Car Washing

Washing vehicles is a common activity for residents and a popular method for community fundraising, but car washing stormwater runoff is a significant source of pollution in urban and suburban watersheds. When a car is washed on a paved surface like a driveway or street, the wash water, laden with soap, dirt, and automotive fluids, flows directly into the nearest storm drain. Because storm drain systems provide no treatment, these pollutants are discharged directly into local streams, rivers, and lakes, harming aquatic life and degrading water quality. This activity is a key focus for municipal pollution prevention programs aimed at reducing non-point source pollution.

The pollutants in car wash runoff are diverse. Soaps and detergents contain phosphates and surfactants. Phosphates act as a fertilizer, contributing to excessive algae growth (eutrophication) that depletes oxygen in the water, harming fish and other aquatic organisms. Surfactants can be toxic to aquatic life by breaking down the protective mucus layer on fish. The grime washed from the vehicle itself contains heavy metals like copper from brake dust, zinc from tire wear, and lead from older vehicle components. It also includes hydrocarbons from engine oil and exhaust residue. This combination of chemicals, nutrients, and metals makes untreated car wash discharge a potent form of pollution, especially during dry weather when it can be a primary contributor to stream contamination.

The cumulative impact of residential car washing is substantial. Surveys in Washington state found that about 60% of residents who wash their own cars could be classified as “chronic car-washers,” doing so at least once a month. Of those, 70% to 90% reported that their wash water drained directly to the street. Extrapolated nationally, this behavior suggests that as much as 25% of the U.S. population may be chronic car washers, translating to potentially 27 million regular sources of residential car wash pollution.

How the car washing stormwater pollution pathway works

The pollution pathway for car washing stormwater is a direct and efficient one. Most residential and charity car washes occur on impervious surfaces such as asphalt or concrete driveways, parking lots, or streets. These surfaces are engineered to shed water quickly. When a vehicle is washed, the water mixes with detergents and the accumulated grime on the car’s surface. This grime is a collection of pollutants from daily driving, including oils, grease, heavy metals from brake and tire wear, and atmospheric deposition. Regular vehicle maintenance can reduce some of these fluid leaks, but road grime is unavoidable.

This polluted mixture flows across the impervious surface and into the gutter, which is part of the municipal separate storm sewer system (MS4). It enters a storm drain inlet and travels through a network of underground pipes. Unlike sanitary sewers, which carry wastewater from homes and businesses to a treatment plant, the MS4 is designed to prevent flooding by conveying stormwater directly to the nearest water body. There is no filter, no treatment process, and no opportunity for pollutants to be removed. The discharge from the pipe into the receiving water is therefore nearly identical in composition to the runoff that left the driveway, delivering a concentrated pulse of contaminants directly into the aquatic environment.

Recommended practices

Adopting better practices for washing vehicles can significantly reduce polluted runoff. The primary goal is to prevent wash water from entering the storm drain system. This can be achieved through site selection, proper disposal of wash water, and the use of appropriate materials.

For Residents

Individual homeowners have several simple options to prevent pollution. These practices are central to effective residential stewardship programs.

• Use a commercial car wash. Commercial facilities are required to drain their wastewater to the sanitary sewer system, where it is treated at a wastewater treatment plant before discharge. Many also recycle their water, making them a more environmentally friendly option.
• Wash on a permeable surface. If washing at home, choose a location like a lawn or a gravel driveway. These surfaces allow the wash water to soak into the ground. The soil acts as a natural filter, trapping pollutants and breaking them down before they can reach groundwater or a nearby stream. This is similar to the principles behind good landscaping and lawn care for stormwater management.
• Use a hose with an automatic shut-off nozzle. This conserves water by ensuring it only runs when needed.
• Use biodegradable, phosphate-free soaps. While the grime from the car is the main source of toxic pollutants, choosing a less harmful soap reduces the overall impact. However, even “green” soaps should not be allowed to enter storm drains.
• Empty wash buckets into a utility sink or toilet. This directs the concentrated, soapy water to the sanitary sewer for treatment.

For Charity and Fundraiser Car Washes

Fundraising car washes often involve washing many vehicles in one location, concentrating the potential for pollution. Organizers should take extra steps to manage their events responsibly.

• Partner with a commercial car wash. Many commercial facilities offer fundraising programs, providing a professional service while ensuring environmental compliance.
• Select a suitable location. If holding the event on-site, choose a spot that drains to a grassy area or use a temporary diversion to direct flow away from storm drains. Avoid locations that drain directly to the street.
• Use a “car wash kit.” Many municipalities offer kits for charity groups to borrow. These kits typically include a storm drain cover or insert, a sump pump, and hoses to redirect the captured wash water to a sanitary sewer access point (like a utility sink drain) or a large landscaped area.
• Pump wash water to the sanitary sewer. The preferred method is to capture all wash water and pump it into a sanitary sewer drain. This requires permission and coordination with the local utility.
• Dispose of water on permeable surfaces as a last resort. If a sanitary sewer connection is not feasible, pump the water onto a large, flat, vegetated area far from any storm drains or surface waters. The area must be large enough to absorb the volume of water without creating runoff.

For Businesses Washing Vehicle Fleets

Businesses that maintain their own vehicle fleets should implement formal good housekeeping and structural controls to manage wash water.

Building a municipal program

A municipal program to address car wash pollution typically focuses on public education and providing resources for community groups. Since many residents are unaware that storm drains lead directly to local waters, outreach is the critical first step. National polling has found that only about one in five Americans knows that stormwater runoff is the most common source of water pollution, highlighting the need for basic education.

Effective programs combine broad public awareness campaigns with targeted, practical support. Storm drain stenciling programs, where volunteers label inlets with messages like “Drains to River,” are a simple way to reinforce the connection between the street and local water bodies. Outreach materials such as brochures, utility bill inserts, and social media campaigns can explain the problem and promote the recommended practices.

A centerpiece of many successful programs is a “water-friendly” car wash kit loaner system for charity fundraisers. Kitsap County, Washington, developed a program that offers two types of equipment for groups to borrow: a catch-basin insert with a sump pump, or a vacuum/boom device called a “Bubble Buster.” Both systems capture wash water so it can be pumped to a sanitary sewer or vegetated area. The county provides the kits free of charge along with training materials.

The cost of such a program is relatively low. For Kitsap County, the Sound Car Wash program requires about 10-15 hours of staff time per week during the 25-week car wash season (April-September). The annual budget for materials and equipment replacement is estimated at $1,500 to $3,000. The City of Tacoma reported that its Clean Bay Car Wash kit program costs less than $2,000 per year and requires less than two weeks of total staff time annually. The equipment itself is a one-time expense; a catch basin insert costs approximately $65, while a vacuum system can cost $2,000 to $2,500.

Effectiveness

Quantifying the direct pollutant load reduction from car wash management programs is challenging due to the diffuse, non-point nature of the activity. However, the effectiveness can be inferred from the high prevalence of the behavior and the known composition of wash water. Given that millions of households wash cars at home and that most of this water enters storm drains, shifting this behavior has a clear and positive effect on stormwater quality.

The primary measure of effectiveness is behavior change. Surveys provide a baseline for the scale of the problem. For example, a 1997 study in Washington found that 56% of residents washed their own cars, 91% of whom reported the water drained to pavement. The same study found that 50% of these residents would be willing to switch to a commercial car wash if offered discounts or free washes, indicating that outreach and incentive programs can be highly effective.

While specific load reduction data for residential programs is limited, the potential can be estimated by considering the pollutants being diverted. Each car wash sends a mix of detergents, oils, and heavy metals toward a stream instead of a treatment plant or filtration through soil. Preventing these discharges directly reduces the pollutant load entering receiving waters. The impact of these loads can be modeled using tools like a Simple Method runoff calculator to understand how preventing impervious-area runoff contributes to overall watershed health goals.

Frequently Asked Questions