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 investigation of water quality conditions in the Tualatin River Basin, the Uni-fied Sewerage Agency and the U.S. Geological Survey measured phosphorus and E. coli concentrations in the Fanno Creek and Bronson Creek subbasins. These measurements were made during September 1996 to characterize summer base-flow conditions in the two subbasins. Documenting phosphorus and E. coli concentrations and understanding their distribution during low-flow periods is important for developing effective subbasin water-management plans as well as understanding sources that contribute to loads in the Tualatin River main stem. Phosphorus.-Studies of water quality in the Tualatin River Basin have shown that ground water in this region is rich in phosphorus. The Tualatin Basin Technical Advisory Committee and the Oregon Department of Environmental Quality have estimated background phosphorus concentrations for a number of subbasins based on ground-water and surface-water data from these studies (http://waterquality.deq.state. or.us/wq/TMDLs/TMDLs.htm, accessed October 22, 1999). Data from most of the sites sampled during the September 1996 study indicate that during summer low-flow conditions in the Fanno and Bronson Creek subbasins, total phosphorus concentrations were near estimated background values associated with groundwater base flow. Some upstream sites in each subbasin had phosphorus concentrations somewhat higher than could be attributed to a ground-water source, suggesting additional sources were contributing to instream phosphorus loads. However, the lower several miles of both Fanno and Bronson Creeks were near background, indicating that anthropogenic or other sources of phosphorus in addition to ground water were not contributing substantially to phosphorus loads to the Tualatin River main stem during this low-flow period. Data collected during this study suggest that ponds associated with some of the tributaries in the Fanno Creek subbasin (e.g., Pendleton Creek and the tributary near Gemini) may be contributing to increased phosphorus levels, but in contrast to earlier studies (Tualatin Basin Technical Advisory Committee, 1997), phosphorus concentrations did not diminish as water passed through large instream ponds located along Bronson Creek between river miles 1.5 and 2.0. This illustrates that the effects of ponds on water quality can be transient. E. coli.-Several sites in the Bronson Creek subbasin and most sites in the Fanno Creek subbasin exceeded the single-sample bacteria criterion for recreational contact in freshwater (406 organisms/100 mL). Most of the elevated levels measured are typical of urbanized areas, and generally higher levels in the Fanno Creek subbasin can probably be attributed to greater urbanization there than in the Bronson Creek subbasin. For comparison, E. coli levels measured during this same low-flow period in two nonurbanized subbasins located just west of the study area (4 samples from Gales Creek and 5 samples from Dairy Creek; fig. 1) were less than the single-sample criterion (USA, unpub. data, 1996). Within the Fanno Creek subbasin, particularly high E. coli levels in the vicinity of Shattuck Road and in Pendleton and Summer Creeks may indicate additional inputs from sources such as domestic pet and wildlife waste, failing septic systems, or improperly managed hobby farms. The tributary near Shattuck Road also had elevated concentrations of chloride and total Kjeldahl nitrogen, which is consistent with input from a sewage source such as a failing septic system. Avian wastes deposited in ponds along Pendleton Creek may also contribute E. coli to the stream. E. coli data collected during this study show that at a few sites-the tributary near Shattuck Road and Pendleton Creek in the Fanno Creek subbasin and the area near Kaiser Road on Bronson Creek-E. coli levels were relatively high. However, there was no evidence in either subbasin of large-scale sources of bacterial contamination to surface water during the summer low-flow period investigated.