Sediment and Nutrient Contributions from Subsurface Drains and Point Sources to an Agricultural Watershed.

Excess sediment and nutrients in surface waters can threaten aquatic life. To determine the relative importance of subsurface drainage as a pathway for movement of sediment and nutrients to surface waters, loading from various tile systems was compared to that from sewage treatment plants (STP) within the same watershed. Movement through tiles comprised 1 to 8% of estimated total (overland plus tile) annual sediment loading from the respective areas drained by the tile. Load during the growing season from five closed drain- age systems without surface inlets averaged 5 kg sediment/ha, 0.005 kg dissolved reactive P (DRP)/ha, 0.003 kg NH₄₋N/ha, and 3.8 kg NO₃₋N/ha; and from two open drainage systems with surface inlets averaged 14 kg sediment/ha, 0.03 kg DRP/ha, 0.04 kg NH₄₋N/ha, and 3.1 kg NO₃₋N/ha. The eight STP contributed about 44 530 kg suspended sediments, 3380 kg total P, 1340 kg NH₄₋N, and 116 900 kg NO₃₋N to the watershed annually. Drainage systems added less NH₄₋N and P, but more NO₃₋N and suspended solids to surface waters than STP. Tile drainage pathways for NO₃₋N, STP in the case of P, and overland pathways for sediment are indicated as targets to control loading in artificially drained agricultural watersheds.