GEOLOGICAL AND GEOPHYSICAL CHARACTERIZATION AS A COMPONENT OF RIPARIAN BUFFER DESIGN

Riparian buffers have been shown to remove nitrate from groundwater, but detailed geologic characterization of these systems is generally lacking. Our 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 objectives of this research are: 1) to assess the role of geology in controlling nitrate fate and transport, and 2) to use innovative techniques to elucidate that relationship for future buffer design. The research area is the Bear Creek watershed, home of the Bear Creek Riparian Management System (RiMS). Restored riparian buffers, consisting of 20-m-wide strips of switchgrass, shrubs, and trees have been in place since 1990. Buffers are typically underlain by about 4 m of fine- and coarse-grained alluvium of the Holocene DeForest Formation. Fining-upward sequences are common in the sand and gravel. The alluvium is underlain by till of the Morgan Member of the Dows Formation and/or shaly limestone of the St. Louis Formation of Mississippian age. Results of water-quality monitoring and a natural gradient tracer test in the buffer suggest that a decrease in nitrate from field edge to stream is the result of denitrification. Nitrate concentrations are also vertically stratified, with lowest and highest concentration occurring near the top and bottom of the aquifer, respectively. Denitrification predominates at the water table and highest nitrate concentrations coincide with maximum groundwater velocities at the bottom of the aquifer, suggesting that denitrification is influenced by aquifer residence time. Traditional installation of monitoring wells and coring to elucidate these relationships in proposed buffers is expensive and time-consuming. In this study, electrical resistivity and shallow seismic surveys are used to delineate the extent of the alluvial aquifer throughout the watershed. Results suggest great variability in the composition of alluvial sediments, which may compromise or enhance the water-quality function of buffers.