Pursuant to a congressional request, GAO reviewed the Environmental Protection Agency's (EPA) risk assessment process for Superfund hazardous waste sites, focusing on: (1) whether these assessments adhere to EPA guidance or vary by region; and (2) how EPA monitors the quality and consistency of these risk assessments.

GAO found that: (1) most of the 20 risk assessments reviewed generally adhered to EPA guidance, were prepared consistently among the 10 EPA regions, identified which contaminants were present, and used similar assumptions in measuring human exposure to hazardous contaminants; (2) some risk assessments were not completely accurate and did not follow EPA guidance for estimating contamination levels, adequately describe the assumptions' inherent weaknesses, or calculate the total risk of exposure; (3) EPA has used many different assumptions in estimating exposure, future site uses, residual contamination amounts, and the ways people absorb contaminants through their skin; (4) although EPA has conducted annual quality assurance reviews to monitor the quality and consistency of risk assessments, these reviews produce only summary descriptive information and do not analyze inconsistencies among risk assessments; (5) EPA could improve its risk assessments and better meet regional staff needs by analyzing assessment inconsistencies; and (6) identifying assessment inconsistencies would not require significant additional funding, since EPA already collects the necessary information in its annual reviews.