Pleistocene sands of the Mississippi River Alluvial Aquifer produce the highest groundwater arsenic concentrations in southern Louisiana, USA

Geostatistical and geospatial analysis techniques were employed to evaluate relationships between high arsenic (As), iron (Fe), and manganese (Mn) concentrations in groundwaters from the Pleistocene Mississippi River Alluvial Aquifer (MRAA) in south-central Louisiana. The MRAA is recharged by the Mississippi River, and its groundwater contains Fe concentrations that range from 0.43 to 179 µmol kg⁻¹ (n = 40) and As concentrations that range from 26 nmol kg⁻¹ up to 8500 nmol kg⁻¹ (i.e., 637 µg kg⁻¹). Arsenic speciation analysis reveals a mix of redox states with As(V) accounting for between ca. 18% and ca. 84% of the total dissolved As (mean ± 1σ = 61.6 ± 24.4%, n = 6). Spatial distributions of As, Fe, and Mn concentrations suggest that the reductive dissolution of Fe/Mn oxides/oxyhydroxides is the likely source of As to the MRAA groundwaters. Geostatistical and geospatial analyses were employed to produce interpolation maps that show concentrations of As, Fe, and Mn decrease with distance from the Mississippi River. Spatial correlations are stronger between groundwater As and Fe concentrations (r = 0.710, P_As*Fe < 0.0005) than those between the As and Mn concentrations (r = 0.404, P_As*Mn < 0.0005). More specifically, regions of the MRAA characterized by high dissolved As concentrations spatially correlate with regions of high dissolved Fe concentrations, and to a lesser degree, high dissolved Mn concentrations. We hypothesize that recharge of the MRAA by Mississippi River water introduces labile organic carbon into the MRAA that subsequently fuels microbial respiration, which reduces Fe/Mn oxides/oxyhydroxides, and mobilizes adsorbed and/or co-precipitated As into MRAA groundwaters. Once mobilized, As is transported away from the riverbank by flowing groundwaters but is rapidly removed from solution by re-adsorption onto Fe/Mn oxides/oxyhydroxides within the regionally oxidized aquifer sediments. Our investigation indicates that oxidized Pleistocene sand aquifers in large river delta plains can also produce groundwater with dangerously high As concentrations if redox conditions shift to suboxic/ferruginous conditions.