The Migration And Transformation Mechanism Of Heavy Metals In Lead-Zinc Mines In Underground Rivers And Battlefield Water Supply

The pollution of heavy metals in karst water has seriously threatened the drinking water safety of residents in karst areas and has done great harm to human health,which has increased the difficulty of battlefield water supply and has become an urgent environmental problem for the country and society.Revealing the characteristics of migration and transformation of heavy metals is helpful to understand the toxicological effects and environmental behavior of heavy metals,but the law of migration and transformation of heavy metals in underground river basins is more complex,and there were few reports on related fields at home and abroad.With the theme of"Mechanism of migration and transformation of heavy metals in lead-zinc deposits in underground river basins and battlefield water supply",this study selected the Sidi underground river in Yangshuo,Guilin as the research target area.Through the collection of water samples,sediment samples,and mineral samples from underground river basins in low and high water periods,and based on the law of water balance and mass conservation,the characteristics of hydrochemistry,hydrogen and oxygen isotopes of water and sulfate sulfur and oxygen isotopes were comprehensively analyzed to explore the migration and transformation of heavy metals from mining areas to the Sidi underground river.Through the health risk assessment of heavy metals in the water body,the selection of battlefield emergency water sources in the study area was determined,and the prevention and control measures for battlefield emergency water sources under the influence of mine wastewater were put forward,which provided data reference for battlefield water supply support.The main conclusions can be drawn as follows:（1）Mine wastewater had a great influence on the hydrological and geo-hydrochemical characteristics of underground river basins.The hydrochemical characteristics of the study area were affected by the discharge of mine wastewater.The average concentration of Pb,Zn and Cd（8.22,348.74,and3.11μg/L）in the water body was much higher than that of the Lijiang River（0.05,14.81,and 0.02μg/L）.the average values of Eh and DO showed karst surface water（208.40mv and 9.56mg/L）>non-karst groundwater（205.55mv and 8.62mg/L）>karst groundwater（191.72mv and 6.65mg/L）,and the average concentrations of Pb,Zn,Cd,As,Cu,Sr,and SO₄^2-showed non-karst surface water(（19.06,871.17,7.67,0.49,1.01,84.66 and40.66mg/L）>karst surface water（9.56,379.54,3.17,0.46,0.99,53.22 and 18.33mg/L）>karst groundwater（0.58,2.81,0.12,0.21,0.56,29.94 and 7.87mg/L）.The hydrochemical types from non-karst surface water to karst surface water and finally karst groundwater were as follows:Ca·Mg-SO₄·HCO₃ and Ca·Mg-HCO₃·SO₄→Ca·Mg-HCO₃·SO₄,Ca-HCO₃,and Ca-HCO₃,Ca·Mg-HCO₃→Ca-HCO₃ and Ca·Mg-HCO₃.The average contents of Pb,Zn and Cd in the sediments were 22.5,19.6 and 17.1 times of the background values of the Xijiang River basin,respectively.Copper（Cu）,Fe,As,Cr and Sr were mainly in residual state,Cd and Zn were mainly in weak acid extracted state,and Mn and Pb were mainly in reducible state.The average contents of heavy metals in sediments showed non-karst surface water>karst surface water>karst groundwater,and the average concentrations of Pb,Zn,Cd and Sr in sediment pore water showed non-karst surface water area（84.52,1632.40,16.36 and 178.27μg/L）>karst surface water area（11.73,364.52,4.94 and 101.67μg/L）>karst groundwater area（9.51,159.64,2.34 and76.72μg/L）.Principal component analysis showed that Pb,Zn,Cd,As,Sr,Cu,Al,Mn and Fe in the water of the study area were mainly from the Laochang Pb-Zn Mine input,and Pb,Zn,Cd,As and Cu in the sediment were mainly from mine drainage input.In the study area,non-karst surface water,karst surface water,and karst groundwater were mainly affected by mine wastewater discharge and carbonate dissolution,while non-karst surface water and karst surface water were mainly affected by mine wastewater discharge.karst groundwater is mainly controlled by regional carbonate weathering.（2）Environmental isotopes indicated the transformation of heavy metals from oxidation of surface water to reduction of underground river water.The characteristics ofδ²H_H2O andδ¹⁸O_H2O values of water bodies in the study area showed that theδ²H_H2O andδ¹⁸O_H2O values of non-karst surface water were relatively negative,followed by karst surface water,and karst groundwater were relatively positive.The enrichment ofδ²H_H2O andδ¹⁸O_H2O in non-karst surface water and karst surface water was mainly controlled by sulfide oxidation of lead-zinc ore and hydrolysis of heavy metals,and karst groundwater was mainly controlled by the dissolution of carbonate rocks.The sulfate in the water body of the study area mainly came from the oxidation of sulphide minerals,the mine wastewater formed sulfate along the flow direction,and theδ³⁴S_SO4 values of sulfide were low,which showed that theδ³⁴S_SO4 values of the water body were gradually negative in the process from non-karst surface water to karst surface water and finally to karst groundwater.The O of SO₄^2-in non-karst surface water mainly came from H₂O,and the proportions of O of SO₄^2-in karst surface water and karst groundwater were relatively weak,which showed that the values ofδ¹⁸O_SO4 in the water body were gradually positive in the process from non-karst surface water to karst surface water and finally to karst groundwater.Bacterial sulfate reduction occurred in karst groundwater,and the values ofδ³⁴S_SO4 andδ¹⁸O_SO4 increased.The mixed proportions of all kinds of water bodies in the underground river were analyzed by using the conservative ions,hydrochemical parameters,isotope characteristics,and mass balance model of hydrochemistry,and it was concluded that the contribution of mine wastewater to SO₄^2-was the highest（>80%）,followed by groundwater（7.2～16.9%）,and the contribution of rainwater to the Sidi groundwater was the least（<1%）.（3）The migration and transformation mechanism of three flow paths of heavy metals in the Sidi underground river basin was revealed,and the conceptual model of heavy metal migration and transformation in the Sidi underground river basin was constructed.The heavy metals in Sidi underground river basin had experienced three kinds of flow paths:from non-karst surface water to karst surface water（flow path A）,from karst surface water to fissure spring（flow path B）,and from karst surface water to karst conduit（flow path C）.In the process of the migration of heavy metals in flow path A with the surface rivers,the heavy metals were mainly affected by the dilution of the river.In this process,the metal particles in the sediments were secondary dissolved under the influence of the flow dynamics in the flat and open part of the riverbed,and the concentration increased.In flow path B,the process of heavy metals entering spring water from surface river water was affected by the karst fissure medium and flow size.Physical and chemical processes such as the dissolution of carbonate rock,microbial sulfate reduction reaction（SRB）,dissolution of iron and manganese oxides,and replacement reaction with carbonate occur in the water body.The dissolved Pb,Zn,Cd,Mn,and Fe formed by the dissolution of iron and manganese oxides migrate to the insoluble state under the combined action of carbonate buffering,microbial sulfate reduction reaction（SRB）,Mn²⁺and Fe²⁺replacement Ca²⁺to form sediment adsorption.The heavy metals in flow path C were affected by the medium of the karst conduit and the size of the flow.The dissolution of carbonate rock and the environment of carbonate particles in the karst pipe were the main reasons for the decrease in the concentration of Zn and Cd,which caused the underground river to become the reservoir of Zn and Cd in the study area.The rapid erosion sediment of karst conduit flow made iron and manganese oxides fully dissolved.It was the main reason for the obvious increase in the concentration of Pb,Mn,and Fe in the conduit.Mine wastewater drainage and soil heavy metal activation promoted the migration and transformation of heavy metals in the Sidi underground river basin.The process of mine wastewater and soil-activated heavy metals entering the underground river of the Sidi has experienced the mixing of non-karst surface water and karst surface water,the mixing of karst surface water and karst fissure water,and the mixing of karst surface water and karst pipe water.By revealing the migration and transformation mechanism of heavy metals in the water flow paths A,B,and C,a conceptual model was constructed about the migration and transformation of heavy metals in the Sidi underground river basin.（4）The flow path B was the preferred target of emergency water supply in the study area and the prevention and control measures of heavy metal pollution were proposed.The maximum concentrations of Pb,Zn and Cd in flow path An in the study area exceeded the standard limits of drinking water in China by 13.2,2.0 and 3.2 times,and the maximum concentration of Pb and Cd in flow path C exceeded 3.8 and 1.3 times of the standard limit of drinking water in China,and the phenomenon of flow path B did not exceed the standard.The total hazard quotient and total hazard index of heavy metals in the three flow paths showed that the flow path B was the smallest（8.24E-01,7.01E-02,8.9E-01）,the flow path C was the second（1.80E+00,1.21E-01,2.01E+00）,and the flow path A was the largest（2.33E+00,1.59E-01,3.10 E+00）.Pb,Cd,Zn and As were potential harmful elements in water body（1>hazard index>0.1）,which should be paid more attention to.Cd and Pb in sediments were the elements with great potential ecological harm in the study area,and the potential hazard index ranges from medium to very strong.The comprehensive potential ecological risk index of heavy metals in sediment showed strong to extremely strong in flow path A（352.7～889.0）,light in water path B（90.6～93.1）,and medium in flow path C（204.7～206.6）.The comprehensive potential ecological risk index of heavy metals in sediments in the study area was as follows:flow path A>flow path C>flow path B.The results of heavy metal risk assessment indexes of water bodies and sediments showed that flow path B was the preferred target for battlefield emergency water sources.For the emergency water source in the study area,a combination of methods such as desulfurization by foam flotation,covering method,waterway guidance method,neutral method and mechanical excavation were proposed to prevent and control heavy metal pollution.Meanwhile,measures to strengthen the concealance of emergency water sources in the battlefield were also proposed,so as to provide water supply guarantee for the smooth progress of military operations and provide case reference for the prevention and treatment of emergency water sources in similar battlefields in karst areas.The innovations of this study were as follows:This study revealed the transfer and transformation mechanism of lead-zinc mine heavy metals in flow path A（non-karst surface water into karst surface water）,flow path B（karst surface water into fissure spring）,and flow path C（karst surface water into karst conduit）in the Sidi underground river basin and constructed a conceptual model of migration and transformation of heavy metals in lead-zinc mines in the Sidi underground river basin.It provided a case study for battlefield water supply in military action.