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Contents
Significant Findings
Introduction
Purpose and Scope
Description of Study Area
Acknowledgments
Methods and Quality Assurance
Discharge and Water Quality
Quality Assurance Results
Results
Storms Sampled
Water Quality
Discharge
Solids
Biochemical
Oxygen Demand
Bacteria
Phosphorus
Nitrogen
Summary
References Cited
Appendix A.-- Quality Assurance Program
Quality Assurance Samples
Quality Assurance Results
Table A1. Replicate sample results and
relative percent
differences during storm
samplings, 1998-99
Appendix B.-- Water quality data from Fanno Creek, Oregon
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Significant
Findings
As part of an ongoing cooperative study between Clean Water Services
of Washington County, Oregon, and the U.S. Geological Survey, water-quality
data were collected from Fanno Creek, Oregon, during three storms from
June 1998 to December 1999. Samples were collected over the discharge
hydrograph from three sites during one summer storm, one fall storm,
and one winter storm. From these data, the following conclusions were
reached for water-quality conditions and processes in Fanno Creek during
storm runoff:
- Discharge was significantly correlated with total solids (TS), total
suspended solids (TSS), total volatile suspended solids (TVSS), turbidity,
and total phosphorus (TP).
- Of the different fractions of TS measured, TS was most directly correlated
with TSS.
- Rising limbs of discharge hydrographs had higher concentrations of sediment
and TP, possibly indicating that sources were nearby (resuspension of
streambed, bank erosion, close upland sources) and that available supplies
limited downstream transport.
- Concentrations of sediment (TS, TSS), TP, and bacteria (E. coli) were
greatest and most variable at the most upstream site. Peak bacterial
loads were similar at upstream and downstream sites, so additional sources
were not evident, or downstream sources were offset by settling or losses
of bacteria from upstream.
- Biochemical oxygen demand during storms was primarily associated with
decomposable materials on particulate matter.
- E. coli concentrations exceeded the State of Oregon single-sample
water-quality standard of 406 colonies/100 mL in almost all samples.
E. coli concentrations measured during the summer storm were
an order of magnitude greater than those measured during the fall
or winter storms, primarily due to warmer water and less dilution
during the summer storm.
- E. coli were correlated with suspended sediment (TSS and
turbidity), indicating that they were either transported to streams
attached to particles bound to resuspended streambed particles, or
they had an affinity for particulate material in water.
- TP concentrations exceeded both the 1998 and 2001 Total Maximum Daily
Load (TMDL) criterion concentrations in almost all samples.
- Soluble Reactive Phosphorus (SRP) in the stream may have originated
primarily from ground-water discharge, whereas TP was mostly associated
with particulates.
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