The best place to wash your car is to use a full or self service car wash. They are designed to recycle used water and filter out many of the harmful chemicals and pollutants washed away from your vehicle.
Although we highly recommend going to a full or self service car wash, an alternative is to park your vehicle on the lawn or gravel. Use biodegradable soaps to wash your vehicle, using as little water as possible. Shut off water while washing your car, then rinse. Remember not to leave your car on the lawn.