The groundwater of Bangladesh is severely contaminated with arsenic. Understanding the source, mechanisms of mobilization, and transport of arsenic is important for evaluating the current threat to public health and for planning future water management. We have conducted an extensive set of hydraulic and chemical measurements at a field site in the badly contaminated Munshiganj district. We have measured both organic and inorganic carbon isotopes as well as concentrations of helium and tritium in an effort to determine the source of carbon and other chemicals involved in arsenic mobilization. We will analyze these results through application of a groundwater flow model constructed with hydraulic measurements from an array of test wells.

We have also conducted injection-withdrawal experiments to test hypotheses concerning the in situ geochemistry of arsenic. These tests elucidate how arsenic mobilization is influenced by mixing of local groundwater with water containing nitrate (an oxidant), molasses (a reductant) and low arsenic water (which drives desorption). Results indicate that arsenic sorption may be modeled with a non-linear isotherm taking into account competition from phosphate and silicate anions, and that redox controls on arsenic mobility are complex. In situ arsenic concentrations are observed to fall with both addition of nitrate (probably by precipitation of iron oxides) and addition of organic carbon (probably by precipitation of sulfide minerals).