HYDROGEOLOGIC SETTING CONTROLS NO3-N REMOVALIN GROUNDWATER BENEATH MULTI-SPECIES RIPARIAN BUFFERS

摘要

Multi-species buffers have been proposed to minimize the impacts of agricultural production and improvewater quality in Iowa. Although it is hypothesized that buffers will also improve groundwater quality, the effect of buffers onnutrients in groundwater is often discussed only in idealized hydrogeologic settings. The hypothesis of improvedgroundwater quality via NO_3~-N removal was tested in the Bear Creek watershed, a 7,656 ha agricultural watershed in centralIowa. Multi-species riparian buffers, consisting of 20- to 40-m-wide strips of switchgrass, shrubs, and trees up to 14 yearsold, have been installed in sections of the watershed. After preliminary electrical resistivity soundings established thegeology, multilevel piezometer transects were installed at seven sites within the watershed to assess the NO_3~-N removalefficiency of each buffer. Hydraulic gradient, hydraulic conductivity, and NO3-N concentration data from individualmultilevel ports were used to calculate NO3-N input and output at each buffer and total NO3-N removal. Two sites wereremoved from the analysis due to unfavorable hydrogeologic conditions. Data from the other sites collected during 2001-02showed that three sites consistently removed greater than 93 percent and two sites removed only about 20 to 30 percent ofNO3-N. Results suggest that NO3-N removal is favored by hydrogeologic conditions of groundwater flow toward the creek,small groundwater velocities, and long groundwater residence times in fine-textured materials. Sufficient organic carbonmust also be present to sustain denitrification. Age of the buffer has the least effect on the amount of NO_3~-N removal.