HYDROSTRATIGRAPHIC CONTROLS ON GROUNDWATER CONTAMINATION NEAR THE MARGIN OF A TYPICAL MID-WESTERN FLOODPLAIN

Throughout the mid-western U.S., agricultural contaminants pose a threat to drinking water aquifers that underlie the floodplains of major river systems. The importance of aquifer recharge along the margins of floodplains bounded by terraces has recently been demonstrated, but details regarding movement and fate of contaminants that enter aquifers by this mechanism have not been evaluated. Data obtained using a combination of high-resolution direct-push methods and monitoring wells provide evidence for surface-derived nitrate and phosphate in an alluvial aquifer near the margin of the Kansas River floodplain. Lateral transport of contaminants away from the floodplain margin along relatively high permeability zones is consistent with a hydrostratigraphic site model developed using direct-push electrical conductivity logging. The distribution of both nitrate and phosphate is influenced by preferential movement along alternating layers of fine and coarse sediments that typify alluvial sequences. Lateral movement of these constituents is observed within the uppermost silt and clay layer that is otherwise thought to protect the aquifer from contamination. Phosphate concentrations generally decrease with depth, but persist where phosphate has been transported laterally within the silt and clay. Nitrate concentrations are highest near the floodplain margin, but appear to be chemically reduced based on d¹⁵N_NO3 patterns and other chemical indicators of anaerobic conditions. Organic carbon concentrations in fine-grained parts of the alluvium have been used to model bulk reduction capacities of aquifers and to predict threshold levels of nitrate attenuation. However, vertical variations in the organic carbon content of cores from the site and correlation of organic-rich zones across part of the floodplain using electrical conductivity logs demonstrate that the use of bulk estimates may not be appropriate where reactive constituents occur in stratified sediments. Recognition of potential contaminant infiltration at floodplain margins and an understanding of the sedimentary and chemical controls on groundwater chemistry in these settings is important to the protection of floodplain aquifer resources.