Arsenic Speciation in Mekong Delta Sediments Depends on Their Depositional Environment

Abstract

Arsenic contamination in groundwater is pervasive throughout deltaic regions of Southeast Asia and threatens the health of millions. The speciation of As in sediments overlying contaminated aquifers is poorly constrained. Here, we investigate the chemical and mineralogical compositions of sediment cores collected from the Mekong Delta in Vietnam, elucidate the speciation of iron and arsenic, and relate them to the sediment depositional environment. Gradual dissolution of ferric (oxyhydr)oxides with depth is observed down to 7 m, corresponding to the establishment of reducing conditions. Within the reduced sediment, layers originating from marine, coastal or alluvial depositional environments are identified and their age is consistent with a late Holocene transgression in the Mekong Delta. In the organic matter- and sulfur-rich layers, arsenic is present in association with organic matter through thiol-bonding and in the form of arsenian pyrite. The highest arsenic concentration (34-69 ppm) is found in the peat layer at 16 m and suggests the accumulation of arsenic due to the formation of thiol-bound trivalent arsenic (40-55%) and arsenian pyrite (15-30%) in a paleo-mangrove depositional environment (∼8079 yr BP). Where sulfur is limited, siderite is identified, and oxygen- and thiol-bound trivalent arsenic are the predominant forms. It is also worth noting that pentavalent arsenic coordinated to oxygen is ubiquitous in the sediment profile, even in reduced sediment layers. But the identity of the oxygen-bound arsenic species remains unknown. This work shows direct evidence of thiol-bound trivalent arsenic in the Mekong Delta sediments and provides insight to refine the current model of the origin, deposition, and release of arsenic in the alluvial aquifers of the Mekong Delta.

As release under the microbial sulfate reduction during redox oscillations in the upper Mekong delta aquifers, Vietnam: A mechanistic study

“…The seasonally flooded sediment was collected at the Quoc Thai commune, An Giang province, Vietnam, in a paddy soil near the Mekong River during the dry season, in 2014 ( Fig. SI-1) where was contained a high As concentration in groundwater (Wang et al, 2018;Van Phan et al, 2017). A sediment core was drilled from a depth of 0-20 m and separated into sections as a function of depth under an argon (Ar) flux and then immediately placed in the heat sealed Mylar® bags under N 2 atmosphere (Wang et al, 2018).…”

“…Concentrations of arsenic in groundwater higher than 50 μg/L have been reported in several Southeast Asian countries including Bangladesh, India, Cambodia, China, and Vietnam (e.g. Wang et al, 2018;Van Phan et al, 2017;He and Charlet, 2013;Fendorf et al, 2010;van Geen et al, 2008;Zheng et al, 2004;Buschmann et al, 2008). In particular, the Mekong Delta floodplain, which stretches over 62,100 km 2 in Southern Vietnam and South Cambodia, harbors numerous wells with elevated As concentrations (Buschmann et al, 2008;Nguyen et al, 2000).…”

“…A national survey, conducted by the Department of Water Resources Management monitored the As concentration in tube wells from 2002 to 2008, showed that large areas of the Mekong Delta were contaminated by arsenic, in nearly 900 wells (Erban et al, 2013). Areas with As concentrations in groundwater exceeding 1600 ppb are located along the main Mekong river branches, from An Giang to Can Tho City (Wang et al, 2018;Van Phan et al, 2017;Hanh et al, 2011;Hoang et al, 2010;Nguyen and Itoi, 2009). The origin of As in groundwater of the Mekong Delta is naturally produced by microbial reduction of arsenic-bearing iron oxides in the alluvial sediments (Polya et al, 2005;Rowland et al, 2007;VanVan Dongen et al, 2008;Kocar and Fendorf, 2012;Polizzotto et al, 2008;Stuckey et al, 2015;Winkel et al, 2010).…”

“…Arsenic can also co-precipitate with iron sulfide minerals to form arsenic-substituted pyrite, known as "arsenian pyrite" (e.g. Rittle et al, 1995;Neuman et al, 1998;Charlet et al, 2011;Langner et al, 2012;Le Pape et al, 2017;Wang et al, 2018). Lastly, arsenite is known to bind strongly onto sulfhydryl groups (thiols) of OM (ThomasArrigo et al, 2016;Langner et al, 2013;R.M.…”

Leaching Behavior of As, Pb, Ni, Fe, and Mn from Subsurface Marine and Nonmarine Depositional Environment in Central Kanto Plain, Japan

“…Several investigations were performed on the chemical speciation, potential mobility, and bioavailability of As and Pb in the core sediment of various characteristics and different depositional environments [7,[17][18][19][20][21]. However, the sediment of different depositional environments (marine, nonmarine, and peat) and their correlation with pore water concentration in a single profile have not been largely considered by researchers.…”