Water Quality Improvement Project
© Joe Mabel, Google Images
In 1981, studies determined that effluent from the Metro-Renton wastewater treatment plant impacted the lower Green-Duwamish River area. The treatment plant outfall was located in the Green River upstream of the confluence with Black River. The plant effluent particularly affected the Green River, upriver of the Duwamish River, during the critical late summer and fall periods, when the effluent made up a substantial portion of the flow in the river. The effluent led to increased water temperature (though not enough to violate water quality standards), low dissolved oxygen, toxic levels of residual chlorine, and increased nutrients.
Ammonia (NH4+) is one measure of nitrogen, a nutrient that can increase the growth of plants and algae in water. The presence of large concentrations of ammonia in a stream or lake can create a large oxygen demand. This demand is caused by the conversion of ammonia to nitrate, called "nitrification". High concentrations of nitrate in wastewater treatment plant effluent can cause algae to grow in large quantities. Dead and decaying algae can cause oxygen depletion problems, which in turn can kill fish and other aquatic organisms in streams. Higher-than-normal levels of nutrients in the water can also lead to changes in the water’s pH and clarity. In addition, increased algae and plants can be ugly, create odor problems when they die, decompose and interfere with recreational activities like boating and swimming.
Biological Oxygen Demand (BOD) is the amount of oxygen required by aerobic microorganisms (organisms that need oxygen to survive) to break down organic matter in water. It can be used to measure the amount of water pollution in a water body.
© EPA, Google Images
To protect water quality, the decision was made to remove the point source (Metro-Renton wastewater treatment plant) from the Green River, and instead discharge the effluent to Puget Sound at Alki Point. Only emergency discharges to the Green River are allowed from the wastewater treatment plant. Discharges for maintenance were authorized only during high flow periods, and only for a few hours. Subsequently, the wasteload allocation (WLA) for the wastewater plant was set to zero for routine discharges. Ecology submitted a TMDL to EPA with an established WLA for the wastewater plant. EPA approved the TMDL on December 29, 1992.
The city of Seattle conducted bi-weekly monitoring at the three Duwamish River and Waterway stations for dissolved oxygen and biological oxygen demand (BOD 5-day). They also monitored water quality in the Green River for ammonia, total suspended solids, and heavy metals during the infrequent plant discharges.
Removal of the wastewater treatment plant effluent resulted in some improvement in Green River water quality below the plant outfall. The Green River showed some improvement for ammonia-nitrogen and heavy metals. The Duwamish portion met water quality standards for ammonia. However, both Green River segments continue to show some impairment for fecal coliform, dissolved oxygen, mercury, lead, and temperature.
Duwamish Waterway Ammonia-Nitrogen TMDL (Ecology publication)
Green-Duwamish River Basin
WRIA 9: Duwamish-Green Watershed Information (Water website)
# of TMDLs - 2
Approved by EPA
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