Dissolved Organic Matter Sources in High Arsenic Groundwater From a Sand‐Gravel Confined Aquifer. Issue 2 (21st February 2023)

• Record Type: Journal Article
• Authors: Gao, Zhipeng; Guo, Huaming; Qiao, Wen; He, Chen; Shi, Quan; Ke, Tiantian; Cao, Yuanyuan
• Is Part Of: Journal of geophysical research. Volume 128:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-21
• Subjects: arsenic -- deep confined groundwater -- DOM sources -- degradation index -- Songnen Basin; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2022JG007178
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: Sources of dissolved organic matter (DOM) triggering arsenic enrichment in deep confined groundwater is a highly debated issue. Along these lines, the spectroscopic and molecular characteristics of DOM were monitored in both shallow unconfined and deep confined groundwater from the Songnen Basin, China. Results demonstrated that deep groundwater with higher dissolved arsenic concentrations was characterized by more abundant labile organic compounds than shallow groundwater. The three‐end‐member mixing model estimated that shallow groundwater had more fraction of surface water, while deep groundwater was primarily recharged by lateral groundwater flow. Groundwater DOM sources were estimated from the fluorescence index (FI) values, which suggests that shallow groundwater DOM was primarily sourced from surface water with stronger terrestrial signatures. However, deep groundwater DOM was mainly sourced from in situ sedimentary organic matter release or by‐products from microbial activity with stronger microbial signatures. Higher values of degradation index (I_deg) indicate that deep groundwater DOM had undergone greater degradation extents than the shallow one. The microbial DOM degradation in anoxic environments mainly occurred via a pathway where more saturated and low‐molecular‐mass compounds were preferentially degraded to produce more mid‐molecular‐mass compounds with unsaturated, aromatic, and microbial signatures. This explained the significant positive correlations between groundwater arsenic concentrations and FI and I_deg values. A stronger DOM degradation state in deep groundwater was conducive to more arsenic enrichment than that in the shallow one. This work successfully identified distinct sources of shallow and deep groundwater DOM, and proposed a paradigm of DOM processing pathways during arsenic enrichment under anoxic environments. Plain Language Summary: Sources of dissolved organic matter (DOM) in high arsenic groundwater has long been a highly debated issue, especially in a deep‐confined aquifer being capped by overlying clay layers. To distinguish the sources of DOM in shallow and deep groundwater, both the spectral and molecular characteristics of groundwater DOM were monitored. A three‐end‐member mixing model was established to estimate the fractions of surface water, polluted water, and fresh lateral recharge in groundwater samples. The acquired results clearly demonstrated that shallow groundwater DOM had a higher proportion of surface water‐derived organic matter with a stronger terrestrial signature, while deep groundwater DOM was primarily supplied by the release of in situ sedimentary organic matter and/or by‐products of microbial activity. Moreover, microbes appear to preferentially utilize low‐molecular‐mass organic compounds with relatively high H/C ratios as substrates under anoxic groundwater environments, during which mid‐molecular‐mass aromatic compounds with microbial signatures were produced. The calculated DOM degradation indices indicate that deep groundwater DOM underwent a stronger extent of degradation than a shallow one, which resulted in a stronger Fe(III) oxide reduction and higher arsenic concentrations in deep groundwater. Key Points: Shallow groundwater dissolved organic matter (DOM) showed more terrigenous fraction from surface water, while that in deep groundwater was mostly in situ supplied Saturated and low‐molecular‐mass DOM compounds were preferentially consumed, producing more unsaturated and mid‐molecular‐mass compounds Deep groundwater DOM was more degraded than the shallow one, which may be responsible for higher arsenic levels in deep groundwater