NITRATE RECYCLING IN A SMALL SUBURBAN WATERSHED

The study was done in Dry Run Creek watershed, a small sub-urban watershed in the northwestern part of Black Hawk County, Iowa. Different hydrologic units like prairie, wetland, agricultural area, urban area, and stream channels were compared in terms of nitrate transport. Water samples and soil samples from different depths were collected during the study period (May to October). Agricultural soil had the highest nitrate concentration (average 44 mg/kg) and wetland soil had nitrate below detection level. Urban soils had higher nitrate (average 14 mg/kg) compared to stream bank soils (average 8 mg/kg). Urban sites along the edge of the residential areas and close to the stream channels had low soil nitrate compared to other urban sites. On the other hand, stream bank soils in agricultural land had the highest nitrate and sites along the edge of the urban areas had nitrate below detection level. Stream water from agricultural lands showed higher nitrate values (average 22 mg/l) compared to the stream water from urban areas (average 8 mg/l). Wetlands had very low nitrate (average 2.3 mg/l) and prairie water had nitrate below detection. In the study area, the major flux of nitrate is from the agricultural lands. Urban areas are contributing a portion to that flow of nitrate. Nitrate coming from the agricultural and the urban areas travels to the stream channels through surface runoff and base flow. Wetlands considerably reduce nitrate from the flow system because topographically wetlands are between the urban areas and the drainage ways. Nitrate comes to the stream channels mainly in the months of May (average 39 mg/l) and June (average 29 mg/l) when there are high agricultural activities. Vertical distribution of soil nitrate in all hydrologic unites indicates effective downward movement of nitrate with water. In groundwater, nitrate concentration was high in urban areas (average 14 mg/l) compared to agricultural areas (average 4 mg/l). Temporal variations in groundwater nitrate within each site were unpredictable as the study period was short. In general, spatial and temporal variations of nitrate concentration in different hydrologic units indicate that nitrate is actively moving through the watershed. Also, different hydrologic units are actively interacting with each other to facilitate this movement.