THE ALLUVIAL AQUIFER OF THE ARKANSAS RIVER IN THE DARDANELLE AREA, CENTRAL ARKANSAS

Ten pairs of shallow and deep monitoring wells were drilled to gather data for refining a numerical groundwater flow model of the alluvial aquifer of the Arkansas River at Dardanelle. The wells provide (1) stratigraphic data through borehole logging, (2) sites for water-level monitoring for model calibration and verification, (3) sites for aquifer testing for hydraulic properties, and (4) sampling points for groundwater geochemistry.

The alluvial sediment varies from about 16 to 22 m thick above Paleozoic shale and sandstone, thinning away from the river. The upper 8-12 m consists of overbank deposits of very fine silty sands with numerous, discontinuous clay-rich layers, which are more continuous in farther reaches from the river. The lower section contains medium to very coarse gravelly sand. The saturated zone is in the lower section in most of the area but extends up a short distance into the upper section in the area farther from the river; in every location we sampled, saturation was below the lowest clay-rich layer, indicating unconfined conditions. The natural hydraulic gradient towards the river is interrupted by composite cones of depression formed by Dardanelle's municipal well field, but a small groundwater ridge remains between the well field and the river most of the time. However, during periods of sustained high river stage the gradient is reversed and the river infiltrates the aquifer and feeds into the well field. Seasonal water level fluctuations indicate that aerial recharge is substantially higher in the region near the river, where clay layers are discontinuous, and highly restricted in the farthest reaches of the flood plain, where back swamps formerly occurred after floods. Hydraulic conductivity in the coarse sediment, measured by pneumatic slug tests, ranges from 35 to 77m/d.

A comparison of the water chemistry of the Arkansas River, the aquifer, and the city supply wells indicates that water in the wells closest to the river is a mixture of aquifer water plus water infiltrated from the river. This is indicated by a gradual trend from a high concentration of Cl in the river to lower Cl concentrations progressively further into the well field. Furthermore, two very distinct trends of Cl/Br ratios characterize water samples from the river and wells near the river as compared to wells removed from the river.