A model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years, Red River floodplain, Vietnam

Abstract

Aquifers on the Red River flood plain with burial ages ranging from 500 to 6000 years show, with increasing age, the following changes in solute concentrations; a decrease in arsenic, increase in Fe(II) and decreases in both pH, Ca and bicarbonate. These changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation, the reduction kinetics of As containing Fe-oxides, the sorption of arsenic, the kinetics of siderite precipitation and dissolution, as well as of the dissolution of CaCO 3 . The arsenic released from the Fe-oxide is preferentially partitioned into the water phase, and partially sorbed, while the released Fe(II) is precipitated as siderite. The reaction network involved in arsenic mobilization was analyzed by 1-D reactive transport modeling. The results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by Fe-oxide reduction. The energy released by Fe-oxide reduction is strongly pH dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the pH. Overall it is the rate of organic carbon degradation that determines the total electron flow. However, the kinetics of Fe-oxide reduction determines the distribution of this flow of electrons between methanogenesis, which is by far the main pathway, and Fe-oxide reduction. Modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in As during the first 1200 years where it reaches a maximum of about 600 μg/L. During this initial period the release of arsenic from Fe-oxides actually decreases but the adsorption of arsenic onto the sediment delays the build-up in the groundwater arsenic concentration. After 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further, but diminishing, release from Fe-oxide being reduced. After 6000 years the arsenic content has decreased to 33 μg/L. The modeling enables a quantitative description of how the aquifer properties, the reactivity of organic carbon and Fe-oxides, the number of sorption sites and the Correspondence to: Dieke Postma. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content.

Quantifying Geochemical Processes of Arsenic Mobility in Groundwater From an Inland Basin Using a Reactive Transport Model

“…These values fall within the range of solubility of crystalline phases (goethite with log K = −1.0) and amorphous phases (lepidocrocite with log K = 1.371). The solubility of Fe (III) oxides in the alluvial fan was close to that reported in D. Postma et al () at log K = 0.391, in which only hematite and goethite were identified.…”

“…The degradation of organic matter is the overall engine for the reaction system and was modeled according to equation to describe the decrease in degradation rates with time (D. Postma et al, ). where k organic is the rate constant (1/yr); C 0 refers to the initial organic matter concentration (mol/L); and the exponent of 2.5 to ( C t / C 0 ) reflects the heterogeneity of the organic matter's reactivity (D. Postma et al, ).…”

“…During dissimilatory reduction, the most critical factors controlling As release are the reactivity of available organic matter and the stability of Fe (III) oxides, which act as electron donor and acceptor, respectively (Neumann et al, 2010;Schaefer et al, 2017;Stuckey et al, 2016). These can be reflected by kinetic processes of organic matter degradation and Fe (III) oxide reduction (D. Postma et al, 2016). In addition to redox processes, As adsorption has frequently been included in models (D. Postma et al, 2007Postma et al, , 2016Postma et al, , 2017Rawson et al, 2017;Pi et al, 2018;Sun et al, 2018).…”

Considering a Threshold Energy in Reactive Transport Modeling of Microbially Mediated Redox Reactions in an Arsenic-Affected Aquifer

“…Simulated H 2 concentrations were generally within 1-10 nM that is a typical range of values observed in natural systems [28,29,42]. Simulated methane concentrations were below 0.4 mM, which are values comparable with those observed in other natural anoxic aquifers worldwide, such as the Red River floodplain in Vietnam [25,26] and the Asserbo [29] and Rømø [43] aquifers in Denmark. Figure 1.…”

“…In the last decades, some research efforts were devoted to developing a quantitative understanding of the processes governing As mobility through the implementation of reactive transport modeling [11,[23][24][25][26][27]. However, none of these previous modeling efforts implemented the role of bacteria in mediating OM degradation and related TEAPs within the numerical simulations of As mobility.…”