Comparison of two field sampling procedures (en core and field methanol extraction) for volatile organic compounds

In-situ Lasagna technology was recently evaluated at a contaminated site at Offutt Air Force Base. The site was contaminated with low levels (<30 mg/kg) of volatile organic compounds (VOCs). Originally, researchers planned to use field methanol extraction for both pre- and post-treatment sampling to evaluate the effectiveness of the technology on contaminant reduction. Precharacterization sampling, however, indicated that concentrations of some contaminants of concern were much lower than expected. Because use of methanol increases the detection limit, it was probable that post-treatment concentrations of these target contaminants would be nondetectable if methanol extraction was used. Project management, therefore, decided to use En Core samplers in addition to field methanol extraction during the pretreatment sampling event. The En Core sampling approach, while yielding a lower detection limit, uses discreet samples along the length of a core, whereas the methanol extraction approach samples the entire length of the core. The concern was that discreet samples may bias results if any "hot spots" were present. The two field sampling procedures, En Core and field methanol extraction, were performed side-by-side during the pretreatment phase of the technology evaluation in order to determine if the concern was valid for this site. Results were compared for four contaminants of concern: trichloroethylene, vinyl chloride, cis-1,2-dichloroethene, and trans-1,2-dichloroethene. The two procedures produced similar results with respect to both the concentration means and the variances, and no bias was evident. This finding supports project management's decision to use only En Core samplers post-treatment due to low concentrations of target contaminants.