Analysis of Regulatory Guidance Values for Residential Surface Soil Arsenic Exposure

Abstract

Residential surface soil regulatory guidance values (RGVs) specify the threshold at which soil contamination requires action. Usually, RGVs are risk-based values based on child ingestion, inhalation, and dermal exposure. The U.S. Environmental Protection Agency, 45 U.S. states, and 27 other nations have developed arsenic surface soil RGVs. Regulating arsenic poses unusual problems because it presents both cancer and noncancer risks, and its background concentration often exceeds health-based risk levels. Statistical analyses are presented to characterize 119 arsenic surface soil RGVs. State values vary between 0.039 and 200 mg/kg. Worldwide values vary between 1.7 and 687 mg/kg. The U.S. and worldwide values resemble lognormal probability distributions but the data cannot be mingled since worldwide values are significantly higher. An analysis of 40 arsenic background studies yielding averages between 1.3 and 45.1 mg/kg is also presented. Monte Carlo simulations of screening model calculations are used to explore the impact of coefficient uncertainty. Results indicate that 95% of cancer-based results should fall between 0.004 and 2.7 mg/kg and 95% of noncancer results should fall between 1.0 and 40 mg/kg. Although U.S. state arsenic RGVs vary by 3.7 orders of magnitude, most values appear to fall within the bounds of plausible risk- and background-based values.