POTENTIAL SOURCES OF SALINE GROUNDWATER IN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER OF SOUTHEASTERN ARKANSAS

The Mississippi River Valley alluvial aquifer (MRVAA) provides 97% of the total groundwater used in southeastern Arkansas. Unfortunately there are many isolated areas of this aquifer with chloride concentrations >100 mg/L in Chicot and Desha counties. Concentrations exceeding 70 mg/L are unsuitable for crops such as rice. Many high chloride areas are less than 5 miles in diameter, with no clear explanation as to their origin. A large area with total dissolved solids (TDS) concentrations exceeding 1,000 mg/L exists in southern Chicot County.

Federal and State agencies are unsure how to manage this problem due to uncertainties about the source of the salinity and the potential of affected areas to spread via high groundwater pumping rates. Previous studies were researched to provide insight into these issues.

Two structural features dominate this area: the Desha Basin, which stretches from Desha County into west-central Mississippi; and the Monroe Uplift.

A small but strong positive magnetic anomaly underlies southern Chicot County. The center of the anomaly is about ten miles northwest of a large area with high TDS ground water. A high degree of offset along a north-south line is associated with this feature. Well logs from the northern margin of the Monroe Uplift suggest a NW-SE trending fault with approximately 100 ft displacement in upper Tertiary formations. The intersection of these two faults coincides with the area of maximum chloride and TDS concentrations in the alluvial aquifer of southern Chicot County. This would provide a mechanism for deep ground water with high TDS concentrations to migrate upwards into shallower aquifers. The Smackover Formation of Jurassic age contains briny water and is encountered at approximately 5,000 feet below sea level in southeastern Arkansas.

In some areas of the MRVAA, intense pumping can create local hydraulic gradients capable of pulling saline water into the alluvium from underlying Tertiary aquifers. Upward flow from below in an area where the Jackson Formation is thin or missing could explain isolated areas of higher salinity groundwater. Thinning of the Jackson surface through channel erosion is known to have occurred in southern Chicot County. The meandering character of the band of high salinity groundwater in the alluvial aquifer also suggests such channeling.