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

Abstract

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 (δ 13 C org ) 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 δ 13 C org distinguished four groups of sediment types that differ in their organic carbon sources. This includes organic carbon originating predominantly from vascular C 3 plants (C/N: 15.4-21.0, δ 13 C org : -28.6 to -26.7 ‰), C 4 plants (C/N: 10.6; δ 13 C org : -14.8 ‰), freshwater derived particulate organic carbon (C/N: ≤8; δ 13 C org :≤-24 ‰) as well as mixtures incorporating both sources. At the high As sites, we found particulate organic carbon (POC) being 1 to 2 ‰ less depleted in δ 13 C org 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 n 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

Biomarker-indicated extent of oxidation of plant-derived organic carbon (OC) in relation to geomorphology in an arsenic contaminated Holocene aquifer, Cambodia

“…This conclusion, that plant derived OC is mostly restricted to the clay layers, is also supported by another proxy, the C/N ratio. Higher C/N ratios indicate that OC is likely to be plant derived 30 and at our site clay layers have higher C/N (see supplementary information Table 1), this is consistent with a study of broadly similar sediments in Vietnam 16,19 .…”

“…Where possible the calibrated ages are presented with a confidence of 95.4% but in some cases where a significantly narrower age range could be calculated from a moderately lower probability, this was accepted (see supplementary information Figures 7 and 8). δ 13 C of sedimentary OC is a bulk proxy commonly used to distinguish the source of different plant groups and marine and terrestrial inputs 19,30 . A sub-sample of CO 2 (from the radiocarbon analysis) was used to measure δ 13 C using a dual-inlet mass spectrometer with a multiple ion beam collection facility (Thermo Fisher Delta V), in order to correct 14 C data to −25‰ δ 13 C VPDB .…”

Biogeochemical phosphorus cycling in groundwater ecosystems – Insights from South and Southeast Asian floodplain and delta aquifers

“…The groundwater of the Pleistocene aquifer is usually characterized by a near-neutral pH and low Fe 2+ , As 3+ , and TDP concentrations, whereas the groundwater of the transition zone and the Holocene aquifer are characterized by elevated total concentrations of Fe 2+ , Mn 2+ , As 3+ , and TDP (Neidhardt et al 2018). Detailed descriptions of the sedimentation history and groundwater evolution at Van Phuc have been presented in previous publications (Berg et al 2008, Eiche et al 2017, Eiche et al 2008, van Geen et al 2013.…”

Investigating Fluorescent Organic-Matter Composition as a Key Predictor for Arsenic Mobility in Groundwater Aquifers

“…2,3,7,[11][12][13] While sedimentary DOM from peat layers, 9,[14][15][16] as well as surfacederived organic carbon, [17][18][19] have been suggested, the source of labile DOM is still a matter of much debate. 20,21 Beyond labile DOM, recent studies also highlight the potential influence of humic and fulvic acids (refractory organic polymers) in As mobilization through electron shuttling, [22][23][24][25] or competition for sorption, 26 and complexation processes. 27,28 Since the chemical composition of DOM defines its reactivity in the environment, 29,30 the ability to track the different chemical fractions of DOM is essential to better understand its role on arsenic mobilization in groundwater.…”