ESTIMATION OF RESIDUAL DENSE NAPL MASS BY INVERSE MODELING

Ground water at many sites is contaminated by chlorinated solvents which are of relatively low solubility and denser than water, as well as other contaminants of similar properties. The presence of a dense nonaqueous phase liquid (DNAPL) in an aquifer can provide continued flux to the dissolved phase long after the initial discharge. The flux may be significant even when the nonaqueous phase has dispersed to a state of residual saturation, in which little or no free product can be detected or recovered. At an industrial site we required an estimate of the magnitude of a DNAPL source without undertaking invasive exploration. In theory, the presence and magnitude of a residual DNAPL source can be inferred from observations of downgradient concentrations by (1) developing an inverse modeling solution to estimate flux, and (2) matching estimated flux to a conceptual model of NAPL dissolution, which yields an estimate of residual mass. Such a method can provide a qualitative indication of the presence of a residual DNAPL source. However, the sensitivity of the solution to assumptions regarding physical characteristics of the source medium are such that a useful quantitative estimate cannot be made without observations over long periods of time.