LITHOLOGIC AND RECHARGE CONTROLS ON THE SPATIAL VARIABILITY OF ARSENIC IN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER, SOUTHEASTERN ARKANSAS

Twenty one of 118 irrigation water wells completed in the shallow (25 - 30 m) Mississippi River Valley alluvial aquifer in the Bayou Bartholomew watershed, southeastern Arkansas had Arsenic (As) concentrations exceeding 10 ppb, ranging from below detection to 77 ppb. Nested monitoring wells at three sites in Jefferson County, Arkansas were installed (10 m and 36 m) in the vicinity of the highest, median, and lowest concentrations of As within the study area. Sediment and groundwater samples were collected to characterize the geochemistry, mobilization, and spatial and vertical distribution of As. A five step sequential sediment extraction (Tessier et al., 1979) was done to differentiate the exchangeable, carbonate, amorphous Fe and Mn oxide, organic, and acid leachable fraction of As, and other compounds. Sequential extraction data indicate about 20% of total As is associated with amorphous Fe and Mn oxides. Goethite (ferrihydrite), hematite, magnetite, etc. were identified in the sediments by XRD and SEM. Positive correlations (r=0.64 and r=0.75) exist between As and Fe in the Fe and Mn oxide extraction, and in the nitric acid extraction. The sediment and groundwater quality data indicate reductive dissolution of iron oxyhydroxides in the presence of organic matter in the zone of seasonal water level fluctuation is responsible for As release. Spatial variability of As is controlled by recharge potential, flow dynamics, and the degree of reducing conditions in the aquifer. The site with the highest As also has the greatest recharge due to the lack of a significant overlying low permeability barrier. The presence of a shallow water level, depleted DO, NO₃-N, mildly suboxic Eh, high dissolved Fe²⁺ , Mn²⁺, As³⁺ and dissolved organic matter indicates an active zone of iron oxyhydroxide reduction. As released in this zone may move downward with recharge. At the site with the lowest As, recharge is retarded due to a thick overlying low permeability layer. Groundwater at this site is highly reducing (negligible DO, absence of NO₃-N, highly anoxic Eh, relatively low Fe²⁺, Mn²⁺, high SO₄^2- in shallow well, etc.) and has passed the active iron oxyhydroxide reducing stage into the sulfate reducing stage which is no longer releasing As from iron oxyhydroxide.