Geochemical and isotopic evidence for upward flow of saline fluid to the Mississippi River Valley alluvial aquifer, southeastern Arkansas, USA

Abstract

Groundwater from the Quaternary Mississippi River Valley Alluvial (MRVA) aquifer in southeasternArkansas (SE AR), USA, has higher salinity compared to other MRVA groundwater. Previous studieshave argued for infiltration of evaporated soil water as a primary source for the elevated salinity, althoughseepage from local rivers and deep groundwater sources also have been considered. Geochemical andisotope data from irrigation, public supply, and industrial wells, as well as subsurface geologic data, areused to demonstrate that upward flow of saline water along regional faults is the primary source of salinityin MRVA aquifer groundwater in SE AR. Sodium, chloride (Cl-) and bromide (Br-) concentrationsillustrate mixing relationships between MRVA aquifer groundwater and Jurassic Smackover Formationbrine, with mixing percentages of <1% Smackover brine being the source of anomalously high Cl-, Br-, andother ions in MRVA groundwater with elevated salinity. Stable oxygen and hydrogen isotope data suggestsubstantial mixing of Paleogene Wilcox Formation water with that of the MRVA aquifer groundwater andvarying degrees of evaporative concentration. Radiocarbon and helium isotope data argue for contributionsof chloride-rich, pre-modern and relatively fresh modern water for recharge to the MRVA aquifer.Chloride concentration in MRVA aquifer waters closely follows the spatial distribution of earthquake-inducedliquefaction features and known or suspected geologic faults in SE AR and northeastern Louisiana.A conceptual model is developed where deep-seated basinal fluids in overpressured reservoirs migrateupward along faults during and following Holocene earthquakes into the overlying MRVA over 100s to1,000s of years