Worldwide Residential Soil Regulatory Guidance Values for Chlorinated Methanes

In many environmental jurisdictions worldwide, surface soil contamination is regulated using guidance values that specify the maximum amount of pollutant that can be present without prompting a regulatory response. Three of the four chlorinated methanes are among the seven most frequently regulated synthetic surface soil contaminants. There are at least 80 U.S. regulatory jurisdictions and at least 30 international jurisdictions with guidance values for at least one of the chlorinated methanes, yielding as many as 106 values (for dichloromethane) for the same regulatory consideration. This analysis explores the variability of these values. Results indicate that the values span at least 5 orders of magnitude and are distributed in patterns similar to a lognormal random variable, but fit distributions that are statistically distinct. The distributions also contain value clusters that may imply emerging consensus about appropriate regulatory levels or demonstrate the impact of regulatory leadership to suppress variability. Simulation based on the current U.S. Environmental Protection Agency risk analysis model are used to estimate the degree to which value ranges may be attributed to uncertainty in exposure models. Approximately 50% of guidance values fall within the 95% confidence interval uncertainty bounds of risk model results.