CONTROLS ON THE MOBILITY OF NATURALLY OCCURRING ARSENIC IN A SHALLOW AQUIFER UNDER VARIABLE CHEMICAL CONDITIONS

Processes controlling mobilization of naturally occurring arsenic (As) have been examined in a quartz sand aquifer on western Cape Cod, Massachusetts, USA, where the groundwater flow system and water and sediment chemistry are well-understood. Spatially and temporally variable chemical conditions result from land-disposal of treated wastewater. Concentrations of naturally occurring As in sediments are 2-8 nanomoles/gram (0.15-0.75 ppm), with 20 to 50 percent occurring as As(V) bound to material coating surfaces of quartz and other mineral grains. Arsenic concentrations are < 3 nanomoles/liter (nM) (below detection) in ambient groundwater, which has ~300 ìM dissolved oxygen (DO), pH 5.3-5.9, and low dissolved salts. In wastewater-contaminated groundwater with low DO but elevated nitrate concentrations, As(V) is the only dissolved species, and concentrations increase with increasing phosphate concentrations. In anoxic groundwater with elevated concentrations of ferrous (Fe(II)) and phosphate, dissolved As is >130 nM (10 ppb) and both As(III) and As(V) are present. Nitrate transported into the Fe(II)-zone owing to changes in wastewater loading was accompanied by disappearance of dissolved As and phosphate, likely due to binding to Fe oxides formed by microbial Fe(II) oxidation. Subsequently, concentrations of Fe(II), phosphate, As(V), and As(III) slowly increased after nitrate was displaced by anoxic groundwater. Laboratory and field experiments showed that: 1) As(III) is rapidly oxidized to As(V) by manganese oxides in coatings on sediment-grain surfaces; 2) adsorption of As(V) is so extensive that As(V) can only be detected when competitively adsorbing phosphate is present; 3) in the Fe(II)-zone, As(V)-reducing microorganisms control reduction to As(III) at a rate that is slow compared to the time-scale of transport and likely a function of As(V) concentration (e.g., via Monod kinetics). Thus, in the DO and nitrate-containing zones, oxidation to As(V) is rapid and concentrations are controlled by competitive adsorption with phosphate, whereas in the Fe(II) zone, there appears to be a steady state between release of adsorbed As(V), via diffusion and reductive dissolution of hydrous Fe(III) oxides, and reduction to As(III), which accounts for the occurrence of both species.