Origin and availability of organic matter leading to arsenic mobilisation in aquifers of the Red River Delta, Vietnam

Groundwater arsenic (As) concentrations in the Red River Delta (Vietnam) are often patchy and related to the microbially induced reduction of Fe oxy-hydroxides. In this study, we explored the influence of the origin, composition and availability of natural organic matter on the hydrochemical variability in the aquifers of Van Phuc. Carbon isotope signatures (δ¹³Corg) and C/N ratios were assessed in combination with lithology, geochemistry, hydrochemistry, hydrology and the distribution of specific biomarkers.The elationship of C/N ratios and δ¹³Corg distinguished four groups of sediment types that differ in their organic carbon sources. This includes organic carbon originating predominantly from vascular C3 plants (C/N: 15.4–21.0, δ¹³Corg: −28.6 to −26.7‰), C4 plants (C/N: 10.6; δ¹³Corg: −14.8‰), freshwater derived particulate organic carbon (C/N: ≤8; δ¹³Corg:≤−24‰) as well as mixtures incorporating both sources. At the high As sites, we found particulate organic carbon (POC) being 1–2‰ less depleted in δ¹³Corg than at low As sites. More importantly, however, our assessment shows that, the availability of organic matter has to be considered decisive with regard to groundwater As contamination. Fine-grained clayey sediments overlaying sands generally protect organic matter from substantial degradation and its leaching into an adjacent aquifer. However, at the sites that are high in dissolved As in Van Phuc, sediment layers rich in organic matter are hydraulically connected to the underlying aquifer. Here, soluble organic matter seeping into the aquifer can induce and/or enhance reducing conditions, thereby mobilising As from Fe oxy-hydroxides. Our study shows that both the clay content as well as the origin of organic matter are largely controlled by the depositional environment of the sediments.