Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: September 28, 2009
Publication Date: December 11, 2009
Citation: Tomer, M.D., Moorman, T.B., Wilson, C., Cole, K.J., Isenhart, T. 2009. Hydrograph Separations can Identify Contaminant-Specific Pathways for Conservation Targeting in a Tile-Drained Watershed [abstract]. Soil & Water Conservation Society (SWCS) Science 2 Solutions Conference. p. 58. Technical Abstract: Water quality issues continue to vex agriculture, partly due to perceived tradeoffs among different contaminants. Yet, agricultural water quality is influenced by contaminant-transport pathways unique to each watershed that are not well defined. More accurate information on contaminant pathways could provide a basis for mitigation through well-targeted approaches. This study determined dynamics of nitrate, total phosphorus (TP), E. coli, and sediment during a runoff event in Tipton Creek, Iowa, in an effort to distinguish the role of key sources and pathways in delivering each contaminant. The watershed, under crop and livestock production, has extensive tile drainage in the upper part, and discharges through a well-defined alluvial valley. A September 2006 storm yielded 5.8 mm of discharge during the ensuing seven days, which was monitored at the outlet (19,850 ha), two upstream tile-drainage outfalls (1856 ha), and a runoff flume draining 11 ha in the lower valley. Hydrograph separations indicated 13% of tile discharge was from surface intakes. Tile and outlet nitrate-N loads were similar, verifying sub-surface tiles dominate nitrate delivery. On a unit-area basis, tile discharge delivered TP and E. coli loads that were about half and 30% of the outlet’s, respectively: rapid, synchronous timing of showed surface depressions, most drained by intakes, are an important pathway for both contaminants. Flume results indicated surface runoff was a significant source of TP and E. coli loads, but not the dominant source. At the outlet, sediment, P, and E. coli were reasonably synchronous. Radionuclide activities of 7Be and 210Pb in suspended sediments showed sheet-and-rill erosion sourced only 22% of sediment contributions; channel sediments dominated and were an important source of P and E. coli. The contaminants followed unique pathways, necessitating separate mitigation strategies. Results, while viewed in context of a late summer storm and mature crop cover, nonetheless inform on conservation efficacy in the watershed. To comprehensively address water quality, erosion-control and nitrogen-management practices currently encouraged could be complemented by buffering tile intakes and stabilizing streambanks. The study also demonstrates how detailed monitoring of rainfall-runoff events can provide contaminant-pathway information that is indiscernible from monitoring even at weekly intervals.