The occurrence, distribution, concentrations, and pathways of agrichemicals in water were investigated in the Sugar Creek watershed, a poorly drained agricultural watershed typical of many watersheds in the midwestern USA. Water samples from Sugar Creek, two tile drains, and 11 wells along a groundwater flowpath to Sugar Creek were collected between May 1992 and August 1996 and analyzed for N and pesticide compounds. Nitrate was the principal N species and pesticides were common in alluvial water-bearing units in the Sugar Creek floodplain. In the confined stratified drift aquifers, ammonia was the principal N species and pesticides were rare. Tile drains directly affected the water quality in Sugar Creek by transporting soil pore water and shallow groundwater containing high concentrations of nitrate (NO3) and pesticides to the creek. When tile drains were flowing (typically December through July), elevated NO3 concentrations (2–10 mg/L NO3N) in the creek correlated with high NO3 concentrations (2–23 mg/L NO3N) in tile drains discharging to the creek. Likewise, with concentrations of atrazine and atrazine metabolites, seasonal trends in the tile-drain effluent were similar to seasonal trends in Sugar Creek. When tile drains went dry, NO3 concentrations in the creek were low, indicating most groundwater discharge to the creek consisted of old or denitrified water. Trace levels of pesticides in the creek at low flow probably were the result of seepage from alluvial water-bearing units.
