Establishing a Relationship Between
Passive Soil Vapor and Grab Sample Techniques for
Determining Volatile Organic Compounds
ALAN D. HEWITT
tity of soil. Analysis of grab samples is performed
INTRODUCTION
using a specified laboratory procedure, which is
Traditionally, passive soil vapor techniques
usually validated by accompanying quality as-
have been used to establish the presence and rela-
surance (QA) and quality control (QC) data.
tive distribution of volatile organic compounds
VOCs in an unsaturated porous medium partly
(VOCs) in the near-surface vadose zone. These
exist in a gaseous state due to their high vapor
methods of screening are often used during the
pressures. Indeed, it is this physical characteristic
initial phase of a site investigation to provide
of VOCs that has led to the success of both pas-
insight for the placement of monitoring wells and
sive and active soil vapor techniques. On the other
to delineate areas for intensive sampling. In con-
hand, because a portion of the contaminant VOC
trast, discrete soil sample ("grab samples") tech-
is present in the gaseous phase, intrusive sam-
niques are used for risk assessment, to establish
pling protocols have to be rapid and minimize
both the horizontal and vertical profiles of con-
disruption of the soil structure to prevent losses.
tamination in areas of concern and to assess the
Hewitt and Lukash (1996) have shown that in less
progress of remediation.
than 40 minutes, more than 90% of the low boil-
To date, we are unaware of a study showing a
ing point VOCs (i.e., trichloroethylene [TCE]) can
strong relationship between the results from a
be lost from an intact soil stored in an uncovered
passive soil vapor survey and soil VOC concen-
core liner. Moreover, procedures that require mul-
trations resulting from grab sample collection and
tiple handling operations often result in underes-
analysis. The task of establishing a relationship
timating soil VOC concentrations by one to three
between these two VOC assessment technologies