河池市金城江区城西岩溶水源地砷污染来源及途径

Sources and pathways of arsenic pollution in the karst water source area in the west of Jinchengjiang district,Hechi City

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55742.5 m^(3),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。

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 m^(2),with an average thickness of 2.5 m.The volume of arsenic contaminated soil is 55,742.5 m^(3).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.