SPATIAL VARIATION OF WATER CHEMISTRY IN THE ALLUVIAL AND SPARTA AQUIFERS OF EASTERN ARKANSAS

The Arkansas Soil and Water Conservation Commission is expanding the network of monitoring wells for the alluvial and Sparta aquifers in eastern Arkansas because of the importance of these aquifers for irrigation and domestic water supplies. Although sediments composing the aquifers generally grade upward to sand, silt and clay, there are discontinuous lenses of these sediments throughout the aquifers. The production zone for the alluvial aquifer is at the base of the aquifer and is composed of coarse grained sand and gravel. The alluvial wells in this study are 42-46 m deep and the Sparta wells are 135-185 m deep.

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.