Laboratory Complex Resistivity Investigation Of Organic Contaminants: Denver Federal Center Soils

Abstract

The Denver Federal Center is a DNAPL contamination site that lends itself to the use of complex resistivity in monitoring the migration and concentrations of contamination. The physical properties of DNAPLs often render monitoring-well data misleading by underestimating actual concentrations present in the liquid and vapor phases. Smectite-rich soil at the site and chemical degradation of volatiles through dechlorination, however, suggests the use of complex resistivity as a monitoring tool sensitive to clay-organic reactions. To aid the interpretation of complex resistivity field data, this study involves controlled contamination of site soils in the laboratory for comparison. Introduction In every geographic subset defined by the Environmental Protection Agency (EPA), volatile compounds comprise the most abundant class of groundwater contamination. The Denver Federal Center remediation site is one such example of groundwater contamination from volatiles originating from the seepage of storage tanks originally housing chemical byproducts of asphalt production. Significant contaminants above regulation standards at the time of investigation included 1,1dichloroethane (DCE), 1,1,1-trichloroethane (TCA), and trichloroethene (TCE). Remediation at the site involved the installation of a gate and funnel reaction wall constructed of iron filings, which acts as a type of chemical filter to the contaminants. Concentrations of contaminants at the site are currently monitored by analyzing groundwater samples obtained from bailing previously purged wells that are located throughout the site and within the reaction wall. The difficulty of this approach lies in the irregular and unpredictable migration pattern and the multi-phase nature of the contaminants existing in liquid, aqueous, and vapor phases. Even if a randomly placed monitoring well is located near a pool of contamination, analysis of the groundwater sample may only yield information regarding the aqueous phase of the contaminant, underestimating the volatile and liquid phases. Complex resistivity has the potential to offer additional information in monitoring efforts. It is a noninvasive geophysical method sensitive to chemical reactions between organic contaminants and clays present in the site soils. For complex resistivity to be used effectively at this site, field data must be correlated to laboratory data of native soils contaminated with known concentrations of volatiles relevant to the site. In this study, soil samples have been contaminated with known concentrations of DCE, TCA, and TCE as well as with native groundwater. Groundwater samples were obtained from inside and down-gradient of Cell 1 in the remediation wall and from a well on the southern flank of the wall where it was suspected that groundwater flow may have been circumventing the funnel and gate system (Table 1). Complex resistivity of these samples is measured over time because the chemical processes involved in the degradation of these volatiles occur over time. If general patterns in the data reflecting these reactions over time may be detected in the laboratory, the assumption is that these patterns may aid in the interpretation of field measurements at the site.