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Groundwater Assessment

Environmental Assessment > Groundwater Assessment in Washington State > Active Studies > Stillaguamish River TMDL Groundwater Support

Stillaguamish River TMDL Groundwater Support

In 2004, Ecology completed a Total Maximum Daily Load (TMDL) study for a water-cleanup plan to resolve a lack of dissolved oxygen (DO) in the Stillaguamish River. The study indicated low DO concentrations in a pool reach downstream of the City of Arlington's wastewater treatment plant. QUAL2Kw water-quality computer-model results indicated that, even with no contributions from human-made sources, DO concentrations would likely drop below the water-quality criterion of 8.0 mg/L to about 7.0 mg/L in some locations.

Ecology's 2004 study could not determine all the sources contributing to low DO conditions in the river. Scientists attributed the downstream DO depression to a combination of nonpoint source pollution inputs or polluted runoff, discharge from the Arlington treatment plant, and unknown factors. Clearly understanding the factors that affect DO levels is critical for meeting water quality standards and for treating and discharging municipal wastewater in a cost-effective manner. For that reason, in 2004 Ecology did not establish nutrient and oxygen-demand wasteload allocations for the Arlington wastewater treatment plant.

The purpose of the groundwater component of the study is to better understand the relative contribution of groundwater to the water and nutrient budgets for the five-mile stretch of the river between the City of Arlington and the I-5 Bridge during the critical low-flow period, typically September-October.

Hydrogeologists installed six piezometers and sampled three times during the August to October 2012 low-flow season. They measured water level relative to surface water, temperature, dissolved oxygen, specific conductivity, and pH. Water-quality samples were collected to be tested for nitrogen (ammonium-N, nitrate+nitrite-N, total N), total and ortho-phosphorus, iron, chloride, organic carbon, and alkalinity. Temperature data loggers measured temperature continuously at three depths in each piezometer to determine whether groundwater was entering or leaving the river.

Stillaguamish Basin Water Quality Improvement Projects is the main basin project-summary page.

Environmental Information Management database Study ID: STILTMDL


Quality Assurance Project Plan: Upper Mainstem Stillaguamish River Dissolved Oxygen Study

For more information:

Barb Carey, barb.carey@ecy.wa.gov, (360) 407-6769