HYDROGEOLOGY AND WATER QUALITY BENEATH MULTI-SPECIES RIPARIAN BUFFERS IN THE BEAR CREEK WATERSHED, CENTRAL IOWA

Riparian buffers have been shown to remove nitrate from groundwater, but detailed hydrogeologic characterization of these systems is generally lacking. Our previous research in the Bear Creek watershed in central Iowa suggests that geology influences groundwater velocity, residence time, denitrification rate, and ultimately how well the buffer functions. The research area is the Bear Creek watershed, a 7,656 ha watershed with > 85 percent row crops. Multi-species riparian buffers, consisting of 20-m-wide strips of grass, shrubs and trees, have been in place for up to 11 years. Buffer sediments typically consist of about 4 m of fine- and coarse-grained alluvium of the Holocene DeForest Formation underlain by till of the Morgan Member of the Dows Formation and/or shaly limestone of the St. Louis Formation of Mississippian age. In this study, a combination of piezometers (multilevel and nested) along with hydraulic gradient and hydraulic conductivity data were used to assess controlling factors on nitrate removal in buffers. Results of previous water-quality monitoring and a natural gradient tracer test suggested that a decrease in nitrate concentration from field edge to stream is the result of denitrification near the water table. However, our data suggest that the distribution of nitrate concentrations within the alluvial aquifer varies in the watershed. In some locations, high nitrate-N concentrations (>15 mg/L) coincide with high groundwater velocities (~1 m/d) in coarse sand units near the bottom of the aquifer. Other locations in deep, aquitard protected sand units are characterized by a lack of nitrate-N (<0.1 mg/L), while above-ambient concentrations (>5 mg/L) occur in overlying sediments exposed to agricultural activity. Electrical resistivity surveys and coring suggest great variability in the composition of alluvial sediments throughout the watershed. Our preliminary data suggest that water quality is primarily dependent on groundwater residence time and flow paths in these sediments.