SOURCES OF NUTRIENT AND ESCHERICHIA COLI CONTAMINATION WITHIN THE OTTER CREEK WATERSHED, MADISON COUNTY, KENTUCKY
We found highest nutrient concentrations immediately below discharge from the Otter Creek sewage treatment plant (STP), which is a point source for nitrate (3.5 – 4.4 mg/L N-NO3) and phosphate (0.8 - 1.0 mg/L P-PO4). Background levels were ~0.4 mg/L N-NO3 and ~0.09 mg/L P-PO4. Nitrate and phosphate values progressively decrease at stations downstream from the STP. Ammonium averages ~0.4 mg/L N-NH4, ranging from 0 to 1.4 mg/L in May, but measurable ammonium occurs only sporadically in June and July. The highest observed value is 1.8 mg/L N-NH4 (station CC, June) with the majority of stations having 0 mg/L. 53% of samples exceeded EPA E.coli concentration standards for human contact (>575 cfu/100 mL) and are distributed throughout the watershed, displaying classic non-point-source pollution.
Phosphate and fecal microbes are the principal contaminants. Compared to a national data set, phosphate contamination is most severe, often exceeding the 90th percentile value. Nitrate is generally below the 25th percentile level. Ammonium concentration is not related to STP discharge but exceeds the 90th percentile value in May; concentrations approach those of pristine streams in June and July. Non-point sources for nitrate, phosphate, and E. coli are likely due to leaky sewage pipes within the town of Richmond, and to pasture runoff in rural areas. Ammonium sources are enigmatic, but seem associated with pasture land and septic systems. Sampling in June and July after rain events saw higher nitrate, phosphate, and E. coli concentrations, but lower ammonium levels relative to measurements in May.