Phosphorus is naturally occurring and present in many clay soils in the study area. Phosphorus concentrations at the 11 Fixed Sites were primarily related to land use and secondarily related to clayey surficial deposits in the basin. Phosphorus concentrations in stream water at agricultural sites were considerably higher than at urban and forested sites during base flow conditions. Nitrate was detected in all 412 stream-water samples collected during the study. Median nitrate concentrations at the four agricultural sites and the mixed agriculture and forested sites were slightly higher than at the urban site and considerably higher than at the forested sites.
Nutrient concentrations vary seasonally in streams throughout the study area.
The highest concentrations occur during stormwater runoff. At two agricultural streams in the study area, more phosphorus was carried during storms in June 1993 than during the next 24 months combined. Highest nitrate concentrations during base-flow periods generally occur during the winter months. This may be due to the seasonal change in contributions from ground-water discharge to streams or to the seasonal cycle of algae growth in streams. The highest nutrient concentrations occurred during periods of precipitation runoff.