I did some research on this a while ago and I was surprised to find that biodegradable soaps can be poisonous to crabs, oysters, and rockfish, damaging gills and depleting the water of oxygen. If you wash your car on the side of the road the storm drains will often carry all the soap from your car (and the oil and dirt) the nearest stream with no pollutant removal. Therefore, I started taking my car to the carwash where all the water is managed and treated properly. If you can't take the car to the car wash try spraying the soapy water on a grass or gravel surface so it has some time to filter before it reaches a storm drain.