The Lower Mississippi River Alluvial Aquifer (LMRAA) is a critical groundwater resource for Arkansas, Mississippi, and Louisiana. It is second only to the Ogallala aquifer in terms of the volume of groundwater pumped for irrigation. High concentrations of salinity, iron (Fe), and arsenic (As) affect several regions of the LMRAA. In this study, long-term geochemical changes in the LMRAA in Louisiana were evaluated to better understand the relationships among salts, Fe, and As. The geochemistry was investigated using historical data collected from the LDEQ and USGS. Data from the LDEQ were collected every three years from approximately 2001 to 2013. Major and some trace element data were available, including concentrations of sodium (Na), chlorine (Cl), magnesium (Mg), calcium (Ca), Fe, and As. These historical data were supplemented with recent (2016/2017) sampling and analysis of the isotopes of oxygen (δ 18O) and hydrogen (δ2H). Geochemical results show that groundwater in the LMRAA in Louisiana can be characterized by two main groupings. The first group is generally characterized by a Na/Cl ratio close to one and/or higher salinity, while the second group is generally characterized by excess Na (relative to Cl) and tends to be more alkaline and rich in Fe. The highest salinity regions are spatially limited, and their extents appear to have remained stable over time. Areas of elevated salinity in the northeast part of the study area may be attributable to mixing of deeper salt-rich waters with the shallow groundwater system, while the salt-rich areas in the southern part of the study area are thought to be attributable to dissolution of salt domes. The waters potentially influenced by brines in the northeast are additionally characterized by higher Mg/Ca ratios. These waters are also enriched in δ18O relative to other areas of the LMRAA. There was no correlation between the areas of potential brine interaction and the concentrations of Fe or As. Instead, areas of high Fe concentration correlated spatially with areas of high alkalinity and the development of waters with excess Na (i.e., waters where Na is in substantial excess relative to the amount of chloride, and instead counterbalanced by HCO3-). Arsenic concentrations varied from below detection to 67.7µg/L at one location sampled by the LDEQ in 2010. Six of the approximately 25 wells historically sampled by the LDEQ as part of the ASSET program consistently had concentrations of As >10 µg/L. These locations generally correspond with the groundwater characterized by higher Fe, alkalinity, and Na-excess, but at the same time appear to be localized and often surrounded by wells with low concentrations of As. The concentrations of Fe and As were not correlated. This rather heterogeneous distribution of As contamination could point to anthropogenic influences or sources. The concentrations and spatial distributions of waters rich in salts, Fe, and As in the LMRAA appear to have remained relatively consistent for the last decade, even though demand for groundwater in the LMRAA of Louisiana has more than doubled over this time to 493 million gallons per day (in 2016).
|Advisor:||Borrok, David M.|
|Commitee:||Broussard III, Whitney P., Duex, Timothy W.|
|School:||University of Louisiana at Lafayette|
|School Location:||United States -- Louisiana|
|Source:||MAI 57/05M(E), Masters Abstracts International|
|Keywords:||Aquifer, Arsenic, Bicarbonate, Geochemistry, Iron, Salinity|
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