U.S. Geological Survey

Phosphorus and E. coli in the Fanno and Bronson Creek subbasins of the Tualatin River Basin, Oregon, during summer low-flow conditions, 1996

By Kathleen A. McCarthy

USGS Water-Resources Investigations Report 00-4062, 31 pages, 12 figures, 6 tables

Report availability

Significant findings

As part of an ongoing cooperative study between the Unified Sewerage Agency of Washington County, Oregon, and the U.S. Geological Survey, phosphorus and Escherichia coli (E. coli) concentrations were measured in the Fanno and Bronson Creek subbasins of the Tualatin River Basin during September 1996. Data were collected at 19 main-stem and 22 tributary sites in the Fanno Creek subbasin, and at 14 main-stem and 4 tributary sites in the Bronson Creek subbasin. These data provided the following information on summer base-flow conditions in the subbasins:

• Concentrations of total phosphorus at 70% of the sites sampled in the Fanno Creek subbasin were between 0.1 and 0.2 mg/L (milligrams per liter), very near the estimated background level of 0.14 mg/L attributed to ground-water base flow. These data indicate that ground-water discharge could account for the phosphorus measured at most sites in this subbasin.
• Concentrations of phosphorus at all but one of the sites sampled in the Bronson Creek subbasin were also between 0.1 and 0.2 mg/L, indicating that ground-water discharge could account for the phosphorus measured at most sites in this subbasin.
• A few sites in the Fanno Creek subbasin had phosphorus concentrations above background levels, indicating a source other than ground water. Some of these sites- Pendleton Creek and the tributary near Gemini, for example-were probably affected by the decomposition of avian waste materials and the release of phosphorus from bottom sediments in nearby ponds.
• Concentrations of E. coli--an indicator of fecal contamination and the potential presence of bacterial pathogens-exceeded the current single-sample criterion for recreational contact in freshwater (406 organisms/100 mL [organisms per 100 milliliters]) at 70% of the sites sampled in the Fanno Creek subbasin.
• Concentrations of E. coli in the Bronson Creek subbasin exceeded the single-sample criterion at one-third of the sites sampled.
• Most occurrences of elevated E. coli levels were probably due to sources such as domestic pet and wildlife waste, failing septic systems, or improperly managed hobby farms. The data did not indicate any large breaks in sewer lines or other large-scale sources of bacterial contamination to surface water in either subbasin during this low-flow period.

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