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
Highlights
Vertical upward migration of groundwater is known to affect water quality in near-surface aquifers
This study did not sample any wells with saline groundwater in the Middle Claiborne aquifer, the Sparta 1 well is in an area with saline water in the overlying Mississippi River Valley Alluvial (MRVA) aquifer (Kresse et al 2014)
This study used existing water chemistry data from wells in the MRVA aquifer in southeastern Arkansas (SE AR) to identify a limited number of wells to sample for chemical and isotopic tracers to determine the source of saline waters, as evident by anomalously high (>100 mg/L) chloride contents
Summary
Vertical upward migration of groundwater is known to affect water quality in near-surface aquifers. The conditions leading to upward migration vary, but most commonly stem from flow along preferential pathways from underlying rock units at elevated pore pressure (Carrillo-Rivera et al 1996; Mehta et al 2000; Frumkin and Gvirtzman 2006; Petitta et al 2011), thermohaline convection (Evans et al 1991; Magri et al 2009), groundwater pumping in shallow aquifers (Howard and Mullings 1996; Cherry 2019), or cross-formational or up-dip flow (Ma et al 2005). Hydrologic conditions leading to upward migration of saline water into shallow aquifers, especially those utilized extensively for agricultural or industrial water supply, are not well understood. Water quality problems are generally mild in the aquifer, especially in northeastern Arkansas, western Tennessee, and southeastern Missouri
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