  |
|
|
The ProblemThe Spokane River and Lake Spokane have low dissolved oxygen (DO) levels during low flow in the summer months. Portions of the river and Lake Spokane violate the Washington State water quality standards for DO, and are listed on the 2004 list of impaired water bodies.
Maintaining good water quality in the river, especially adequate concentrations of DO, is vitally important for supporting fish, invertebrates and other aquatic life.
Like you and me, aquatic animals need oxygen to live. As air or water moves past an animal’s breathing apparatus (gills or lungs) oxygen is transferred to its blood. It is much more difficult to transfer oxygen from water to blood than it is to transfer oxygen from air to blood. Therefore, it is critical that an adequate amount of oxygen is maintained in the water for this transfer to take place efficiently and sustain aquatic life.
In addition to being required by aquatic organisms for respiration, oxygen is necessary to help decompose organic matter in the water and bottom sediments. It is also necessary for other biological and chemical processes.
Oxygen is produced during photosynthesis and consumed during plant and animal respiration and decomposition. While photosynthesis occurs only during daylight hours, respiration and decomposition continue throughout the night. Therefore, streams with significant plant life can have DO levels that fluctuate greatly over a 24-hour period. DO concentrations also increase in surface water wherever the water flow becomes turbulent, such as in a riffle area, or waterfall. This turbulence brings more water into contact with the air thus increasing the amount of atmospheric gas (including oxygen) that is dissolved into the water. This process is called reaeration.
Temperature, flow, elimination of rapids and riffles, and pollution can all have an impact on the amount of DO in the water. Cold water can hold more dissolved gas, such as DO, than warm water. Therefore, during the summer months when stream water is warmer, oxygen can be limited by the ability of the water to “soak up” atmospheric gases. During late summer, stream flows can get very low. When there is less water in a stream it can heat up more rapidly. It also has less opportunity for reaeration as it moves slowly through areas that were once riffles during higher flows or when it stagnates in reservoirs and then is diverted around waterfalls through hydropower penstocks.
Wastewater discharges and stormwater runoff often carry oxygen-demanding substances (pollution) to streams and lakes. Most conventional pollutants (nutrients, organic matter, and other chemicals) require oxygen for decomposition and/or other chemical reactions. The amount of oxygen required for some of these processes is called the Biochemical Oxygen Demand, or BOD. As BOD concentrations increase and if there is a lack of reaeration, the river DO concentration decreases. Nutrients also stimulate algae growth which can contribute to long-term DO depletion when dying algae decomposes in the sediment.
These substances can come from both point sources and nonpoint sources. Point sources are end-of-pipe discharges such as from wastewater treatment plants or stormwater collection systems. Nonpoint sources come from diverse sources where the exact source is usually not easily determined. Some examples include runoff containing fertilizer and pesticides from lawns and croplands, organic debris from forested land, soil erosion, and faulty septic tanks.
Facilities that discharge effluents (point sources) to the Spokane River are required to have a National Pollutant Discharge Elimination System (NPDES) permit. Currently, these permits tell the discharger how much of each pollutant they are allowed to discharge to the river. The NPDES permit limits the amount of BOD, phosphorus and other compounds that can be discharged. For the most part, the current permits have limits that are called technology-based limits, which were to be applied consistently across the country without consideration to water quality. As permits are renewed, limits may become more stringent if it is necessary to protect the beneficial uses of the receiving water.
The Department of Ecology has contracted with the Corps of Engineers to
develop a water quality computer model for the Spokane River and Lake
Spokane using data obtained from recent and past studies. This
model will be used to predict water quality changes in response to
varying river flow and types, quantity, and locations of wastewater
discharges. The model and its predictions will be used in a future
public process to establish the total maximum daily load (TMDL) of
pollutants that can be discharged to the river.
See our Frequently Asked Questions for more information about this TMDL. (Ecology Publication)
Next: The Solution
Contact us for more information
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.
