Characteristics And Compound-specific Carbon Isotope Compositions Of Lipids In Arsenic-rich Aquifer Sediments And Groundwater Of The Hetao Basin,inner Mongolia

Organic matter,as an electron donor,plays a vital role in As mobilization mediated by microorganisms during reductive dissolution of Fe/Mn oxides in shallow aquifers.However,the types and sources of organic matter involved in biogeochemical processes accelerating As mobilization are still controversial.In addition,the types and sources of dissolved organic matter in groundwater and the differences between dissolved organic matter and sedimental organic matter are still unknown.In order to solve those two problems,both sediment and groundwater samples were collected along the groundwater flow line at different depths of different boreholes from the Hetao Basin,a typical inland basin hosting high As groundwater.Lipids and their compound-specific carbon isotope ratios were analyzed to evaluate characteristics and sources of organic matter and its role in the As mobilization process.The key findings of this work include:(1)Sedimentary As was well correlated with iron-manganese oxides and total organic carbon,suggesting that As exists as Fe/Mn oxide bound forms or combination with organic matter.Groundwater As far exceeded the drinking water guide value of 10 ?g/L,indicating that dissolved As may be mainly released into groundwater from Fe/Mn oxides in sediments.(2)The lipid extracts and analysis show that lipids in clay samples were mainly originated from terrestrial higher plants,while those in fine sand samples were derived from terrestrial higher plants,microorganism and petroleum.Lipids in groundwater were mainly originated from microorganism and degradation products of sedimentary organic matter.(3)The spatial variation of sediment lipids was mainly affected by lithology.Fine sand samples had low lipid contents and high bioavailability,while clay samples had high lipid contents but low bioavailability.The variation of sediment lipids along the groundwater flow direction were relatively weak.The types and contents of groundwater lipids changed little spatially,indicating that this typeof organic matter was universal in As hotspot areas.(4)Biomarker parameters show that the fine sand samples and groundwater samples were also characterized by evident in-situ biodegradation,with similar intensity of microbial degradation.Compound-specific carbon isotope compositions of sedimentary lipids showed that short-chain n-alkanes and n-alkanoic acids had more positive ?13C values compared to long-chain compounds.In particular,??13C between short-chain and long-chain n-alkanes were evident in groundwater samples with high As concentrations.?13CTOC were also low in high-arsenic layer fine sand samples.These results jointly indicate that these lipids in fine sand samples and groundwater acted as carbon source for indigenous microorganism and induced the release of As,during which the short-chain components were particularly more vulnerable to biodegradation.