Sources and pathways of arsenic pollution in the karst water source area in the west of Jinchengjiang district, Hechi City
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摘要: 为研究岩溶水源地污染物来源和运移途径，以河池市金城江区城西水厂岩溶水源地为研究区，依据水文地质学分析方法，开展城西水厂砷异常原因调查，利用示踪试验和取样检测等手段，查明砷污染源处于城西水厂水源地准保护区范围外，为三境村砒霜厂旧址及其东面岩溶洼地，并评估出砷污染土层方量为55 742.5 m3，其污染途径长12.25 km，主要通过岩溶管道，经欧洞地下河污染城西水厂，这进而证实欧洞地下河的另一水源方向。建议调整城西水厂水源地准保护区南部边界，以地下水分水岭为界，并适当增设水质长期监测点，以实施污染源风险管控。Abstract:
Karst groundwater is an important water resource in karst areas of China and even the world, which plays an important role in people's life and industrial and agricultural production. A karst aquifer is composed of interconnected karst pores, cracks, caves, pipelines and other multiple media, which has strong capabilities for water collection, storage and drainage. Because soil layers in karst areas are generally thin, pollutants can directly enter groundwater aquifers through soil layers, sinkholes and karst cracks, which makes karst groundwater extremely vulnerable to pollution. Due to several reasons in the past, some old industrial slag yards were built on karst caves and cracks, but no effective anti-seepage measures were taken. Many incidents of karst groundwater pollution caused by leakage of slag piles occurred in China. In order to study the characteristics of sources and transporting routes of pollutants in karst underground water sources, this study took the karst water source of Chengxi water plant in Jinchengjiang district, Hechi City as a study area. By investigating the causes of arsenic anomalies in this plant and conducting hydrogeological analysis, this study identified pollution sources and sampled in the contaminated site to assess the arsenic pollution sources. The sources of arsenic pollution were found out, that is, the former site of arsenic factory and the karst depression to its east in Sanjing village, Wuxu town, Jinchengjiang district. The waste slag piled on the site of Sanjing arsenic factory was alluvial with the flood in the gully and bottom depression. Under the long-term action of rain, sunlight, air and microorganisms, the waste residue underwent complex chemical reactions, releasing heavy metal ions. Leachate seeped into groundwater through sinkholes and karst cracks, forming pollution sources. Arsenic concentrations in the contaminated site range between 7.17 and 25,200 mg·kg−1, with an average concentration of 1,296.3 mg·kg−1. The area where the arsenic concentration is higher than the risk control value of other agricultural land (120 mg·kg−1) covers about 22,297 m2, with an average thickness of 2.5 m. The volume of arsenic contaminated soil is 55,742.5 m3. Two tracing tests of underground water found that arsenic flows to Chengxi water plant through underground karst runoff, and the pathway is as follows: karst depression in the east of Sanjingcun arsenic factory → Xingdong in Xingdong village of Liuwei town → Fandong → Oudong → Nalong reservoir (underground runoff zone) → Chengxi water plant. The runoff is 12.25 km long with a groundwater flow rate of 0.86 km·d−1 and a permeability coefficient of 0.995 cm·s−1. The long-term monitoring of water quality shows that the arsenic content of Chengxi water plant is at risk of exceeding the permitted level of Class III (0.01 mg·L−1) during the flood season. Therefore, it is necessary to control the pollution source (karst depression in the east of the arsenic factory) and to strengthen the monitoring of groundwater quality in the water source area of Chengxi water plant by setting up monitoring points for long-term observation of water quality at key points in the runoff area. For example, new monitoring points can be established at Lingxiao spring, estuary of Dula underground river, Xingdong skylight and Fandong skylight. The pollution source is located outside the quasi-protected area of the water source of Chengxi water plant. Considering that the water source mainly consists of karst water, and the groundwater primarily flows through karst pipelines, the pollutant runoff path is long and the attenuation is slow, it is suggested to expand the quasi-protected area of the water source of Chengxi water plant, with the southern boundary defined by groundwater divide of Longjiang karst groundwater system and Diaojiang karst groundwater system. -
Key words:
- karst area /
- groundwater /
- water source area /
- causes of pollution /
- tracer test
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图 2 城西水厂地下水砷污染事件时间轴线示意图
Figure 2. Time axis of arsenic pollution events in groundwater of Chengxi water plant
图 3 三境砒霜厂东面岩溶洼地采样布置及结果分布图
Figure 3. Layout and distribution of sampling in the karst depression in the east of Sanjing arsenic factory
表 1 城西水厂岩溶水源地系统
Table 1. System of karst water source area of Chengxi water plant
城西水厂岩
溶水源地次级系统 介绍 都腊地下河 发源于内丈一带，从都腊地下河出口排出地表 候洞地下河 发源于勒从、铭润山庄一带，从候洞屯排出地表汇入凌霄泉地表水流 欧洞地下河 发源于兴洞村外塘屯一带，向北径流，主要通过外塘屯、兴洞屯、凡洞屯；水位高时从
欧洞屯排出地表，水位低时则通过地下岩溶管道向北径流到达城西水厂溶潭凌霄岩溶泉 在凌霄村廷榄屯遇阻水断层后，以上升泉的形式出露地表 六塘岩溶泉 发源于凉水坳一带，流量较小 
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表 2 历史遗留砒霜厂信息
Table 2. Information of the arsenic factory
厂名 地理位置 地下水流向分析 丁峒砒霜厂 都安县三只羊乡丁峒 厂址地表水和地下水流向南东方向，可排除丁峒砒霜厂为城西水厂污染源的可能性 白尾砒霜厂 金城江区六圩镇白尾 厂址地下水总体径流方向为北东向，可排除白尾砒霜厂为城西水厂污染源的可能性 三境砒霜厂 金城江区五圩镇三境村 厂址地表水和地下水流向北东方向（城西水厂水文地质单元方向）
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表 3 采取土样信息表
Table 3. Sampling information
采样布置 编号 样品数 备注 槽探取样 TC1~TC13 65 采集深度为25 cm、50 cm、100 cm、150 cm和200 cm 浅钻取样 ZK1、ZK2 10 采样深度为300 cm、400 cm、500 cm、600 cm和650 cm 塘底取样 ST1~ST6 6 洼地水塘底部土样 洼地淹没区取样 YMQ1~YMQ28 28 布置在洼地边缘至1.5 m高的范围内周边山体，主要为控制污染源经洪水
浸泡后污染物浸出的二次污染情况周边山体取样 DB1~DB6 6 布置在洼地边缘以上6 m，主要为检测未受污染土壤砷浓度的背景值，以
及控制污染范围
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