In 2002 Congress enacted amendments to the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, commonly referred to as Superfund) that outline landowner liability protections. The CERCLA amendments also provided statutory defenses such as the third party defense in § 107(b) of the statute.
There are also special provisions for state and local governments. A good source for a general discussion of liability concerns is the "Revitalizing Contaminated Sites: Addressing Liability Concerns (Revitalization Handbook)."
Additionally, EPA’s Office of Site Remediation Enforcement has developed a comprehensive approach to defining liability issues and providing guidance and enforcement tools to address potential liability concerns. Information is available on the Agency's landowner liability web page and from the liability-landowner category in the Superfund enforcement policy and guidance database.