UTILIZING HYDROGEOLOGIC SETTING FOR SITING RIPARIAN BUFFERS IN AREAS OF INTENSIVE AGRICULTURE

Riparian Management Systems (RMS) have been proposed to minimize the impacts of agricultural production and improve water quality in Iowa. They comprise a toolbox of Best Management Practices (BMPs) that address Nonpoint Source (NPS) pollution and include multi-species buffers, constructed wetlands, stream bank bioengineering, and, more recently, rotational grazing and boulder weirs. As part of RMS, multi-species riparian buffers have been shown to decrease nutrient, pesticide, and sediment concentrations in runoff from adjacent crop fields, add biomass carbon to the soil, and improve wildlife habitat. Although there is a widespread perception that buffers will also improve groundwater quality, the effect on nutrients in groundwater has been discussed only in idealized hydrogeologic settings. Since 1990, our research team has been characterizing the hydrogeology of existing and proposed riparian buffers in the Bear Creek watershed - a 7,656 ha agricultural watershed in central Iowa. Multi-species riparian buffers, consisting of 20 to 40-m-wide strips of switchgrass, shrubs, and trees have been installed in sections of the watershed as part of the Bear Creek RMS. At the Risdal site, buffers are constructed over about 4 m of fine- (loam) and coarse-grained (sand and gravel) alluvium of the Holocene DeForest Formation. An aquitard composed of till and shale/limestone underlies the alluvium, which effectively channels groundwater through a 2-m zone beneath the buffer. Although this would appear to be an ideal hydrogeologic setting, buffer effectiveness is limited due to a zone of high velocity, nitrate-laden groundwater at the base of the aquifer. To complicate matters, many new buffers in the watershed are being constructed not on alluvium but on terrace gravel and till deposits. We are using field investigation (including geophysics), water-quality sampling, and groundwater modeling to characterize the various hydrogeologic settings in the watershed and to suggest the most favorable sites for future buffers.