HYDROCHEMISTRY IN RELATION TO GROUND-WATER LEVEL DECLINES OF THE ALLUVIAL AND SPARTA AQUIFERS, GRAND PRAIRIE REGION, EASTERN ARKANSAS

Saline water contamination of major aquifers in eastern Arkansas has been an historic problem and will likely expand. In the Grand Prairie region, high demands from multiple water users result in significant areas of ground-water level decline. During a period of maximum aquifer stress, 77 water samples collected from the shallow Quaternary alluvial aquifer and 40 from the underlying Tertiary Sparta aquifer were analyzed for major inorganic constituents and selected trace elements. Both aquifers, bound on the west, south, and east by major rivers, were examined for interconnectivity and spatial distribution of selected parameters in relation to ground-water level declines. Southwest to northeast trending sampling transects across regional cones of depression exhibit linear concentration gradients from the outer edges toward centers of water-level decline. Chloride ranged from 11 mg/L near the outer edge to 120 mg/L near the center of decline within the alluvial aquifer and from 2 mg/L to 56 mg/L within the Sparta aquifer. Spatial distributions of chloride for both aquifers suggest that increased concentrations of this parameter are not a direct result of recharge from local and regional rivers. Instead, increased concentrations in both aquifers are products of rock-water interaction along ground-water flowpaths that result in part from upconing due to significant water-level declines. Concentrations of iron are relatively high, with a marked difference between aquifers. Concentrations of 0.002-43 mg/L iron and 2-172 mg/L chloride in the alluvial aquifer are nearly four times concentrations of 0.013-11 mg/L iron and over three times concentrations of 2-56 mg/L chloride found in the Sparta, indicating little or no interaction between the two aquifers. Parameters such as lithium, sulfate, zinc, and copper also show no inter-aquifer similarities and provide further evidence against significant aquifer connectivity.