Car Wash Runoff - Dangerous Impact on Watershed

A recent report on the severe impact of driveway car washing on the Puget Sound watershed in Washington State appeared in the October 2007 Stormwater Journal1, “Measuring Toxicity of Car Wash Runoff.” Water from driveways or fund-raising car wash events typically runs down the street or parking lot and into the nearest storm drain. This wastewater (effluent) may carry detergents, trace amounts of metals, and small amounts of fuels and automotive fluids. Because it goes untreated into the nearest stream, this runoff has the potential to harm fish and other marine life in the streambed.

Car Wash Runoff – Test Description

The impact of chemicals in car wash runoff was measured by assessing the death rate in young rainbow trout. This seemed to be a very practical way to show how harmful car wash runoff can be in our streams. These tests are termed “practical” fish toxicity tests because the runoff solutions were collected and prepared to represent the actual runoff water that would be expected to enter into the stormwater drains and eventually enter the streams and rivers of Puget Sound.

Toxicity Test Water Samples – Fish Toxicity Test Results

Cars were washed on an asphalt surface at an oil change service facility. The asphalt condition was typical of a parking lot; its surface had numerous dark spots indicating leaks of petroleum product. Wash and rinse water that dropped to the asphalt ran about 30 feet across the asphalt to a storm drain grate.

Results of the two tests conducted both used a common car wash detergent concentrate, which is commercially available and marketed specifically as a car wash detergent.

The Setting

Puget Sound is home to 3.8 million people. The Sound is the second largest estuary in the United States. Its watershed covers nearly 16,500 square miles and includes more than 10,000 rivers and streams that drain into the sound. Mostly all of the stormwater that falls on developed areas enters storm drains and flows untreated into the sound.

As of 2006, the number of registered vehicles to the 3.8 million people in the Puget Sound area was approximately 3.7 million vehicles. By 2020, an additional 1.4 million people are expected to settle around the Sound.

Hypothetical Assumptions

Assumptions were made and calculations performed for a hypothetical urban or suburban Puget Sound setting in which a small stream is subjected to car wash runoff via storm drains. The testing conditions were based on typical circumstances that would be expected to apply in a real-life situation.

The specific setting tested was a small stream, about 10 to 20 miles long, which represents a typical stream’s flow during the tested time. This flow is typical of many small streams in the area.

Approximately 100,000 people are assumed to live in this watershed area. One percent of the cars of this population are washed in driveways during the time period, making a total of approximately 1,000 vehicles that will be washed. A consumer car wash detergent is used to wash the cars, and 75 gallons of water flows to the storm drain and, subsequently, to the small stream, for each car washed. A simple “bathtub” calculation was performed in which all of the stream flow and car wash runoff was pooled for a 48-hour period and the resulting detergent concentration calculated. This was similar to the concentration that was found to be lethal to 50% of juvenile rainbow trout tested. Therefore, some fish in the stream could be killed and it would be likely that the detergent would wash protective mucus from the gills of some of the surviving fish. The “survivors” would then be more susceptible to other contaminants that may exist or be introduced later into the stream. It is also possible that oxygen uptake necessary for fish survival may be impaired and that other physiological impacts to fish survival may occur. Other freshwater organisms living in the stream would also likely be affected, depending on individual species’ sensitivities.

Minor changes to the assumptions made in the above analysis could increase the calculated detergent concentration and make significant impacts to fish and other freshwater organisms more likely. For instance, increasing the percentage of cars washed from 1% to 1.5% increases the total amount of detergent flushed to the stream by 50% and raises the calculated detergent concentration in the stream for the low-flow situation. Dilution by the stream is the most important factor in the calculated detergent concentration.


For the Puget Sound, during September and October, most salmon are returning to area streams to spawn the next generation, and it typically represents the lowest stream-flow time of year. Although adult fish are found in the streams, they have been severely stressed by the long return migration and are likely more susceptible to damaging impacts of detergents and pollutants in stream water. A case can be made that during this pivotal time of the year, driveway car washing runoff that reaches streams via storm drains is a real detriment to salmon survival.

Adapted from Stormwater: The Journal for Surface Water Quality Professionals, October 2007.

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