A FRAMEWORK FOR ASSESSING RISK REDUCTION DUE TO DNAPL MASS REMOVAL FROM LOW-PERMEABILITY SOILS

Many emerging remediation technologies are designed to remove contamin ant mass from source zones at DNAPL sites in response to regulatory re quirements, There is often concern in the regulated community as to wh ether mass removal actually reduces risk, or whether the small risk re ductions achieved warrant the large costs incurred. This paper sets ou t a proposed framework for quantifying the degree to which risk is red uced as mass is removed from DNAPL source areas in shallow, saturated, low-permeability media. Risk is defined in terms of meeting an altern ate concentration limit (ACL) at a compliance well in an aquifer under lying the source zone, The ACL is back-calculated from a carcinogenic health-risk characterization at a downgradient water-supply well, Sour ce-zone mass-removal efficiencies are heavily dependent on the distrib ution of mass between media (fractures, matrix) and phase (aqueous, so rbed, NAPL). Due to the uncertainties in currently available technolog y performance data, the scope of the paper is limited to developing a framework for generic technologies rather than making specific risk-re duction calculations for individual technologies. Despite the qualitat ive nature of the exercise, results imply that very high total mass-re moval efficiencies are required to achieve significant long-term risk reduction with technology applications of finite duration. This paper is not an argument for no action at contaminated sites, Rather, it pro vides support for the conclusions of Cherry et al. (1992) that the pri mary goal of current remediation should be short-term risk reduction t hrough containment, with the aim to pass on to future generations site conditions that are well-suited to the future applications of emergin g technologies with improved mass-removal capabilities.