Hydrograph Separations Can Identify Contaminant-Specific Pathways for Conservation Targeting in a Tile-Drained Watershed.

Water quality issues continue to vex agriculture. Understanding contaminant-specific pathways could help clarify effective water quality management strategies in watersheds. Hypothesis: If conducted at nested scales, hydrograph separation techniques can identify contaminant-specific pathways that conservation should target. This study’s setting was Tipton Creek, a tile-drained watershed (20,000 ha) in Iowa under intensive agricultural production. A September 2006 runoff event was monitored for nitrate-N, total P, and E. coli at the watershed outlet, a field flume in the lower watershed (10 ha), and an upstream tile outfall draining 1700 ha . At the outlet, sediment loads and Be/Pb nuclides were measured to estimate bed and bank sources. On a unit-area basis, nitrate load from the tile and outlet was similar, tile P load was about half of the outlet’s, and tile E. coli load was about one third that at the outlet. Early timing of P and E. coli from the tile indicates tile inlets draining surface water from potholes are an important pathway not targeted by current conservation practices. Runoff from steeper fields near the stream also contributes these two contaminants. Sediment loads were dominated (~80%) by bed and bank sources. Results indicate a comprehensive approach addressing these contaminants would include: a) buffers for tile inlets (to filter E. coli and P), b) riparian practices (e.g., buffers) that stabilize streambanks by fields and pastures (reducing P, E.coli, and sediment), c) practices to reduce and intercept tile nitrate losses (e.g., wetlands), and d) optimized erosion control on fields near the stream.