HYDROGEOLOGIC CONTROLS ON THE EFFICIENCY OF NITRATE REMOVAL BENEATH MULTI-SPECIES RIPARIAN BUFFERS IN THE BEAR CREEK WATERSHED, CENTRAL IOWA

Riparian buffers have been shown to remove nitrate from groundwater, but the processes controlling removal are not well documented. Previous research at the Risdal Farm 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 for this study is the entire Bear Creek watershed, a 7,656 ha watershed with > 85 percent row crops. Reconstructed riparian buffers, consisting of 20-m-wide strips of grass, shrubs and trees, have been in place for up to 11 years. Hydraulic gradient and hydraulic conductivity data from 67 piezometers (multilevel and nested) in 6 buffers were used to assess controlling factors on nitrate removal in buffers. Data collected over a 14-month period indicate that the distribution of nitrate concentrations within the buffers 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 below the buffer. In other locations, aquitard-protected sand units below the buffer are characterized by a lack of nitrate-N (< 0.1 mg/L), while above-ambient concentrations (> 5 mg/L) occur in overlying sediments. Nitrate removal efficiency varies from 0 to over 95 percent. Buffers at the Jon Risdal South and Strum West sites both consistently removed over 95 percent of nitrate. The Tesdal West and Risdal South sites varied between 40 and 90 percent removal. The Strum East and Tesdal East sites often had little effect on nitrate removal, but occasionally achieved reductions up to 50 percent. Nitrate removal was favored in locations with available dissolved organic carbon and low groundwater velocities (long residence time). Lack of dissolved oxygen in these locations suggests denitrification as the removal mechanism. Based on data from the 6 sites, the water quality benefits of buffers are most dependent on geology, groundwater residence time and geochemical environment and least dependent on the age of the buffer.