Runoff, solids, and contaminant losses into surface tile inlets draining lacustrine depressions

Surface tile inlets connected to subsurface tile lines provide a direct pathway for pollutants to enter water bodies. This study was conducted to evaluate the quantity and quality of surface runoff entering surface file inlets draining natural depressions of lacustrine landscapes. Surface runoff, solids, and nutrient losses via surface tile-inlets in two lacustrine watersheds in the southern Minnesota River Basin were investigated from 1995 to 1998. Tillage and crop rotations were: (i) fall NH₃-N injection after soybean [Glycine max (L.) Merr.] or navy bean (Phaseolus vulgaris var. vulgaris) harvest and (ii) fall chisel plowing after corn (Zea mays subsp. mays) harvest, followed by spring field cultivation, planting, harrowing, and row cultivation. Maximum annual runoff loss via surface inlets was less than 5% of annual precipitation. Flow-weighted total P (TP) concentration ranged from 0.2 to 2.9 mg L^-1 in snowmelt and 0.7 to 6.5 mg L^-1 in rainfall runoff. Nitrate N concentrations were less than 10 mg L^-1. Three-year maximum cumulative pollutant loads were: 138 kg ha^-1 total solids (TS), 20 kg ha^-1 chemical oxygen demand (COD), 363 g ha^-1 TP, 205 g ha^-1 dissolved molybdate reactive P (DMRP), 1342 g ha^-1 total dissolved inorganic N (TDIN), and 1126 g ha^-1 nitrate N. Most of the dissolved pollutants were associated with snowmelt runoff and most of the particulate pollutants were associated with major storms. Losses of particulate pollutants via surface tile inlets are smaller than expected due to the dynamics of ponding in natural landscape depressions.