I blasted mine at a carwash and so far all is well. Sunlight may help. Since it is safe to use low concentration of soap directly on plants as a method of insect control I can't see how soil is harmed using the same method.
I am sure someone will post if this is a taboo issue.
David

Hi Fellow Gardners,
I just obtained a 55 gallon barrel from a local car wash and I am interested in using it as a rain barrel. I have read on this forum that it is safe to use a barrel from a car wash. However, it says on the label "DANGER Contains Phosphoric Acid". Would it be safe to use the rainwater on my vegetable garden? No matter how well you rinse out the barrel, doesn't the plastic absorb some of the chemicals and then leach into the water?
I've received some excellent advice on this forum in the past and I am hoping someone can help me this time.

Hi All,

I looked up H3PO4. Phosphoric acid is a food additive to give a sour taste to foods according to the article I read. It seems the extreme low concentration of any residue in the plastic would not be a concern since the plants utilize the chemical as a fertilizer. Check it out yourself in google.

http://www.fenghaichem.com/template/p3.htm

Aloha