COMPARISON OF POTENTIOMETRIC MAPS PRODUCED BY DENSELY SPACED MEASUREMENT POINTS, AND USGS 7.5' MANUAL ELEVATION PICKS FOR THE ALLUVIAL AQUIFER OF EASTERN ARKANSAS

A potentiometric surface map of an alluvial aquifer has been produced based on measured surface elevations utilizing survey grade global positioning system (GPS) for densely spaced measurement points (158 positions in 15 mi X 15 mi area) near Pine Bluff in southeastern Arkansas. The fast static method was used for GPS elevation measurement, and the data were processed using National Spatial Reference System (NSRS) through National Geodetic Survey (NGS), Online Positioning User Service (OPUS) for the base station and Trimble GPS software for each measured position. The elevation data and resulting potentiometric map were compared to a U.S. geological survey (USGS) potentiometric map of the alluvial aquifer, and a potentiometric map based on manual elevation picks from the 7.5' quadrangle topographic maps (5 ft contours). With the exception of 10 outliers in the 158 position data set, there are no significant differences (more than 5 ft differences) between the 7.5' quadrangle topographic map extracted elevations and survey-grade GPS measurements. However, the densely spaced measured data allowed identification of more subtle ground water/surface water interactions. For example, the USGS potentiometric map and other publications indicate the major recharge source of the alluvial aquifer in this area is the Arkansas River. It is hypothesized that smaller streams have less hydraulic connections with the aquifers than the larger rivers in the area. But, the potentiometric map produced based on the high density GPS data shows that there are significant hydraulic connections between small streams, including Bayou Bartholomew, and the alluvial aquifer. The results indicate that it is necessary to have more densely spaced monitoring points (e.g. at least one point per square mile) for appropriate investigation of hydraulic connections between surface water and alluvial aquifers. In areas with complex surface water components, and where hydraulic gradients are low, accurate elevation and potentiometric data are much more important to define the intricacies of the groundwater flow system.