Aqueous concentration ratios to estimate mass of multi-component NAPL residual in porous media

Abstract

A method was developed and evaluated for estimating the mass of chemicals contained in NAPL residual zones based on temporal changes in the ratios of the dissolved contaminants derived from the dissolution of multi-component NAPL residual.

The Effective Solubility Model (ESM) was developed as a tool to describe the changes in aqueous concentration ratios of organic contaminants emitted from the dissolution of a multi-component zone of NAPL residual. The model is based on the principle .&.hat temporal changes in the ratios of aqueous-phase concentrations in groundwater can be related to the degree of chemical mass depletion of NAPL residual. The ESM utilizes a series of linked equilibration cells to calculate the dissolution of multi-component NAPL according to Raoult's Law.

ESM simulations compared favourably to the results of three published laboratory dissolution experiments and two controlled field experiments. A method was developed using the ESM, together with groundwater monitoring data, to estimate the quantity of chemical mass contained in NAPL zones at sites of NAPL contamination. As an example application, monitoring results of the groundwater pump-and-treat system at the Emplaced-Source experiment were used to estimate the mass of the NAPL source. The NAPL mass estimated using the ESM method was within 20% of the actual NAPL mass. The mass estimate was made using only the measured aqueous concentrations in extraction well PW-2, and the physical and chemical properties for the NAPL components and aquifer. No specific assumptions were required regarding the dimensions or geometry of the source zone, groundwater flow conditions, or dissolution mass transfer coefficients. This research has illustrated the potential for the ESM method to provide useful estimates of the chemical mass contained in multicomponent NAPL residual zones in porous media.