Washing a car in your driveway uses around 80 to 140 gallons of water that runs into storm drains and from there straight into creeks and rivers. This water is contaminated with oil, gasoline and fumes from the exhaust as well as with the cleaning agent. Commercial car washes use less water (about 45 gallons per car) and are in many countries, including USA and Canada required by law to drain their waste-water into septic system where it gets treated properly. Many of them also recycle and reuse the rinsing water - find a green car wash in your area and try to take your car there whenever needed. If you must wash your car yourself, use biodegradeable soap and do it on your lawn instead of the asfalt, to give the soil a chance to soak up the water.