| As an important source of water supply,karst water resources play a crucial role in the social and economic development of many countries and regions worldwide.Guizhou Province,which is the representative and typical region in the southern karst regions,the karst groundwater is an indispensable resource for human survival and social sustainable development.Karst water is highly susceptible to human activities due to the unique hydrogeochemical characteristics,outstanding geochemical sensitivities,and ecological vulnerability.Moreover,Guizhou Province is enriched in coal resources,it is the most important production base of large-scale coal mining,and most coal mines are located in the karst mountain areas.Long-term coal mining activities lead to the persistent emission of acid coal mine waste water,posing a certain threat to quality and safety of extremely fragile karst water.The Zhijin country,located in the Midwestern Guizhou Province,is a typical karst region and an important coal mining area in China.Karst water is the main source of water supply.The waste water produced from the contiguous coal mining activities is a serious threat to the quality of the water and soil environment,which gradually affects the local drinking-water supplies and the life and production of local residents.Based on this,a typical karst watershed affected by coal mining activities in Zhijin County is chosen as the research object,the major elements geochemistry,trace elements,hydrogen and oxygen isotopes in water,sulfur and oxygen in dissolved sulfate are comprehensively applied to analyze the chemical composition,trace element geochemical characteristics and isotopic composition of karst water in the watershed.The purpose of this study is to reveal the characteristics of water quality and evolution rules of karst water under the influence of acidic mine waste water,to clarify the related geochemical processes affecting the karst water quality under the influence of acidic mine waste water,and to provide scientific basis for the protection of karst water and effective management of karst groundwater pollution in the karst mountainous areas.We come to some of the most important conclusions of this study:Affected by the discharge of acid coal mine waste water,the chemical characteristics of karst water in the study area have undergone major changes.The chemical compositions of natural surface water and groundwater are mainly Ca2+,Mg2+,and HCO3-,and the seasonal variation is not significant,which show strong correlation with the geological background.It is mainly related to the weathering dissolution of carbonate rocks in the karst area and the water-rock process carried out in the karst water system.While,the main chemical compositions in karst water affected by acid coal mine waste water are Ca2+,Na+,Mg2+,SO42-,Fe and Al.The contents of these components in water change seasonally,showing a feature that the wet season is higher than the dry season.The water quality evolution of the karst basin is mainly reflected in:the pH is significantly reduced,and the contents of major chemical components such as TDS,SO42-,Ca2+,Na+,Mg2+,and trace elements?Fe,Mn,Al,F-,and REE?are obviously increased.The most important is that the hydrochemical types of karst water gradually change from the primary Ca?Mg?-HCO3type to Ca-SO4,Na-SO4 and Mg-SO4 types.Different geochemical processes under the influence of acid coal mine waste water are important factors controlling the concentration,distribution,species,migration and transformation of trace elements in the karst water environment.The F-in surface water is originally derived from the dissolution of inorganic fluoride-bearing minerals in coal,the concentration and species are controlled by p H,metal concentrations,hydrochemical types as well as the dissolution and precipitation of minerals in the water.F-is predominantly concentrated in Mg-SO4 and Ca-SO4types water with low pH,while the F-concentration in neutral alkaline Ca-HCO3water is very low.In surface waters,the fluoride is dominated by the free ion F-and complexes of Mg F+,Ca F+,Na F,and Al-F,while in groundwater,the free ion F-is dominant.The high concentration of TDS,Fe,Mn,and Al in surface water reduces the concentration and mobility of F,while the concentration of F-in groundwater is affected by the dissolution/precipitation of carbonate rocks.Besides,the enrichment of Ca2+and Mg2+inhibits the release of F-in water.The REE in the surface water of the karst basin is mainly derived from the acid dissolution and release of the surrounding rocks.Under the acid conditions,MREE is preferentially released from the surrounding rocks,which is an important reason for the enrichment of MREE in the surface water.The pH and the concentrations of Fe,Mn,Al and SO42-are important factors influencing the REE concentration,species and distribution patterns in surface waters.The REE is mainly in the forms of sulfate complexes(Ln?SO4?+and Ln?SO4?2-),and the Ln3+is the secondary species in the surface water.With the increase of p H,the adsorption and co-precipitation of REE by hydroxides and secondary minerals formed by Fe,Mn and Al are the main reasons leading to the decrease of REE in the water.In addition,the MREEs is preferentially captured by poorly crystalline Fe-Al hydroxides,which favors that sediments also preserve NASC-normalized patterns with MREE enrichment in the basin.Results of hydrogen and oxygen isotope show that the surface water and groundwater not affected by the acid coal mine waste water are mostly located near the global atmospheric precipitation line and the local atmospheric precipitation line,are mainly supplied by atmospheric precipitation.Most of the coal mine effluents and surface water samples affected by acid coal mine waste water deviated from the atmospheric precipitation line to a different extent,and had a heavier?2H and?18O compositions,indicating that they were subjected to strong water-rock interactions and physicochemical reactions during runoff and recharge.A series of reactions and geochemical processes,including isotope exchange reactions between water bodies and coal seams and carbonate rocks during coal mining,oxidation reaction of pyrite,Fe hydrolysis and strong post-stage evaporation,collectively led to the enrichment of?2H and?18O in the coal mine water and surface water affected by the acid coal mine waste water in the study area,and thus exhibited significant 2H and 18 O drift characteristics.The isotopic compositions of dissolved sulfate in surface water show clear variation,with?34SSO4O4 and?18OSO4O4 values ranging from-18.28‰to-8.08‰and-4.38‰to+1.21‰,respectively.Sulfate in surface water is mainly derived from oxidation of sulfide minerals.In addition,atmospheric precipitation is also one of the sources of sulfate contribution in surface water.The sulfur and oxygen isotopic compositions of dissolved sulfate in groundwater range from-14.24‰to-4.68‰and-2.77‰to+4.75‰,respectively.The sulfate in groundwater exposed in carbonate formations is mainly attributed to the contribution of atmospheric precipitation,while the sulfate in groundwater exposed to coal-bearing strata is mostly originated from oxidation of sulphides in coal seams.Oxygen isotopes geochemical information of water and sulfate show that anaerobic oxidation of pyrite is an important cause of depletion of?34SSO4O4 and?18OSO4in water.Overall,the chemical compositions of karst water in the study area are mainly affected by the weathering of carbonate rocks and the discharge of acid mine waste water.The hydrogeochemical processes are dominated by carbonate weathering,pyrite oxidation and Fe hydrolysis.The chemical compositions of karst water in the study area are controlled by atmospheric precipitation,dissolution of carbonate rocks,the discharge of acid mine waste water.The mixing effect among different end-members is the main reason leading to the evolution of chemical composition of karst water in the area. |
PDF Full Text Request