Heavy Metal Pollution Characteristics And Risk Assessment Of The River And Reservoir In Typical Thallium Mineralized Area

The Lanmuchang in southwest Guizhou is a typical thallium mineralized area.The historical mining activities have caused serious water pollution.In the past 30 years,many scholars have carried out systematic studies on the occurrence characteristics,migration and enrichment regularity of Thallium（Tl）in the epibiotic environmental media of Thallium mineralized area of wood mill,and achieved fruitful results.However,on the one hand,most of the existing studies focused on the thallium mineralized core area,and most of the studies focused on the mine ecological restoration before,and after the ecological reclamation of the core mining area and the treatment of the historical waste residue in 2017,The spatial and temporal variation of thallium in the water environment of the surrounding and downstream river reservoir,the pollution status,and the improvement of thallium and other heavy metal pollution in the water before and after restoration are lack of systematic research,which fails to reflect the restoration effect of mine ecological remediation.On the other hand,due to the fact that Tl has not been included in the standard of water quality monitoring of drinking water source for a long time,and due to the lack of accuracy of instrumental analysis,there is a lack of studies on Tl enrichment characteristics and whether Tl exceeds the standard in drinking water source downstream of the mill.Based on this,this study takes Qingshui Stream and Nashangang Reservoir of Thallium mining area in Lanmuchang,Southwest Guizhou,as the research object.Samples of water,surface sediments and sediment cores in different periods are collected systematically.Through hydrochemical analysis,element content and occurrence form analysis,combined with pollution status assessment and environmental risk assessment,The spatial and temporal variation characteristics of water chemistry and heavy metals such as thallium in regional water environment were systematically identified,the pollution status of regional water environment was clarified,and the health risks of regional water environment were assessed.The study obtained the following findings:（1）The water bodies in Qingshui Stream Basin are nearly neutral to weakly basic.The cations are mainly Ca²⁺and Mg²⁺,and the anions are mainly HCO₃^-and SO₄^2-.The hydrochemical type is Ca·Mg-HCO₃·SO₄（SO₄·HCO₃）type.The seasonal variation and spatial difference of main ion concentration are small.The hydrochemical components are mainly controlled by weathering of carbonate rocks and evaporative karst decomposition.Tl concentration decreased after ecological restoration,but Tl pollution was still serious,and the seasonal difference was obvious.The Tl concentration was relatively stable in flood and normal period,while the Tl content fluctuated greatly in dry period due to the influence of underground spring water and historical waste residue,and the maximum was 4.036μg/L.The results of Nemerow pollution index method showed that Tl of Qingshui Stream body was heavily polluted.（2）Heavy metals such Tl、As、Sb and Ba are enriched to different degrees in the sediments of Qingshui Stream system,and the spatial difference is large,indicating that the river is greatly affected by historical industrial activities.The sediment high content points are mainly located in the upper reaches of Qingshui Stream and decrease rapidly with the increase of distance from the core mining area.The occurrence of Tl in the sediments of Qingshui Stream system is residue state>Reducible state>oxidized state>exchangeable state,and the reducible state is relatively high,and the exchangeable state is relatively high at individual sites,indicating that there is a certain risk of environmental thallium in the sediments of Qingshui Stream,while other heavy metals（except Cd）mainly exist in the form of residual state,with low activity and low environmental risk.The results of enrichment factor,ground accumulation index and ecological risk assessment showed that Tl was the main pollutant in Qingshui Stream sediment,presenting severe pollution level,followed by As and Sb,presenting moderate or mild pollution level.（3）The water of Nashangang Reservoir is nearly neutral to weakly alkaline,similar to that of Qingshui Stream.The cations and anions are mainly Ca²⁺,Mg²⁺,HCO₃^-and SO₄^2-,respectively.The hydrochemical type is Ca·Mg-SO₄·HCO₃,and the seasonal and spatial differences of main ion concentrations are small.Tl in Nashangang Reservoir exceeded the limit（limit:0.1μg/L）,and the Tl in flood season（mean:0.876μg/L）was higher than that in dry season（mean:0.637μg/L）,and the content of other elements were lower than the limit set in the Standard for Drinking Water Quality（GB 5749-2022）.Tl content in reservoir water is stratified,showing that the upper layer is similar to the middle layer,while the bottom layer is low.Tl was mainly enriched in the sediments of Nashangang Reservoir,and Tl increased gradually from the surface layer to the bottom layer in the core and stratified sediments,indicating that there was a certain release in the surface sediments.The occurrence phase of Tl in the sediments of Nashangang Reservoir is similar to that in the sediments of Qingshui Stream,showing residual state>reducible state>oxidizable state>exchangeable state,which has certain environmental Tl risks.（4）Tl was heavily polluted in both flood period and dry period,while Tl was heavily polluted in sediment during flood period.Tl was the main contributing factor of sediment pollution,and other elements were moderately or mildly polluted.The heavy metal pollution level of the reservoir was lower than that of Qingshui Stream sediment.The results of human health risk assessment showed that drinking water exposure was an important exposure route for local residents,which was more than two orders of magnitude higher than skin exposure.The non-carcinogenic risk is slightly higher in children than in adults.