GROUNDWATER FLOW AND CHEMISTRY IN ALLUVIUM OF THE ARKANSAS RIVER IN CENTRAL ARKANSAS: A PROGRESS REPORT

In 2002 a study was initiated to determine if water from the Arkansas River was infiltrating the alluvial aquifer at Dardanelle, Arkansas and contributing to water drawn from the city's well field. A numerical groundwater flow model was constructed with parameters estimated from published regional studies. The model indicated a composite cone of depression drawing part of its water from the Arkansas River. A shortcoming of the early model was a paucity of data suitable for model calibration. A second phase of the study is now under way to gain a more comprehensive understanding of the aquifer. Ten pairs of observation wells, one screened near the water table (7-13 m depth) and one screened near the base of the aquifer (18-21 m depth), are being installed. Data from borehole logging and from these wells will be used for obtaining better stratigraphy, local hydraulic conductivity, water chemistry, and head distribution for model calibration. Initial observations confirm that there is a downward head gradient from the river to the well field and thus infiltration from the river. Furthermore, field pH and conductance measurements of water from these wells, city pumping wells, and the Arkansas River are consistent with the general flow pattern indicated in the early model. Higher pH and conductance in the river water contrasts with lower values of these parameters in wells isolated from the river, and intermediate values consistent with degrees of mixing occur in wells within the cone of depression. However, water levels under the flood plain farther out from the river are lower than the model had predicted, implying that recharge to the aquifer is not uniform and is probably reduced in that area. Observations in Smiley Bayou, a creek near the outside flank of the alluvium, indicate that there is widespread impermeability in the upper stratigraphic horizons of the alluvium of that area, which would agree with the apparent lower recharge rate. Further study of seasonal head variation and water chemistry will be employed to better understand these matters and to refine the model.