A Health‐Based Approach for Evaluating Soil at Depth Contaminated by Hazardous Waste: Utilizing the AALsoil contact Criterion

Contaminated soil at depth can pose a threat to public health upon excavation. Following the excavation of contaminated soils, toxicants which once resided at depth become available for exposure due to direct contact. A scenario is developed that describes the excavation and redistribution of soil to the surface. Based on typical excavations occurring in a residential setting in California, soil to a depth of 10 feet is considered available for future exposure due to direct contact. The capabilities of various earthmoving equipment were employed to establish that the average toxicant level in 12 cubic yards of soil is the appropriate measure for evaluating contaminated soil at depth. This allows the evaluation of future risk to public health due to direct contact exposures following the excavation of contaminated soils at depth. SUMMARY Direct contact exposures can result if soil contamination initially at depth is excavated and distributed to the soil surface. The evaluation of future exposure to soil at depth due to direct contact should be a component of many hazardous waste site investigations. A scenario that accounts for the excavation and redistribution of soil at depth has been established. The manner in which soil would be handled by earthmoving equipment typically employed for residential construction activities, established 12 yd3 of soil as a reasonable and appropriately conservative minimum amount of soil to be placed uniformly over a 1000 ft2 area. In California, 10 ft is the maximum depth to which soil on a residential property is likely to be excavated. In the absence of overriding site‐specific considerations, the appropriate sampling objective for evaluating future direct contact exposures to soil contamination initially at depth is to determine average levels of toxicants in subsurface soils within 10 ft of the existing ground surface. Results of such sampling are amenable to interpretation as described in the California Site Mitigation Decision Tree(1) to determine if contamination initially at depth could present a future risk to public health.