A Method for Determining Ground Water-Protective Soil Cleanup Levels (GPLs) for VOCs

Soil gas sampling provides a more reliable and cost effective technique for delineating contamination and tracking remedial progress at most VOC contaminated sites than soil sampling.   Therefore, the Arizona Department of Environmental Quality (ADEQ) revised their soil remediation rule in 2007 to admit soil gas sampling of VOCs as a surrogate for soil sampling of VOCs.  A theoretically rigorous but simple to use screening model was developed and implemented in Excel.  The model uses soil gas VOC concentrations measured in the vadose zone of the source area to predict the groundwater VOC concentration expected at a down gradient POC monitoring well.  Therefore, the model can be used to develop Remedial Action Objectives (RAOs) for VOCs in the vadose zone.  These soil gas RAOs for VOCs are far more appropriate than soil RAOs for most site conditions.

Application of the model is limited to sites with relatively low net infiltration and where vadose zone soil gas concentrations are expected to persist for times significantly greater than the groundwater travel time beneath the contaminated area.  Application of the model to a hypothetical site is provided, and the sensitivity of model predictions to input parameters is also discussed.