A Comprehensive Approach to Developing Soil Cleanup Levels Based on Protection of Groundwater Quality and Indoor Air Quality. J. S. Smith, J. M. Farr, and L. Chau, ICF Kaiser Engineers, Inc., 4 Gateway Center, Pittsburgh, PA 15222; and ICF Kaiser Engineers, Inc., 1800 Harrison St., Oakland, CA 94612
Historically, the greatest concern associated with the presence of volatile organic compounds (VOCS) in soil is the potential for these chemicals to migrate through the vadose zone and adversely impact groundwater quality. More recently, the potential for VOCs to migrate vertically upwards and infiltrate the living space of buildings has also received attention. A comprehensive approach has been developed that simultaneously accounts for both transport pathways in the development of site-specific cleanup levels for VOCs in soil. Existing approaches to developing cleanup levels for VOCs in soil range from generic, default criteria to complex models based on site-specific information. The former approaches typically result in unrealistically low cleanup levels, leading to unnecessary remediation. The latter approaches result in more realistic cleanup levels, yet tend to be costly and require a lot of site-specific data. We present a comprehensive approach to developing soil cleanup levels that is both site-specific and practical. This approach involves three integrated chemical fate and transport models. The central model involved is a revised version of VLEACH. VLEACH is a one-dimensional finite-difference chemical transport code for simulating vertical transport within the vadose zone. Recent modifications to this code allow for the incorporation of (1) first-order chemical degradation; (2) depth-specific soil properties; and (3) an immobile NAPL phase. The modifications also allow for the evaluation of surface vapor flux estimates. The boundary chemical flux estimates from VLEACH are coupled to a shallow aquifer mixing-cell model and an indoor air model to estimate groundwater and indoor air concentrations. Trial depth-specific soil concentrations are entered into VLEACH in an iterative manner until the exposure point concentrations in groundwater and indoor air fall below critical target concentrations (e.g., MCLs for groundwater and 10-6cancer risk levels for indoor air). In this manner, site-specific information can be incorporated into the development of reasonable soil cleanup levels that will simultaneously protect groundwater and indoor air quality.