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

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.

Numerical Modeling of Arsenic Mobility during Reductive Iron-Mineral Transformations.

Millions of individuals worldwide are chronically exposed to hazardous concentrations of arsenic from contaminated drinking water. Despite massive efforts toward understanding the extent and underlying geochemical processes of the problem, numerical modeling and reliable predictions of future arsenic behavior remain a significant challenge. One of the key knowledge gaps concerns a refined understanding of the mechanisms that underlie arsenic mobilization, particularly under the onset of anaerobic conditions, and the quantification of the factors that affect this process. In this study, we focus on the development and testing of appropriate conceptual and numerical model approaches to represent and quantify the reductive dissolution of iron oxides, the concomitant release of sorbed arsenic, and the role of iron-mineral transformations. The initial model development in this study was guided by data and hypothesized processes from a previously reported,1 well-controlled column experiment in which arsenic desorption from ferrihydrite coated sands by variable loads of organic carbon was investigated. Using the measured data as constraints, we provide a quantitative interpretation of the processes controlling arsenic mobility during the microbial reductive transformation of iron oxides. Our analysis suggests that the observed arsenic behavior is primarily controlled by a combination of reductive dissolution of ferrihydrite, arsenic incorporation into or co-precipitation with freshly transformed iron minerals, and partial arsenic redox transformations.