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.