Pursuant to a congressional request, GAO provided information on the parties responsible for contaminating sites on the Environmental Protection Agency's (EPA) National Priorities List (NPL), focusing on the number of: (1) de minimis parties that may have contributed only a relatively small amount of contamination at nonfederal NPL sites; and (2) potentially responsible parties at nonfederal municipal co-disposal landfill sites.

GAO found that: (1) EPA data collection limitations and reliance on project managers' judgment to identify parties make it difficult to accurately determine the number of de minimis or potentially responsible parties; (2) at least 8,500 to more than 25,000 de minimis parties were associated with 175 nonfederal NPL sites; (3) about 15,000 to more than 40,000 potentially responsible parties were associated with 245 nonfederal municipal co-disposal landfill sites; and (4) the estimates may be understated because of the data limitations and because they do not include sites that may be added to NPL.