SPATIAL VARIATION OF WATER CHEMISTRY IN THE ALLUVIAL AND SPARTA AQUIFERS OF EASTERN ARKANSAS
The purpose of this project was to evaluate the spatial variability of ground-water chemistry from the alluvial and Sparta aquifers on relatively small scale using irrigation wells less than 1.3 km from the monitoring wells. Preliminary data from five recently installed monitoring wells and 20 existing irrigation wells indicate that there is considerable variation of ground-water chemistry in these aquifers for Ca, Na, Fe, HCO3, Cl, SO4, NO3, conductance and pH. Ion concentration ranges for the irrigation wells sometimes approach or exceed the concentrations for the monitoring wells. These differences may be demonstrated by comparing the concentration of the monitoring wells to the variation of ion concentrations of the nearby irrigation wells. For example, the monitoring well concentration is listed first and the irrigation well concentration range is given in parentheses for the following data: Ca 116 (108-166), Na 49 (35-117), Fe 2.1 (2.8-7.1), HCO3 295 (185-311), Cl 23 (13-41), SO4 61 (33-88), NO3 0.03 (0-3.99), conductance (uS/m) 606 (257-520) and pH 6.8 (6.7-7.7). Potassium, Mg, Mn, Pb, Cu, Zn, Br, F, and PO4 concentrations were low and exhibited minor variations. Heterogeneous distribution of minerals and organic carbon and the character of discontinuous sedimentary lenses that affect ground-water residence time contribute to the variation in water chemistry.
The results of this study indicate caution should be used in drawing isoconcentration maps for these aquifers. Information of the type obtained from this study can be used to determine the required well density and selection of contour intervals for constructing isoconcentration maps.