Biodegradation of dissolved organic matter and sedimentary organic matter in high arsenic groundwater system: Evidence from lipid biomarkers and compound-specific carbon isotopes

Organic matter plays important roles in arsenic mobility. Whether sedimentary or dissolved organic matter is the predominant electron donors is still on debate. To fill this gap, both sediments and groundwater were collected from the Hetao Basin, a typical arid-semiarid area hosting high arsenic groundwater, to characterize diagnostic biomarkers and compound-specific carbon isotopes of lipids in dissolved organic matter (DOM) and its depth -matched sedimentary organic matter (SOM). Results showed that SOM in both clay and sand layers were mainly sourced from terrestrial higher plants, but sand SOM had additional sources from microbial and petro-leum OM. Compound-specific carbon isotope compositions of DOM and SOM showed that 813C of low molecular weight (LMW) n-alkanes (813CL) were more positive than those of high molecular weight (HMW) n-alkanes (813CH). The 813CH in groundwater (813CH-DOM) and sediments (813CH-SOM) fall in the range of C3 plants, indi-cating that DOM and SOM had similar origins. More positive 813CL than 813CH suggests preferential degradation of LMW compounds in both sediments and groundwater. In comparison with SOM, more microbial-derived OM was observed in groundwater DOM, especially in high-arsenic groundwater. High arsenic groundwater was accompanied by the low ratio of n-alkanes to hopanes in depth-matched SOM, and the great difference between 813CL and 813CH and low terrestrial/microbial ratios (TAR) in groundwater DOM. This indicates that SOM and DOM should be utilized by indigenous bacteria to fuel the reductive dissolution of Fe oxides for arsenic release into groundwater. Furthermore, more abundant LMW compounds and weaker odd-to-even predominance in DOM than those in SOM support that DOM encountered stronger biodegradation. This study highlights that preferential biodegradation of DOM (especially LMW components and petroleum sourced OM) would provide electrons for the reductive dissolution of arsenic-bearing Fe/Mn oxides and thus induce arsenic enrichment in groundwater.