Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2007
Publication Date: 7/15/2007
Citation: Schilling, K.E., Tomer, M.D., Zhang, Y.K., Weisbrod, T., Jacobsen, P., Cambardella, C.A. 2007. Hydrogeologic controls on nitrate transport in a small agricultural catchment, Iowa. Journal of Geophysical Research-Biogeosciences. 112, G03007. Available: http://www.agu.org/journals/jg/jg0703/2007JG000405/. Interpretive Summary: Riparian soils are known to have significant amounts of organic carbon deep in the soil profile that can promote denitrification of shallow groundwater, especially where buffers have been established. This research showed that upland ephemeral waterways can also exhibit soil and groundwater conditions that are conducive to denitrification. Within a small upland watershed, we found large differences in nitrate concentrations between groundwater collected in uplands and that collected along an ephemeral waterway. Groundwater flow models confirmed that denitrification could eliminate virtually all the nitrate contributed from the uplands. Tile drainage can short-circuit this natural treatment process. Results are of interest to conservation planners and researchers who seek alternatives to manage nitrate in shallow groundwater.
Technical Abstract: Effects of subsurface lithology on nitrate loss in stream riparian zones are recognized but little attention has been focused on similar processes occurring in upland agricultural settings. In this paper, we evaluated hydrogeologic controls on nitrate transport processes occurring in a small 7.6 ha Iowa catchment. Subsurface lithology in the catchment consisted of upland areas of loess overlying weathered pre-Illinoian till, whereas two lowland waterways consisted of Holocene-age silty and organic rich alluvium. Water tables in upland areas fluctuated more than 4 m per year compared to less than 0.3 m in the waterway. Water quality patterns were significantly affected by lithology, with groundwater in lowland waterways having lower nitrate concentrations (<0.5 mg/l) compared to upland areas (>10 mg/l) as wells as lower pH, dissolved oxygen and redox, and higher ammonium and dissolved organic carbon levels. Several lines of evidence suggested that conditions are conducive for denitrification of groundwater flowing from uplands through the waterway. Field-measured nitrate decay rates in the waterway (~0.02 day-1) were consistent with other laboratory studies and regional patterns. Results from MODFLOW and MT3DMS simulations indicated that waterway soils could process all upland groundwater nitrate flowing through them. However, model-simulated tile drainage increased both water flux and nitrate loss from the upland catchment. Study results suggest that of waterways draining uplands can provide a natural nitrate treatment system in our upland glaciated catchments, offering management opportunities to reduce nitrate delivery to streams.