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

Abstract

The poisoning of rural populations in South and Southeast Asia due to high groundwater arsenic concentrations is one of the world’s largest ongoing natural disasters. It is important to consider environmental processes related to the release of geogenic arsenic, including geomorphological and organic geochemical processes. Arsenic is released from sediments when iron-oxide minerals, onto which arsenic is adsorbed or incorporated, react with organic carbon (OC) and the OC is oxidised. In this study we build a new geomorphological framework for Kandal Province, a highly studied arsenic affected region of Cambodia, and tie this into wider regional environmental change throughout the Holocene. Analyses shows that the concentration of OC in the sediments is strongly inversely correlated to grainsize. Furthermore, the type of OC is also related to grain size with the clay containing mostly (immature) plant derived OC and sand containing mostly thermally mature derived OC. Finally, analyses indicate that within the plant derived OC relative oxidation is strongly grouped by stratigraphy with the older bound OC more oxidised than younger OC.

Impact of sedimentation history for As distribution in Late Pleistocene-Holocene sediments in the Hetao Basin, China

“…7b), suggesting a considerable influence from terrestrial C3 plant input. These findings go in line with previous studies in the Red River and Mekong River delta (Eiche et al 2017;Magnone et al 2017). The increasing δ 13 C org signatures towards the surface in the top silty clay or clay layers can be related to C4 plant debris input, as maize is the main C4 crop in the Hetao Basin nowadays.…”

Sub-surface Biogeochemical Characteristics and Its Effect on Arsenic Cycling in the Holocene Gray Sand Aquifers of the Lower Bengal Basin

“…This is an important distinction between the GSA and BSA wells, and implies causality between OM sources and subsurface microbial communities and processes associated with mobilization of As, and the safety of drinking water resources. Such biomarker signatures have also recently been reported from the other As-contaminated aquifers of SE Asia, indicating their critical role in As-mobilization (Magnone et al, 2017). This inference however, needs further investigations involving microbial degradation of OM in sediments on similar lines as established in DOC extracted from these wells (Ghosh et al, 2015a), using tracer compounds in laboratory and in situ measurements of degradation products and As levels in the microbial cultures.…”

Hotspots of geogenic arsenic and manganese contamination in groundwater of the floodplains in lowland Amazonia (South America)

“…Availability and reactivity of organic matter as a driver for redox-reactions plays a crucial role (Lovley, 1995;Nickson et al, 2000;McArthur et al, 2004;Meharg et al, 2006;Glodowska et al, 2020). This implies that local heterogeneities of the aquifers in organic matter supply and hydraulic conductivity controls the spatial variability in aqueous arsenic-concentrations (McArthur et al, 2004;Papacostas et al, 2008;Donselaar et al, 2017;Magnone et al, 2017;Stopelli et al, 2021;Lightfoot et al, 2022). Recent studies in South and Southeast Asia advanced the idea that the most dynamic parts of floodplains and delta's including meandering belts and active floodplain, are most prone for arsenic contamination (e.g.…”

“…It has been suggested that fluvial dynamics causes irregular patterns in the local stratigraphy on a meter to kilometer scale, which affects the groundwater arsenicdistribution. Fluvial dynamics has a direct control on sediment burial age (Postma et al, 2012), sediment deposition rate, flow rate, and indirectly on the availability of reactive organic matter and iron (hydr)oxides (McArthur et al, 2004;Meharg et al, 2006;Kocar et al, 2008;Papacostas et al, 2008;Quicksall et al, 2008;Stuckey et al, 2016;Magnone et al, 2017;Donselaar et al, 2017;Jakobsen et al, 2018), factors that control arsenic mobility in aquifers. Within a single aquifer, local flow patterns, and heterogeneous aquifer sediments can cause vertical and horizontal concentration gradients (McArthur et al, 2004;van Geen et al, 2013;Radloff et al, 2017).…”