Modeling TPH Desorption in Unconsolidated Dune Sand during Remediation Using Dual-Equilibrium Desorption (DED) Model

The verification of the cleanup of dune sands from a dissolved phase petroleum hydrocarbon plume at an active service station site undergoing ozone treatment is being accomplished through soil and groundwater sampling. We hope to use modeling by the dual-equilibrium desorption (DED) model in order to determine when remedial goals negotiated with the regulatory agencies have been met. The site is located in the harbor area of Crescent City, California in an area underlain by dune sands of marine origin. During remediation of sorbed-phase gasoline and diesel range organics (GROs and DROs) and dissolved-phase methyl tertiary butyl ether, target compounds, intrinsic indicators, by-products, and bacterial composition were monitored in the area groundwater and soil. At present, this monitoring has been employed to establish the degree of cleanup of the sands. In this paper the use of the DED model to verify the degree of cleanup of the sands is presented. Results indicate rapid degradation of the target compounds from the dissolved phase with gradual degradation from the sorbed phase. Degradation of the sorbed phase GROs and DROs was indicated by periodic spikes in dissolved-phase concentrations, which are interpreted as representing desorption events. Co-located soil and groundwater samples were collected at intervals of approximately six to nine months. Results from this verification sampling were modeled using the DED. We hope to find a tool that an investigator at a typical underground storage tank site can use to gauge the success of remediation systems. So far, the DED appears to correlate with samples collected using standard field techniques. The use of this methodology has application in establishing the degree of cleanup of contaminated sandy dredge sediments.