Biogeochemical Processes In High Arsenic Groundwater In The Northwestern Hetao Basin, Inner Mongolia:Evidences From Hydrogeochemistry And Stable Isotopes

High-As groundwater is widely distributed all over the world,which is one of the most serious environmental problems.Hetao Basin is a typical inland basin hosting geogenic high-As groundwater in China.Previous research showed that microbially meditated iron oxide reduction triggered by organic matters obviously promotes As enrichment and increases As solubility in groundwater.However,related controlling factors need to be further studied.Fractionation characteristics of carbon stable isotope and iron isotope and their correlations with released Fe and As from Fe oxides/hydroxide reduction remain openThe northwest Hetao Basin was selected as the study area.Based on hydrogeological setting,groundwater and sediment samples were taken and analyzed for chemical components and isotope composition(?¹³C_DIC,?¹³C_DOC,?⁵⁶Fe,?¹⁸O,and?D).The combination of field monitoring and laboratory experiments allows to characterize stable carbon isotopes and iron isotopes in groundwater and to evaluate the related biogeochemical processes during As enrichment.Results showed that 36 in 49 groundwater samples?73.5%?had As concentrations greater than 10?g/L.Groundwater As concentrations ranged between1.32 and 384?g/L.High-As groundwaters occurred under weakly alkaline-reductive environment.In general,groundwaters had relatively high dissolved Fe concentrations,mid-high SO₄^2-concentrations,and low NO₃^-concentrations.Groundwater?¹³C_DIC values ranged between-5.34‰and-11.8‰and showed a decreasing trend from piedmont zone to flat plain area.Groundwater?¹³C_DOC values ranged between-19.2‰and-22.9‰.Groundwater?⁵⁶Fe had the range between-3.22‰and-0.88‰?median-1.45‰?,which generally decreased from the piedmont zone to the flat plain area,with?⁵⁶Fe difference of 0.78‰along the flow path.Vertically,a general increase in groundwater As downward was observed.There was a similar trend in groundwater?⁵⁶Fe to groundwater As.Carbon stable isotope evidences and variations of redox sensitive components showed that processes of groundwater As enrichment were different between shallow?<42 m?and deep?>42m?groundwaters.Arsenic enrichment in shallow groundwaters mainly resulted from dissimilatory reduction of As-bearing Fe?III?oxides/hydroxides minerals.For deep groundwaters,As enrichment may also be affected by bacterial sulfate reduction,during which As may coprecipitate with dissolved Fe?II?as Fe?II?secondary minerals and dissolved Fe concentrations decreased.Bacterial reduction experiments showed that,during reduction of As-bearing goethite in the presence of Clostridium bifermentans strain?lasted for 336 h?,around3.5‰and 1.8‰carbon isotope fractionation were observed in DIC and DOC,respectively.About 0.58‰iron isotope fractionation in dissolved Fe simultaneously occurred,which was similar to isotope fractionation in groundwater samples.Combination of groundwater analysis and experiment results showed that microbial degradation of organic matter produced inorganic carbon in groundwater systems,resulting in decreasing?¹³C_DIC and increasing?¹³C_DOC,and simultaneously mediated reduction of Fe?III?oxide mineral,increasing dissolved Fe concentrations and decreasing?⁵⁶Fe_aq.Therefore,the combination of carbon isotopes and Fe isotopes provides an efficient method for characterizing As mobilization processes under the reducing conditions,which presents a new evidence for high As groundwater genesis.