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An analysis of historical water-quality data for surface and ground water collected in the Willamette and Sandy River Basins during the 1980-90 water years was performed. For surface water, most data were concentrated at sites on the main stem Willamette River or near the mouths of major tributaries. All seasons were represented. Data for nitrogen and phosphorus species were readily available, but simultaneously collected discharge measurements frequently were not. Seven primary sites were used for a quantitative analysis of nutrient data in surface water. At six of the seven primary sites, median concentrations of nitrite plus nitrate and total phosphorus were less than 0.5 milligrams per liter (mg/L) as nitrogen (N) and 0.1 mg/L as phosphorus (P), respectively. These concentrations were lower than national median concentrations for basins with similar land uses. At the Pudding River site, which received significant point-source and agricultural nonpoint-source inputs, median values for nitrite plus nitrate and total phosphorus were 1.4 mg/L as N and 0.2 mg/L as P, respectively. Nitrite-plus-nitrate concentrations at this site were seasonally dependent, with the highest concentrations occurring during winter. Forested sites had significantly lower water temperatures and nutrient concentrations than urban or agricultural sites. Evidence of diel variations in dissolved oxygen concentrations and pH values suggest that, at some sites, low dissolved oxygen concentrations may be a problem during early morning hours in the summer. Historical nutrient data for ground water were limited primarily to nitrate and nitrite-plus-nitrate determinations for wells completed in the basin-fill and alluvial aquifer. Nitrate and nitrite-plus-nitrate concentrations in this aquifer showed a weak inverse relationship with depth.

Analysis of nutrient and ancillary water-quality data for surface and ground water of the Willamette Basin, Oregon, 1980-90