Quantifying Reactive Transport Processes Governing Arsenic Mobility after Injection of Reactive Organic Carbon into a Bengal Delta Aquifer

Abstract

Over the last few decades, significant progress has been made to characterize the extent, severity, and underlying geochemical processes of groundwater arsenic (As) pollution in S/SE Asia. However, comparably little effort has been made to merge the findings into frameworks that allow for a process-based quantitative analysis of observed As behavior and for predictions of its long-term fate. This study developed field-scale numerical modeling approaches to represent the hydrochemical processes associated with an in situ field injection of reactive organic carbon, including the reductive dissolution and transformation of ferric iron (Fe) oxides and the concomitant release of sorbed As. We employed data from a sucrose injection experiment in the Bengal Delta Plain to guide our model development and to constrain the model parametrization. Our modeling results illustrate that the temporary pH decrease associated with the sucrose transformation and mineralization caused pronounced, temporary shifts in the As partitioning between aqueous and sorbed phases. The results also suggest that while the reductive dissolution of Fe(III) oxides reduced the number of sorption sites, a significant fraction of the released As was rapidly scavenged through coprecipitation with neo-formed magnetite. These secondary reactions can explain the disparity between the observed Fe and As behavior.

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

“…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). Surface complexation models (SCMs) are the most logical approach to describing As adsorption and its responses to changes in groundwater chemicals.…”

Spatial Variability of Groundwater Arsenic Concentration as Controlled by Hydrogeology: Conceptual Analysis Using 2‐D Reactive Transport Modeling

“…Reactive transport modeling lends itself to the evaluation of integrated effects of variations in geology, hydrogeology, and geochemical processes on the groundwater arsenic distribution. Comprehensive reactive transport modeling that includes all the main geochemical processes has only been applied in a few studies of As mobility (Kocar et al, ; Postma et al, , ; Rawson et al, ). A prerequisite for performing meaningful comprehensive reactive transport modeling is the availability of data sets that comprise both the detailed geology and burial age of the deposits, groundwater flow patterns, and groundwater ages as well as a detailed description of the groundwater chemistry.…”

Source and Mobilization Mechanism of Iron, Manganese and Arsenic in Groundwater of Shuangliao City, Northeast China

“…This is the most accepted mechanism for high As groundwater in many places in the world [7,[12][13][14]. To investigate the influence of organic matter on the mobility of Fe, Mn, and As, in situ experiments [13,15] were conducted to inject reactive carbon into shallow groundwater. In addition to the increase in groundwater reducibility, the concentration of nitrate and sulfate decreased [13].…”