Abstract

Concentrations of nutrients, major ions, organic carbon, suspended sediment, and the nitrogen isotope ratio of nitrate (δ¹⁵N) were collected at surface-water sites in rivers and drainage basins discharging to the southern part of Hood Canal, Mason and Kitsap Counties, Washington. Base-flow samples were collected from sites on the Union, Tahuya, and Skokomish Rivers from June to August 2004.

Concentrations of nutrients at all sites were low. Ammonia and orthophosphate were less than the detection limit for most samples, and nitrate plus nitrite concentrations ranged from less than the detection limit of 0.06 to 0.49 milligram per liter (mg/L). Nitrate plus nitrite concentrations were near the detection limit of 0.06 mg/L in the North Fork, South Fork, and mainstem of the Skokomish River. The concentration of nitrate plus nitrite in the Tahuya River system above Lake Tahuya was 0.17 mg/L, but decreased to 0.1 mg/L or less downstream of Lake Tahuya. Overall, the Union River contained the highest nitrate plus nitrite concentrations of the three large river systems, ranging from 0.12 to 0.28 mg/L.

δ¹⁵N generally was within the range that encompasses most sources, providing little information on nitrate sources. Most nitrogen was in the dissolved inorganic form. Dissolved inorganic nitrogen in Lake Tahuya was converted into particulate and dissolved organic nitrogen. Dissolved organic carbon concentrations generally were less than 1 mg/L in the Tahuya and Skokomish Rivers and averaged 1.3 mg/L in the Union River. Dissolved organic carbon concentrations of 2.6 to 2.7 mg/L at sites just downstream of Lake Tahuya were highest for the three large river systems, and decreased to concentrations less than 1 mg/L, which was similar to concentrations in the Skokomish River.

Total nitrogen concentrations near 0.5 mg/L were measured at two sites: Unnamed Creek at Purdy-Cutoff Road (site S2b) and downstream of Lake Devereaux (site SP5). Concentrations of nitrate plus nitrite were highest at site S2b (0.49 mg/L), and dissolved organic carbon concentrations (3.3 mg/L) were highest at the outlet of Lake Devereaux. However, the overall impact of these sites on the nutrient loading to Hood Canal probably is negligible because of the low streamflow and small loads.

Springtime samples were collected from the Union River, Tahuya River, Mission Creek, and three smaller drainage basins in March 2004. Samples were collected during spring rain events to determine if increased runoff contributes larger amounts of sediment and nutrients from the land into the surface water. There was little difference in nutrient concentrations between samples collected in the spring and base-flow samples collected in the summer. This is likely due to the fact that the springtime samples were collected during a rain event and not necessarily during a peak in the hydrograph.

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