Evaluation of mass flux to and from ground water using a vertical flux model (VFLUX): Application to the soil vacuum extraction closure problem

Site closure for soil vacuum extraction (SVE) application typically require s attainment of specified soil concentration standards based on the premise that mass flux from the vadose zone to ground water not result in levels e xceeding maximum contaminant levels (MCLs). Unfortunately, realization of M CLs in ground water may not be attainable at many sites. This results in so il remediation efforts that may be in excess of what is necessary for futur e protection of ground water and soil remediation goals which often cannot be achieved within a reasonable time period. Soil venting practitioners hav e attempted to circumvent these problems by basing closure on some predefin ed percent total mass removal, or an approach to a vapor concentration asym ptote. These approaches, however, are subjective and influenced by venting design. We propose an alternative strategy based on evaluation of five comp onents: (1) site characterization, (2) design, (3) performance monitoring, (4) rate-limited vapor transport, and (5) mass flux to and from ground wate r. Demonstration of closure is dependent on satisfactory assessment of all five components. The focus of this paper is to support mass flux evaluation . We present a plan based on monitoring of three subsurface zones and devel op an analytical one-dimensional vertical flux model we term VFLUX. VFLUX i s a significant improvement over the well-known numerical one-dimensional m odel, VLEACH, which is often used for estimation of mass flux to ground wat er, because it allows for the presence of nonaqueous phase liquids (NAPLs) in soil, degradation, and a time-dependent boundary condition at the water table interface. The time-dependent boundary condition is the centerpiece o f our mass flux approach because it dynamically links performance of ground water remediation to SVE closure. Progress or lack of progress in ground w ater remediation results in either increasingly or decreasingly stringent c losure requirements, respectively.