Publication Summary

The 303(d) fecal coliform bacteria, dissolved oxygen, pH, and arsenic listings in the Stillaguamish River watershed and Port Susan were assessed. Fecal coliform load reductions are recommended for freshwater reaches and Port Susan. Stormwater wasteload allocations for fecal coliform and BOD were developed for three jurisdictions, and wasteload allocations for fecal coliform were developed for three municipal wastewater treatment plants. Seasonal dissolved oxygen targets, some below the Class A criterion of 8 mg/L, are proposed as minimum potential concentrations after pollutants sources are removed. Data suggest that both arsenic and mercury are from natural sources in the basin.

The Stillaguamish River basin has significant salmon habitat and recreational uses. The basin empties into Port Susan, a potential commercial shellfish harvest area.

The federal Clean Water Act 1998 303(d) listings for fecal coliform bacteria included the mainstem Stillaguamish River and its two major forks, several tributaries, and Port Susan. The listings for dissolved oxygen, pH, and arsenic were distributed in fewer tributary and mainstem reaches.

This technical assessment used water quality data collected during 2000 to 2002 by the Washington State Department of Ecology (Ecology), combined with data collected by local tribes, Ecology, and other agencies since the late 1980s. All of the 303(d) parameters mentioned above were addressed in the total maximum daily load (TMDL) assessment.

Many reaches of the Stillaguamish River, most tributaries, and Port Susan had seasonal fecal coliform problems. Freshwater fecal coliform load reductions recommended in the TMDL assessment should bring both freshwater reaches and Port Susan back into compliance with criteria.

Stormwater wasteload allocations were necessary for three NPDES stormwater permit jurisdictions. Load and wasteload allocations were developed for fecal coliform and biochemical oxygen demand.

Many tributaries experience seasonally low dissolved oxygen concentrations and low pH values. Groundwater and wetland inputs are partially responsible. Minimum dissolved oxygen targets were proposed after pollutants sources are removed. Pollutant load reductions should allow pH values to return to natural levels. A minimum dissolved oxygen target also was recommended for a reach of the Stillaguamish River where complex hyporheic exchange processes are thought to be occurring during extended low-flow conditions.

Arsenic concentrations are consistently elevated compared to EPA human-health criteria, but far below aquatic toxicity and drinking water standards. Mercury concentrations exceeded chronic toxicity criteria to protect aquatic life during events with elevated total suspended solids concentrations. Data suggest that both arsenic and mercury are from natural sources in the basin.

Link to EIM data for User Study ID STILTMDL