Risk Assessment Of Heavy Metal Pollution In Centralized Groundwater Sources In The Karst Area Of Tangdian,Yunnan

The socio-economic development of an area is often restricted by water resources.The groundwater environment in karst areas is easily affected by human activities because of its relatively sensitive ecosystem.The centralized groundwater source in the karst area of Tangdian is threatened by the tailing pond located in the karst depression,because the pollutants are very likely to enter the groundwater with the leachate.Based on different groundwater pollution risk assessment models at home and abroad,this paper selects the karst area of Tangdian as the research object to conduct groundwater pollution risk research.Through field investigation,groundwater sampling,detection and analysis,the current situation of groundwater pollution in Tangdian area is analyzed.At the same time,a numerical model of groundwater flow and solute transport is established to predict the transport of lead pollutants into the aquifer after the leakage of tailings pond leachate.Using AHP,the weight of each index is recalculated.Through drastic model and pollution source load evaluation,the groundwater vulnerability risk in the study area is analyzed,and the risk control suggestions are put forward.According to the detection data of groundwater samples,the hydrochemical types of groundwater in the study area are mainly HCO₃-Ca,HCO₃-Mg,and their formation is mainly controlled by the dissolution of carbonate rocks.The single factor index method and Nemerow index method were used to evaluate the current status of groundwater quality.The results show that the groundwater quality in the study area presents a good trend.Except for the water samples in Tangdian Village（S03）that were polluted by NO₃^-as Class IV water,the other eight monitoring points reached Class III water or better.It can meet the requirements of centralized drinking water sources and industrial and agricultural water.The results of sample analysis also show that the groundwater quality is affected by agricultural activities and residents’life.Based on the results of hydrogeological surveys in the study area,Visual Modflow software was used to establish a numerical model of groundwater flow and solute migration,and to simulate and predict the migration of lead,a characteristic pollutant of tailings ponds.Numerical simulation results show that after the tailings pond leachate leaks,the pollution plume migrates eastward along the groundwater flow.On the 100th,500th,1000th,5000th and 7300th days predicted by the model,the mass concentration of lead in the center area of the pollution halo is 0.006,0.03,0.05,0.1 and 0.1 mg/L respectively,and the maximum migration distances are about 30,80,210,380 and 440 meters respectively.During the20-year simulation period,the concentration of lead pollutants only exceeded the standard around the tailings pond,which had little impact on the surrounding sensitive points.Aiming at the pollution risk of groundwater source in Shiliuba area caused by leachate leakage from tailings pond,a groundwater pollution risk assessment system based on groundwater inherent vulnerability assessment and pollution source load assessment was established.The analytic hierarchy process was used to determine the index weight,and the DRASTIC model combined with Arc GIS was used to obtain the groundwater vulnerability zoning map of the study area.The area of low-and medium-vulnerability areas accounted for14.26%,low-vulnerability areas accounted for 22.40%,moderate-vulnerabilities accounted for 22.76%,high-vulnerabilities accounted for 21.27%,and high-vulnerability areas accounted for 19.31%.The groundwater pollution source load is evaluated for a single tailing pond,and the groundwater pollution source load is medium.The groundwater pollution risk index in the study area presents three levels:medium-low,medium and medium-high,and its area accounts for 37.97%,27.91%,and 34.12%,respectively.The medium-high pollution risk is concentrated in the Tangdian area,and the centralized groundwater source is a medium-high risk.Finally,relevant suggestions on the prevention and control of regional groundwater pollution risks are put forward.