Scientific Investigations Report 2006-5073
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5073
A total of 23 base-flow samples were collected during summer of 2004 from sites along the Union, Tahuya, and Skokomish Rivers. Samples were analyzed for major ions, nutrients, organic carbon, suspended sediment, and δ15N of nitrate to obtain a better understanding of the nutrient concentrations in the rivers that are discharging into the southern part of Hood Canal, as well as possible sources of those nutrients. Six samples also were collected in the spring during rain events, along the Union River, Tahuya River, Mission Creek, Stimpson Creek, Twanoh Falls Creek, and an unnamed creek from Lake Devereaux, to assess whether nutrient concentrations are elevated in surface water due to increased runoff. Water sampled at all sites was well oxygenated.
Seven sampling sites were selected along the length of the Union River at points where the predominant land use type near the river changed in an attempt to assess the effects of land use on nutrient concentrations (fig. 1). The farthest upstream site (U7) was located in an area dominated by forest with each site farther downstream having additional urban and agricultural influences. Samples were collected on three consecutive days (June 28 to 30, 2004). Discharge generally increased downstream of site U6 (fig. 2A), after the large decrease between sites U6 and U7 due to withdrawals for drinking-water purposes.
Concentrations of the major ions and bicarbonate generally increased downstream, likely due to increased weathering of surrounding substrate (figs. 2B and 2C). Manganese and iron showed a sharp increase in concentration between sites U5 and U4 (fig. 2D), which was likely caused by increased ground-water discharge to the river in that region. Concentrations of particulate organic nitrogen and carbon (fig. 2F), and suspended sediment also showed slight increases towards the mouth of the river (table 1).
Nutrient concentrations were low throughout the river (fig. 2E). Total nitrogen concentrations were fairly constant with concentrations ranging from 0.26 to 0.29 mg/L from sites U5 to U1. Most of the total nitrogen in the river consists of nitrate plus nitrite, with little being in organic form.
The δ15N increases slightly from 1.32 per mil at site U6 to 4.20 per mil at site U2 (fig. 2G), but all values fall within the range covered by most sources of nitrate (Kendall, 1998), thus precluding the identification of nitrogen sources.
The eight sites sampled on the Tahuya River also were selected based on changes in predominant land use. Although most of the Tahuya River drainage basin is forested, a few sites were selected based on increased urbanization (fig. 1, sites T7, T4, and T3). The farthest upstream site (T8) was near the headwaters of the Tahuya River in a predominately forested area, with subsequent sites downstream having additional urban areas. Site T7 is at the outlet of Lake Tahuya, a fairly populated area. Samples were collected on 3 days between July 1 and July 6, 2004.
Discharge generally increased downstream (fig. 3A). On July 2, 2004, discharge increased from 0.11 ft3/s above the Lake Tahuya (site T8) to 1.6 ft3/s at the outlet of Lake Tahuya (site T7). When sampling began again on July 6, 2004, discharge 4.5 mi downstream of Lake Tahuya (site T6) was 0.27 ft3/s, but increased 4-fold to 1.1 ft3/s at site T5. Causes of the decrease between T7 and T6 may include sampling on different days (for example, proximity of rain event), diversion for irrigation and/or agriculture, or a losing reach of the stream may have occurred. There also was little increase in flow between sites T3 and T2.
Similar to samples from the Union River, major ion concentrations in the Tahuya River tended to increase downstream (fig. 3B and table 2). However, there was a significant decrease in calcium, magnesium, and bicarbonate between sites T6 and T5, both sampled on July 6, 2004, that was concurrent with the 4-fold increase in discharge. The significant decrease in specific conductance (from 57 to 46 µS/cm) and pH (from 7.6 to 7.2) suggests that the 0.83 ft3/s of water that was added between sites T6 and T5 was less weathered and more acidic (more like rain water) than the 0.27 ft3/s flowing past site T6.
Biogeochemical processes in Lake Tahuya (between T8 and T7) significantly modified the chemistry of the surface water (figs. 3B-3F). The concentration of nitrate plus nitrite was highest at site T8 upstream of Lake Tahuya (0.17 mg/L), but then decreased to non-detectable values at site T7 downstream of the lake, probably as a result of biological uptake by phytoplankton. Increased growth of phytoplankton also could explain the observed response in DOC concentrations. Biogeochemical coupling with the organic matter produced in Lake Tahuya, also could explain the increased concentration of the redox-sensitive ions manganese and iron at sites T7 and T6, just downstream of Lake Tahuya. Concentrations of DOC and iron decreased to significantly lower concentrations downstream of site T6 although manganese concentrations increased slightly downstream of T5. Total nitrogen values only decreased slightly from upstream to downstream of Lake Tahuya suggesting that most of the nitrogen flowing from Lake Tahuya was dissolved organic nitrogen. The concentrations of particulate organic carbon remained relatively unchanged downstream of site T5.
The δ15N fell within the range covered by most sources of nitrate, thus precluding identification of the nitrogen source (fig. 3G). The slight increase in the δ15N from upstream of Lake Tahuya (1.96 per mil at site T8 and 3.72 per mil for site T6) probably was due to photosynthetic uptake of low δ15N within Lake Tahuya, leaving the residual nitrate with a higher δ15N. Downstream of site T6, δ15N decreased to about 2.6 per mil.
The eight sites sampled on the Skokomish River also were selected based on changes in predominant land use. Six of the eight sites are located on a transect that included the North Fork and mainstem of Skokomish River. The farthest upstream site is located above Lake Cushman (site S6), which is a reservoir that regulates flow to the river within the pristine Olympic National Park, with subsequent sites being affected by agricultural practices. The two additional sites are located on the South Fork Skokomish (S4b) and Unnamed Creek at Purdy-Cutoff Road (S2b), a small tributary just upstream of site S2. Samples were collected on 4 days between July 9 and August 4, 2004.
Streamflow generally tends to increase downstream of Lake Cushman. Discharge increased significantly in the mainstem downstream of the confluence of the North and South Forks.
Several trends in the concentrations of major cations are apparent. Concentrations of ions that increased in the downstream direction include sodium, magnesium, chloride, and silica, while calcium and sulfate decreased (figs. 4B and 4C and table 3). Dissolved manganese and iron concentrations increased significantly downstream of the confluence of the North and South Forks (fig. 4D).
The concentrations of nitrate plus nitrite were less than the detection limit at all sites in the North Fork and mainstem of the Skokomish River. Concentrations of particulate nitrogen and suspended sediment peaked at site S5, just downstream of Lake Cushman diversion, and decreased downstream (table 3). Concentrations of total nitrogen ranged from the detection limit of 0.03 to 0.06 mg/L (fig. 4E). Concentrations of DOC ranged from 0.4 to 0.7 mg/L (fig. 4F).
Concentrations of silica, calcium, magnesium, sodium, bicarbonate, and chloride were higher in samples from the South Fork Skokomish River (site S4b) relative to the North Fork (site S4a). The only ionic species with higher concentrations in the North Fork were sulfate and manganese. Inorganic nutrients were not detected in the South Fork, however, the concentration of total nitrogen was higher in the South Fork (0.09 mg/L) relative to the North Fork (0.06 mg/L). The concentration of dissolved organic carbon was 0.7 mg/L, which was similar to the North Fork.
Concentrations of most constituents at the Unnamed Creek at Purdy-Cutoff Road (S2b) were higher than the Skokomish River, except for sulfate and dissolved organic carbon. However, the overall contribution by this tributary on Skokomish River loads was negligible because of its low discharge (0.84 ft3/s).
During periods of rainfall and increased runoff, larger amounts of sediment and nutrients may be transported from the land surface into the surface water. Therefore, samples were collected in the spring (March) 2004 at six sites, the Tahuya River, Union River, Mission Creek, Stimpson Creek, Twanoh Falls Creek, and an unnamed creek below Lake Devereaux (fig. 1). Sites were selected because of their proximity and contribution to Lynch Cove. Ideally samples should be collected during periods of heavy rainfall and increased streamflow, however, due to time constraints, springtime samples were collected during moderate rain events and not necessarily during peaks in the hydrograph.
Concentrations of nutrients and suspended sediment collected in the spring at all six sites were low. The nitrate plus nitrite concentrations ranged from 0.11 to 0.4 mg/L. Dissolved organic carbon concentrations ranged from 1.0 to 3.3 mg/L (table 4). The site downstream of Lake Devereaux (SP5) contained the highest nutrient concentrations of all sites sampled in March, with nitrate plus nitrite and DOC concentrations of 0.40 and 3.3 mg/L, respectively.
Because of the timing of sampling sites U1 and T1, that are at the mouths of relatively large rivers (Tahuya and Union), little to no difference was observed in the nutrient concentrations between samples collected in the spring and in the summer.
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