DETERMINING THE SOURCE OF SALINE GROUNDWATER FROM THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER IN SOUTHEAST ARKANSAS

Groundwater quality problems exist within specific areas of the Mississippi River Valley Alluvial (alluvial) aquifer in southeastern Arkansas (SE AR). Water samples from the alluvial aquifer in Chicot and Desha counties have historically yielded elevated chloride concentrations with some as high as 1,639 mg/L. Considering the alluvial aquifer is the principle source of irrigation water in SE AR, the saline condition needs to be addressed to ensure the sustainability of crop, groundwater, and soil resources in the area. USGS studies into the source of the salty groundwater remain inconclusive. Spatial relationships between chloride concentrations in alluvial groundwater and soil textures suggest the saline groundwater could be due to evaporative processes in clay-rich, backswamp soils whereby chloride-rich water infiltrates the aquifer, but another possibility is that chloride-rich fluids were injected into the aquifer through faults during past earthquakes and continue to migrate into the aquifer today.

Factor analysis of geochemical data, hydrologic tracer analysis of sampled groundwater, and spatial statistical analysis of salinity and tectonic features in the area were used to test these hypotheses to determine which is most likely responsible for the water quality conditions specific to SE AR or if other phenomena are at work. Existing tracer data suggest a component of evaporatively evolved crustal water of pre-modern age has mixed with younger, fresher meteoric sources in the area to create the saline condition in the alluvial aquifer. Stable hydrogen and oxygen isotope values of waters sampled from the Tertiary Sparta and alluvial aquifers deviate from the global and local meteoric water lines along an evaporative trend (slope=4.4); strong covariance between the values indicates linear mixing between young fresh and older evaporative sources. Increasing chloride content with Carbon-14 age in alluvial groundwater supports this idea. Helium isotope ratios and other noble gas data will confirm or reject our preliminary interpretations and will help determine where the evaporative component of groundwater is sourced.