Phosphorus and E.coli in the Fanno and Bronson Creek Subbasins of the Tualatin River Basin, Oregon, During Summer Low-Flow Conditions, 1996.

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.

Data collected during the September 1996 sampling of the Fanno and Bronson
Creek subbasins are present ed. Summer base-flow phosphorus and E. Coli
concentrations in the two subbasins and their potential impact on water-quality
conditions inthe Tualatin River main stem are discussed.

The area of study was the Tualatin River Basin which drains a 712-square mile
basin in northwestern Oregon, just west of the Portland metropolitan area.
Tualatin River Basin includes the Fanno Creek and Bronson Creek subbasins

Water-quality and stream-discharge data representing base-flow conditions were
collected at 19 main-stem and 22 tributary sites in the Fanno Creek subbasin on
September 10 and 11, 1996, in the Bronson Creek subbasin, 14 main-stem and 4
tributary sites were sampled on September 24, 1996.

Collection of water-quality samples was a joint effort by USA and USGS
personnel. Water temperature, specific conductance, pH, and dissolved oxygen
were measured in the field using Hydrolab multiparameter water-quality sensors.
Grab samples were collected and analyzed for major ions, trace elements,
nutrients, and bacteria at the USA water-quality laboratory. Methods used for
sample collection, processing, and analyses of chemical constituents: and for
field and laboratory quality control are described by Doyle and Caldwell
(1996). Bacterial samples were processed and analyzed using U.S. Environmental
Protection Agency method 600/4-85-076.

Duplicate samples were collected at two sites in the Fanno Creek subbasin and
at one site in the Bronson Creek subbasin to help quantify the overall
precision of the sampling and analytical methods. Results of duplicate
analyses (presented with the regular data in the appendix) indicate acceptable
precision. Laboratory quality-control procedures included method blanks,
travel blanks, spikes, and duplicate samples. The USA water-quality laboratory
also participates in several performance-evaluation programs (e.g., the USGS
Standard Reference Sample project, a monthly quality-assurance program run by
the USGS Oregon District, an annual regional laboratory intercomparison study,
and periodic reviews by the USGS Branch of Quality Systems) and has a record of
high-quality performance (Kelly et al., 1999; Rounds et al., 1999).

USGS personnel conducted instantaneous flow measurements at the time of
water-quality sample collection. Flow in small streams is inherently difficult
to measure, and the accuracy of the data range from excellent to poor,
depending on site-specific conditions. The best discharge measurements
typically were made by collecting the entire flow into a container and
measuring the volume collected over a specific period of time. Other
streamflow measurements were performed using standard USGS techniques and
calibrated type AA or pygmy flow meters (Buchanan and Somers, 1969). The
largest errors were associated with very shallow stream reaches and rock
streambeds. The accuracy of each streamflow measurement is indicated in talbes
A1 and A4 (appendix) of McCarthy, K. , 2000.

The information for this metadata was taken from the Online Publications of the
Oregon District at http://oregon.usgs.gov/pubs_dir/online_list.html .