Abstract
Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (δ13C-CH4, δ13C-DOC, δ13C-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 µg/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in δ13C-CH4 from -87‰ to +47‰, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers.
Highlights
The health of tens of millions of people worldwide is affected by chronic exposure to arsenic-polluted groundwater resources (WHO, 2011; Karagas et al, 2015; Podgorski and Berg, 2020)
The highest dissolved As concentrations are found in the wells that show the largest concentrations of dissolved organic carbon (DOC), ammonium (NH4+) and dissolved methane (CH4), that is 5–7 mg DOC/L, 50–65 mg NH4-N/L and 40–58 mg CH4/L, respectively (Fig. 2 and Fig. S6 for NH4+)
The water isotope signatures in these wells (δ18O −6 ± 1‰) are indicative of evaporative water that locally infiltrates from the aquitard into the aquifer (Fig. 2), rather than of water originating from Red River bank infiltration
Summary
The health of tens of millions of people worldwide is affected by chronic exposure to arsenic-polluted groundwater resources (WHO, 2011; Karagas et al, 2015; Podgorski and Berg, 2020). Diseases caused by this exposure are common in the floodplains and deltas of the large East and South Asian river systems (Smith et al, 2000; Harvey et al, 2002; McArthur et al, 2004; Berg et al, 2007; Fendorf et al, 2010; Zhang et al, 2017). In that context the role of methane (CH4) cycling has rarely been considered, especially in natural settings (Polya et al, 2019), several studies detected CH4 in As-contaminated aquifers (Liu et al, 2009; Postma et al, 2012, 2016; Sø et al, 2018) and indicated that elevated CH4 and As concentrations might be related (Buschmann and Berg, 2009; Sracek et al, 2018)
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