IRON, ARSENIC, AND ELEVATED SALINITY IN THE LOWER MISSISSIPPI RIVER ALLUVIAL AQUIFER OF LOUISIANA

The Lower Mississippi River Alluvial Aquifer (LMRAA) is a critical groundwater resource for Arkansas and parts of Mississippi and Louisiana. It is second only to the Ogallala aquifer in terms of the volume of groundwater pumped. High salt, iron (Fe), and arsenic (As) concentrations affect several regions of the LMRAA. In this study, we evaluate long-term geochemical changes in the LMRAA in Louisiana to understand the sources and distributions of these contaminants. Bulk geochemical data (pH, T, salinity, Na, Mg, Ca, HCO₃, Cl, S, Fe, As, etc.) were compiled from samples collected every three years over a ~20 year period from more than 20 wells in the LMRAA. Initial geochemical results show that groundwater in the LMRAA in Louisiana falls into two main groups. The first group is generally characterized by a higher Mg/Ca ratio and tends to be more alkaline, while the second group is generally characterized by a Na/Cl ratio close to one and high salinity. Measured salinities spanned several orders of magnitude, ranging from 0.01 parts per thousand (ppt) up to 1.14 ppt. The areas of highest salinity are localized and their spatial extents appear to have remained stable over time. Iron concentrations varied from 7.6 μg/L to 21,700 μg/L over the study area. Iron and salinity concentrations did not correlate. Instead, Fe generally correlated with high alkalinity (geochemical group 1) and barium. Arsenic concentrations varied from levels below detection to 82 µg/L. Five of the sampled wells consistently had As concentrations > 10 µg/L. Iron was not correlated with As. The spatial distributions of the two geochemical groups of groundwater appear to be related to differences in the geology of the LMRAA. These differences in geology also appear to influence the cycling of Fe in the aquifer system, while the heterogeneous distribution of As contamination could point to anthropogenic sources. Areas of high salinity in northeast Louisiana are likely attributable to mixing of basinal brines with the shallow groundwater system. Additional work is underway to further evaluate these relationships, including integration of geochemical data available for the LMRAA in Arkansas and Mississippi.