PHYSICAL AND CHEMICAL HETEROGENEITY IN TWO MID-CONTINENT, ALLUVIAL AQUIFERS

Accurate assessment of ground-water contamination and resource management in floodplain aquifers requires a knowledge of physical and hydrochemical conditions. The complexity of such systems is illustrated in a comparison of high-resolution data from two, mid-continent, alluvial aquifers. Although sedimentary fill in the Neosho and Kansas River valleys originates from different sources, these systems are similar to the many mid-continent alluvial aquifers composed of a fining-upward, gravel and sand unit that is capped by fine-grained a confining layer. Fine-scale hydrostratigraphic features and sediment compositions, however, differ at the two sites. A larger range in hydraulic conductivities is observed in the stratified sand and gravel layers of the Kansas River sediments than in lower Neosho River unit. Higher clay concentrations and fewer sandy lenses exist within the confining unit of the Neosho sediments than in the Kansas River sequence. Despite these differences, field-measured chemical parameters (dissolved oxygen, redox potential, pH, specific conductance, iron, manganese, chloride, sulfate) and laboratory analysis of samples collected every 0.61 m in 11- to 22-m vertical profiles at both sites revealed steep, vertical, chemical and redox gradients. Lateral variations in nitrate and phosphate across short distances (<50 m) at both sites indicate horizontal flow along relatively high conductivity layers. Temporal variability in these parameters suggests that both systems respond more rapidly than expected to recharge events. Such small-scale chemical and physical variations are often overlooked in monitoring and modeling studies of alluvial aquifers, and can contribute to significant misinterpretations.