I checked http://www.cleanuplevel.com and it has styrenen cleanup levels ranging from 0.02 ppm up to 14,000 ppm. It appears that the higher levels are being used at locations where they used direct exposure to humans and that the lowere levels are where they were worried about leachability to...
http://www.cleanuplevel.com
This is an online database and mapping system for cleanup levels at Superfund sites. You can search by compound or by site. This is a good way to determine the methods and levels that are being used elsewhere.