OCCURRENCE, DISTRIBUTION AND SOURCE OF ELEVATED CHLORIDES IN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER IN SOUTHEASTERN ARKANSAS

The Mississippi River Valley alluvial aquifer in eastern Arkansas is a primary source of irrigation water and has the largest withdrawals of any aquifer in the state. Problems encountered in the use of high salinity waters for irrigation in Arkansas include both acute impacts, such as the burning of crop foliage, and chronic impacts including a reduction in a the uptake of water as a result of an increase in the osmotic pressure of soils. Additionally, high sodium concentrations encountered in high salinity water can cause soil structure deterioration and water infiltration problems.

Over 2,000 well sites with chloride concentrations were compiled from the U.S. Geological Survey National Water Information System and state agency data bases for southeastern Arkansas. Several GIS layers, including soil type, confining layer thickness, alluvial geology, sub-alluvial surface, and potentiometric surface maps, were overlain on the chloride concentration data to visually inspect for correlations between the data sets.

The distribution of elevated (>100 milligrams per liter (mg/L)) chloride concentrations were divided into two major areas based on concentrations and source mechanisms. One area (Area I) includes a southeast trending band of elevated chloride concentrations from southeastern Jefferson County into central Desha County, with maximum chloride concentrations dominantly below 350 mg/L. Another area (Area II) is a narrow, north-south trending band of elevated chloride concentrations in western Chicot County with maximum concentrations that exceed 1,600 mg/L. Elevated chloride concentrations correspond very well to the distribution of backswamp deposits in Area I, and suggest that poor recharge coupled with evapotranspiration are the source for the slightly elevated chloride concentrations in this area. The present study implies that deep basinal brines may be the source of elevated chlorides in the alluvial aquifer in Area II and provides evidence that the distribution and migration of these high salinity waters are closely tied to the deeper subsurface geology.