Investigating the Relationship Between Manganese Reactivity and Arsenic Adsorption Capacity of Pleistocene Sediments from Southeast Asia.

Arsenic (As) contamination of well water is a major threat to human health across the globe. In Southeast Asia specifically, increased withdrawals of As-contaminated groundwater have prompted a search for signals showing that an aquifer is at risk of dangerous concentrations of allogenic As, or the As in groundwater pulled from an area with a high concentration to a previously uncontaminated well. We hypothesized that due to their higher redox potential and propensity to oxidize the more mobile As(III) to less mobile As(V), reactive Mn oxides of sediments would control the allogenic As pollution potential of an aquifer. Accordingly, this study sought to develop an indicator of aquifer vulnerability to future As contamination by determining the relationship between reactive manganese (Mn) concentrations and the As sorption potential of aquifer sediments. A variety of sediment samples from a Pleistocene aquifer in Cambodia were put through a set of parallel extractions to quantify the fractions, solubilities, and reactivities of Mn present within solid phases. The Citrate-Bicarbonate-Dithionate (CBD) extraction resulted in Mn concentrations ranging from 39.6 to 387 mg/kg. Arsenic adsorption isotherms on the sediments were conducted and modeled with both Freundlich and Langmuir models, yielding Freundlich KF values from 11.8 to 420 (ug/kg)(L/ug)^1/n and Langmuir maximum adsorption (QM) values from 3.5 to 58.7 mg/kg. A linear correlation between the concentration of crystalline Mn oxides, extracted by the CBD method, and the modeled adsorption parameters was determined, with R² of .873 and .930 for Freundlich and Langmuir parameters, respectively. These results suggest that the Mn oxide content of sediments could serve as an easily measurable indicator to quickly assess the potential for future well contamination following the input of allogenic As.