Characterizing grain-scale availability of aromatic hydrocarbons in MGP site soils and assessment of the impact on the dynamics of soil and pore water concentration during and after remediation
To predict groundwater concentration levels which emanate from a contaminated source, early release models assumed local equilibrium partitioning. As the validity of this assumption has been questioned, models have been developed which incorporate kinetic release. To further this work, two complementary studies were performed. The first involved applying developed laboratory and analytical techniques for measuring availability during the remediation of an existing contaminated site. The second involved the application of a two-site equilibrium/rate-limited release model to illustrate the impact of specific degrees of limited availability. For the first study, as an initial measure of contaminant availability, short-term batch tests were performed before, during and after the remediation effort. Test samples which indicated limited availability were selected for long-term testing to quantify the availability parameters. The following conclusions could be made concerning the site post-remediation: (1) despite significant reduction of the contamination levels, the chemicals remained available (at the grain-scale) for additional treatment; (2) the groundwater concentrations in monitoring wells near the source could be predicted to rebound significantly; (3) despite the absence of remediation to an endpoint, methods were developed to determine environmentally acceptable soil concentration endpoints in the laboratory; (4) confirmation of short-term results with long-term experiments is necessary; and (5) the apparent Kd can used to justify soil concentrations endpoints significantly higher than those predicted using typical soil screening level foc*Koc values. For the second study, simulated and actual availability data were used with simulated site-conditions (i.e. velocity, porosity, etc.) to generate groundwater concentration vs. time curves. The curves illustrate a two-step process consisting of aggressive remediation, followed by a post-remediation step where natural hydrologic conditions are re-established. Release rate constants of about 10"3 day -l or faster allow remediation at a practicable, albeit slower, rate. Upon the return to natural conditions, the concentration rebounds to a level determined by the apparent Kd. Slower rate constants significantly reduce the effectiveness of remediation efforts, but result in postremediation concentrations lower than predicted by the apparent Kd.