DO THE MUNICIPAL WELLS OF DARDANELLE, ARKANSAS RECEIVE RECHARGE FROM THE ARKANSAS RIVER THROUGH ITS ALLUVIAL AQUIFER?

The municipal wells of the City of Dardanelle tap the alluvial aquifer of the Arkansas River in an area where the alluvial plain is about 2 km wide and 21 m thick, flanked by areas of Paleozoic shale bedrock outcrop. The city planners were concerned about potential contamination of their water-supply system if the river were to be polluted. We conducted a study of the local hydrology in order to construct a MODFLOW groundwater model of the system using available data. Based on available driller’s logs from the city’s wells, which show an upper interval of silty and clayey fine sand and a lower interval of coarse sand and gravel, a two-layer model was developed. Hydraulic conductivity, K, of the upper layer was estimated from grain-size analysis of soil auger samples taken at 1.5 m depths, with comparison to estimated permeability published in a county soil survey. A range of K values for the model’s lower layer were used, based on aquifer tests published in a USGS report from a reconnaissance study (Bedinger and others, 1963). Aerial recharge for the model was based on an estimate given in the same report. MODFLOW’s stream package was applied to model the involvement of Smiley Bayou in the system, using a stream discharge based on flow measurements made in the field. The Arkansas River was modeled as a constant head boundary based on an average low-flow condition. Areas of bedrock outcrop form a no-flow boundary almost parallel to the Arkansas River, and groundwater flow lines form no-flow lateral boundaries. A steady state model using annual pumping rates of the city’s wells indicates clearly that the wells are being significantly recharged by inflow from the Arkansas River. The conclusion is the same for any reasonable K value picked for the lower part of the aquifer, where the city wells are screened. Model heads reasonably match water levels in wells accessible to measuring devices.