Assessing Health Risks Associated with DDT Residues in Soils in California: A Proposition 65 Case Study

Population growth in California has increased the pressure to convert agricultural land to commercial, industrial, or residential uses. In the ensuing property transactions, buyers and sellers must address the presence of toxic materials in soils such as pesticides, several of which are known to the State of California to cause cancer under Proposition 65. While this statute does not specifically address soil contaminants, the potential scope of its enforcement is sufficiently broad that owners of former agricultural properties may be obliged to provide warning of exposure to potential buyers, occupants, or construction workers about exposure to residues in soil from pesticide applications. However, Proposition 65 provides no guidance on how to assess exposures to chemicals in soil. The U.S. EPA Risk Assessment Guidance for Superfund (RAGS) provides a method for assessing soil-related exposure pathways that is consistent with the intent of Proposition 65. Using this approach, we have calculated the lifetime average concentrations of DDT in soil corresponding to the no-significant-risk level stipulated under Proposition 65 (1 × 10⁻⁵) for a hypothetical residential exposure scenario. The concentration of DDT in soil corresponding to a no-significant-risk ranges from 7.9-18.8 mg/kg, depending upon which exposure pathways are deemed to be complete for residential land use. It is argued that Proposition 65 forces the assessment and possible cleanup of such a situation through the threat of creating a health risk perception that could affect the market value of a property.