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 southeastern Arkansas (SE AR), USA, has higher salinity compared to other MRVA groundwater. Previous studies have argued for infiltration of evaporated soil water as a primary source for the elevated salinity, although seepage from local rivers and deep groundwater sources also have been considered. Geochemical and isotope data from irrigation, public supply, and industrial wells, as well as subsurface geologic data, are used to demonstrate that upward flow of saline water along regional faults is the primary source of salinity in MRVA aquifer groundwater in SE AR. Sodium, chloride (Cl-) and bromide (Br-) concentrations illustrate mixing relationships between MRVA aquifer groundwater and Jurassic Smackover Formation brine, with mixing percentages of <1% Smackover brine being the source of anomalously high Cl-, Br-, and other ions in MRVA groundwater with elevated salinity. Stable oxygen and hydrogen isotope data suggest substantial mixing of Paleogene Wilcox Formation water with that of the MRVA aquifer groundwater and varying degrees of evaporative concentration. Radiocarbon and helium isotope data argue for contributions of 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-induced liquefaction 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 migrate upward along faults during and following Holocene earthquakes into the overlying MRVA over 100s to 1,000s of years