发育的裂隙进入浅循环径流带,形成间歇性的垂直和陡倾快速管隙流,并在溶隙汇水等作用下不断混合,储存于强富水性的裂隙与溶穴中;部分下渗水进入深循环径流带并沿管隙扩散。与此同时,深部来源的高浓度二氧化碳气体沿裂隙释出,并溶解至深循环、浅循环径流带内的下渗水中,使下渗水变得具有侵蚀性,从而对周围石灰岩进行侵蚀;最终,因一近东西向大断裂的阻隔,地下水沿导水断裂露头以泉群的形式排泄[38-39]。这一径流过程表明白水台泉水的化学成分是当地大气降水和上覆土壤层孔隙水、下覆石灰岩浅循环岩溶水以及深部深循环岩溶水综合作用的结果。
白水台泉水常微量元素主要呈现出两类动态变化特征:以Ca、Mg、Sr、Ba、Si等元素为代表的一类在雨季初期均有不同程度的降低,雨季后期至雨旱季过渡时上升,而旱季则保持平稳或略有起伏;以Fe、Al、Mn等元素为代表的一类在雨季和旱季均有峰值出现。对白水台泉水中各元素浓度变化进行系统聚类分析(数据标准化采用Z-score法)发现(图6),当距离小于等于10时,10种元素可分为4类,Mg、Sr、Ba、Si、Ca为一类,Fe、Al、Mn为一类,K、Na各成一类。其中Mg、Sr、Ba间以及Fe、Al、Mn间的距离都小于5,表明两类间内部各元素存在相近的物质来源或迁移路径,K、Na与其余元素的组间距离大于15,说明两组元素迁移规律之间存在较大差异[40−43]。这与元素呈现出的变化特征相一致,表明元素间存在不同的物质来源或迁移路径。
岩溶区地下水中常微量元素主要来自降水对上覆土壤的淋滤以及地下水迁移过程对母岩的溶解[44-45]。白水台地区泉水中Ca、Mg、Sr、Ba等元素浓度明显高于大气降水,同时泉水中的Ca/Na比值变化范围在24~64 之间,与典型的碳酸盐岩中的Ca/Na比值(Ca/Na = 45±25)[46]相近,表明白水台地区泉水中的Ca、Mg、Sr、Ba等元素主要来自下覆的中三叠纪石灰岩。由于白水台地区地下深部来源的CO2在高压下溶解于地下水系统,使得水体对围岩的侵蚀能力进一步增强,在水-岩-气相互作用下,Ca2+、Mg2+、Ba2+、Sr2+作为灰岩溶解的产物以游离态补给泉水。已有研究显示,元素在一定程度上能够响应气候环境的变化[47-49],Ca2+浓度受控于温度和降水量的季节变化,Mg2+浓度主要受到气候环境因素与基岩岩性的影响,而Sr2+、Ba2+浓度与地表植被与土壤性质相关。在旱季过渡至雨季期间,地表植被与微生物产生的CO2进一步加剧水体溶蚀能力,土壤中的Sr2+、Ba2+在淋溶作用下溶解迁移,基岩中的Mg2+溶蚀速率加快,地下系统保存的大量元素含量较高的滞留水被雨季初期的降水以活塞作用的方式推入泉水运移路径,由于降雨量突然增大,雨水较快通过洼地中的落水洞入渗至地下裂隙、管道中,未能充分溶解土壤或深部来源CO2,导致水-岩-气相互作用相对减弱,从而使得这些元素浓度呈现短暂降低的趋势[50]。进入雨季后,水-岩相互作用强度在降雨稀释下减弱,基岩空隙被水填充,使得CO2脱气作用减小,导致Mg/Ca、Sr/Ca和Ba/Ca降低[51-52]。而在旱季,降水减少一方面促进了CO2脱气过程的进行,导致碳酸钙沉淀作用增强,使Mg/Ca、Sr/Ca和Ba/Ca值增大[53],另一方面,土壤与基岩裂隙中的水分较少受到新水的干扰,水的运移速度减慢导致水岩作用的增强,Sr与Ba的优先淋滤引起元素含量差异性变化,使得Mg/Ca、Sr/Ca和Ba/Ca在旱季呈现上升[54] (图7)。此外,研究发现可溶性Si多以分散硅酸盐的形式存在于天然水中[55]。在地下水径流过程中,CO2对石灰岩中少量硅酸盐矿物的溶解作用可产生偏硅酸。因此,在稀释效应的作用下,硅元素在雨季初期呈现下降趋势,旱季较为平稳。尽管前人研究发现白水台地区暴雨前后泉水中的微量元素含量的动态变化基本稳定[19],表明泉水中微量元素并不能迅速响应降水量的变化,这可能与地下岩溶含水层储水空间体积较大,储存调节能力较强,单次暴雨导致的元素浓度变化被均一化有关。本次研究则发现在季节尺度上泉水中的Ca、Mg、Ba、Sr、Si等元素含量响应当地降水量变化,表明白水台泉水中这类元素含量的变化可能指示降水量大小的变化。
图 7 2018-2022年云南白水台S3号泉泉水微量元素比值动态变化趋势图((a) Mg/Ca;(b)Sr/ Ca;(c) Ba/Ca;注:黄实线为雨/旱季平均值,阴影部分代表雨季(5-10月),2020年1月数据缺失是由于该月未采样)Figure 7. Dynamic trend of trace element ratio in S3 spring water in Baishuitai, Yunnan Province from 2018 to 2022((a) Mg/Ca; (b)Sr/Ca; (c) Ba/Ca; Note: The yellow solid line is the average value in the rain/dry season; the shadowed portion indicates the rainy season from May to October; the data of January, 2020 is absent because smaples were not collected)相反,白水台地区泉水中的Fe、Al、Mn等元素则主要来源于大气降水对上覆土壤的淋滤作用[56]。当雨水降落至地表时,被击碎的土壤分解并混入下渗流,沿着山体发育的裂隙、落水洞、竖井等水流通道,携带入地下水系统。亚热带湿润气候条件下,研究区成土母质中碳酸钙大量淋失,残留于土壤中的成土母质所含有的铝锰铁以及粘土物质一同被带入地下水系统,使地下水中含有的Mn2+、Al3+、Fe3+等离子浓度发生变动[44]。雨季泉水中Fe、Al、Mn等元素浓度的增加,可能与雨季各月份降水强度及其分配有关。分析发现在2018年和2021年的雨季降水量相对较多,同时各月份降水差异较大,这期间泉水中的Fe、Al、Mn元素浓度显著增加;而在2019、2020和2022年的雨季降水量相对较少,各月份降水比较均匀,这期间泉水中Fe、Al、Mn元素浓度只呈现小幅度增加。旱季泉水中Fe、Al、Mn等元素通常在12月附近出现峰值,这可能与单次降水强度有关,瞬时峰值的出现可能指示了强降水引起强烈的脉冲作用,引起Fe、Al和Mn胶体、颗粒物等向下迁移,发生强烈淋失[57]。因为旱季降水量不到全年的25%,降水频次少;一旦出现强降水,可以对地表进行强烈的冲刷和侵蚀,携带的土壤颗粒和易溶元素快速下渗补给泉水,导致泉水中元素浓度增加[58]。因此,白水台泉水中Fe、Al、Mn等元素的变化可能指示当地降水强度的变化。
5. 结 论
(1)云南白水台地区雨水中稳定同位素组成具有明显的季节变化特征,表现为旱季偏高,雨季偏低,主要受到水汽来源与蒸发条件的影响。旱季水汽主要来源于性质稳定的大陆性气团,加之蒸发作用强烈,使得剩余水汽团中富集D、18O。雨季则受西南季风与东南季风的影响,水汽从海洋运移到陆地的过程中发生多次分馏和冷凝过程,降水过程中D、18O被严重冲刷,从而使得雨水中δD、δ18O偏轻。
(2)云南白水台地区泉水主要受到当地大气降水补给。石灰岩基层中不同通道、裂隙中的新、老水混合导致的同位素调蓄平滑作用及其与深部来源CO2的氧同位素交换作用是导致泉水与雨水的氢氧稳定同位素组成差异的主要原因。这些结果表明泉水中的δD、δ18O修正性继承了雨水δD、δ18O的部分特征,同时暗示了白水台地区泉域范围大,地下通道、裂隙多,岩溶构造较为复杂。
(3)云南白水台地区泉水微量元素的含量主要呈现两类动态变化特征,并存在不同的来源:以Ca、Mg、Sr、Ba、Si等元素为一类,主要来源于喀斯特水体侵蚀下围岩的溶解,其含量在季节尺度上的变化可能指示降水量的变化;以Fe、Al、Mn为一类,主要来源于大气降水对上覆土壤的淋滤作用,通过下渗进入地下水系统,其浓度变化可能反映了降水强度的变化。
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图 2 2018—2022年白水台地区雨水与S3号泉水氢氧稳定同位素动态变化趋势图
a. 丽江站站点月平均降水量 b. δD c. δ18O d. 氘盈余(d-excess)注:阴影部分代表雨季(5-10月),2020年1月数据缺失是由于该月未采样。
Figure 2. Dynamic trend of hydrogen and oxygen stable isotopes of rainwater and S3 spring water in the Baishuitai region from 2018 to 2022
a. average monthly precipitation of Lijiang station b. δD c. δ18O d. deuterium excess (d-excess). Note: Shaded portion indicates the rainy season from May to October; the date of January, 2020 is absent because samples were not collected.
图 3 云南白水台地区雨水、泉水δD-δ18O关系图
a.白水台雨水线与全球、昆明雨水线对比 b. 图(a)中方框部分放大图,箭头为泉水δD、δ18O相对于白水台雨水线的偏移方向
Figure 3. δD-δ18O relationship between rain water and spring water in Baishuitai, Yunnan Province
a. Comparison of Baishuitai rainwater line with global rainwater line and Kunming rainwater line; b is the enlarged picture of box portion in figure (a), and the arrow indicates the deviation direction of δD and δ18O of spring water relative to the rainwater line of Baishuitai.
图 4 2018-2022年云南白水台S3号泉泉水微量元素动态变化趋势图
a. 丽江站站点月平均降水量 b. Ca c. Mg d. Sr e. Ba f. Si g. Na h. K i. Fe j. Mn k. Al,注:阴影部分代表雨季(5—10月) ,2020年1月数据缺失是由于该月未采样)
Figure 4. Dynamic trend of trace elements in spring water of S3 spring, Baishuitai, Yunnan, from 2018 to 2022
a. average monthly precipitation of Lijiang station; b. Ca; c. Mg; d. Sr; e. Ba; f. Si; g. Na; h. K; i. Fe; j. Mn; k. Al. Note: Shaded portion indicates the rainy season from May to October; the date of January, 2020 is absent because samples were not collected
图 7 2018-2022年云南白水台S3号泉泉水微量元素比值动态变化趋势图
((a) Mg/Ca;(b)Sr/ Ca;(c) Ba/Ca;注:黄实线为雨/旱季平均值,阴影部分代表雨季(5-10月),2020年1月数据缺失是由于该月未采样)
Figure 7. Dynamic trend of trace element ratio in S3 spring water in Baishuitai, Yunnan Province from 2018 to 2022
((a) Mg/Ca; (b)Sr/Ca; (c) Ba/Ca; Note: The yellow solid line is the average value in the rain/dry season; the shadowed portion indicates the rainy season from May to October; the data of January, 2020 is absent because smaples were not collected)
表 1 2018-2022年云南白水台雨水和泉水中常微量元素浓度分布
Table 1. Concentration distribution of normal and trace elements in rainwater and spring water of Baishuitai, Yunnan, from 2018 to 2022
元素
种类雨水(n=21) 泉水(n=55) 极小值 极大值 均值 变异系数 极小值 极大值 均值 变异系数 Ca/mg·L−1 0.00 6.65 2.71 0.77 147.56 219.07 190.20 0.07 Mg/mg·L−1 0.32 11.32 1.13 2.74 11.74 15.79 14.29 0.06 Na/mg·L−1 0.12 10.38 0.67 3.17 4.03 6.28 5.22 0.10 Si/mg·L−1 \ \ \ \ 1.32 2.54 1.80 0.15 Sr/mg·L−1 0.00 0.06 0.01 1.16 1.46 2.16 1.95 0.07 K/μg·L−1 35.56 7 361.82 1 055.81 1.74 500.21 984.13 603.85 0.12 Fe/μg·L−1 0.00 256.47 61.54 0.99 0.79 510.88 55.27 1.42 Ba/μg·L−1 0.12 200.98 33.27 1.84 43.67 69.21 62.11 0.07 Al/μg·L−1 5.32 184.24 52.59 0.89 0.00 126.57 19.39 0.96 Mn/μg·L−1 0.42 29.31 7.27 1.05 0.28 26.73 2.55 1.57 -
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