Hydrology and geochemistry of an alluvial aquifer near a floodplain margin

The Kansas River valley alluvial aquifer is an important source of drinking water for northeastern Kansas and is similar to other large floodplain alluvial aquifers in the Mississippi River system. In this study, the hydrology and geochemistry of the alluvium at the Kansas River floodplain margin were evaluated over an 18-month period that included a 100-year flood event. The major recharge event associated with the 1993 flood caused a large-scale shift in flow direction and gradient at the 1370 m2 monitoring site. This change, along with smaller-scale fluctuations in ground-water flow due to variable amounts of rainfall between monitoring periods, resulted in significantly variable hydrologic conditions at the floodplain margin. The combination of fluctuations in flow direction and gradient, with mixing constraints due to stratified sediments, caused significant spatial and temporal variability in ground-water chemistry over a short (20 to 50 m) lateral distance. Recharge to the site is predominantly by lateral flow. The relative proportions of recharge sources that enter sedimentary layers in the aquifer vary in response to the recharge fluctuations associated with rainfall. Fine-grained sedimentary layers in the upper part of the aquifer are characterized by low flow velocities, and support greater chemical evolution of ground water than do coarser-grained, intermediate and deep strata. Variable rates of recharge and mixing occurred in the deeper strata and caused high temporal variability in ground-water chemistry in these units. The fate of ground-water nitrate at the site is controlled by a combination of chemical reduction in the fine-grained alluvium and dilution through mixing in the lower more transmissive units. The complex hydrochemical conditions that exist in alluvium near floodplain margins confound efforts to monitor the fate of contaminants in alluvium near floodplain margins during large-scale recharge events. The finding suggests that closely spaced, vertical and lateral monitoring points are required to assess accurately the natural variability that exists in alluvium during periods of high recharge. Such conditions should be considered in the design of hydrogeologic investigations of alluvial aquifers on floodplains bordered by terraces.