Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2010
Publication Date: May 1, 2010
Citation: Tomer, M.D., Wilson, C.G., Moorman, T.B., Cole, K.J., Herr, D., Isenhart, T.M. 2010. Source-pathway separation of multiple contaminants during a rainfall-runoff event in an artificially drained agricultural watershed. Journal of Environmental Quality. 39:882-895. Interpretive Summary: Water quality studies are typically conducted on single contaminants, even though several different types of contaminants are important to consider in most watersheds. Evaluating the sources of different contaminants in watersheds simultaneously could better define comprehensive mitigation strategies and provide accurate information about any trade-offs among individual pollutants. This study was based on detailed comparisons of the quantities and timings of four contaminants during a single rainfall-runoff event, conducted simultaneously at three different spatial scales within a tile-drained watershed. The four contaminants were nitrate, phosphorus, E. coli, and sediment. As expected, nitrate loads were dominantly carried via subsurface (tile) drainage. But sediment was dominated (78%) by channel sources, and sheet-and-rill erosion only contributed 22%. Phosphorus and E. coli are usually believed to originate from near-channel sources and surface runoff, but these results highlighted the critical role that surface intakes play as they drain surface water from glacial depressions and road ditches. About half the phosphorus and a third of the E. coli loads delivered during this event could be attributed to surface intakes. Currently, conservation strategies in this landscape are focused on erosion control and nutrient management. These results show that buffering of tile intakes and streambank stabilization are also important. These additional practices could be implemented with appropriate conservation cover without removing large areas from crop or livestock production. Water quality assessments can be significantly aided through short-term but detailed evaluations of quantities of water and contaminants discharged at nested spatial scales. This study is of interest to conservation and environmental professionals and policy makers, as well as agricultural stakeholders, who seek to develop new strategies and approaches to comprehensively address agricultural water quality issues.
Technical Abstract: A watershed’s water quality is influenced by contaminant-transport pathways unique to each landscape. Accurate information on contaminant-pathways could provide a basis for mitigation through well-targeted approaches. This study determined dynamics of nitrate, total P, Escherichia (E.) coli, and sediment during a runoff event in Tipton Creek, Iowa. The watershed, under crop and livestock production, has extensive artificial (tile) drainage discharging through an alluvial valley. Monitoring was conducted at the watershed outlet (19,850 ha), two tile-drainage outfalls (total 1856 ha), and a runoff flume (11 ha) within the sloped valley. A September 2006 storm resulted in 5 mm of discharge during the ensuing seven days. Hydrograph separations provided reasonable water balance among the three scales, and indicated 12% of tile discharge was from surface intakes. Tile and outlet nitrate-N loads were similar, verifying tiles dominate nitrate delivery. On a unit-area basis, tile total P loads were half the outlet’s, and tile E. coli loads were about 30% of the outlet’s; their rapid, synchronous timing showed surface intakes are an important pathway for both contaminants. Flume results indicated surface runoff in the lower valley delivered about 25% of total P, but probably <10% of E. coli loads. At the outlet, sediment Be:Pb nuclide ratios showed sheet-and-rill erosion sourced only 22% of sediment contributions, and that channel sources dominated. The contaminants followed unique pathways and therefore require separate mitigation strategies. To comprehensively address water quality, erosion-control and nitrogen-management practices now being encouraged could be complemented with buffered tile intakes and streambank stabilization.