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
orders of magnitude (Urban et al. 1989, Hewitt
is confounded by both environmental and opera-
1994a, Hewitt et al. 1995).
tional variables. Among environmental variables,
In recognition of the problems with past intru-
temperature, soil moisture, organic carbon con-
sive soil sample collection, handling and analysis
tent, grain size, mineralogy, texture, and baro-
techniques, in-vial methodologies are now being
metric pressure all influence VOC concentrations
promulgated. The U.S. Environmental Protection
in both the soil gas and sorbed phases (Conant et
Agency is currently proposing two in-vial meth-
al. 1996). Among operational variables, these two
ods, 5021 and 5035, for the third update of the
approaches to VOC characterization have very
Test Methods for Evaluating Solid Waste, SW-846
little in common. Passive soil gas survey methods
(U.S. EPA 1986). Using an in-vial approach re-
require a long-term exposure (several days to a
quires that a discrete soil sample be collected and
couple of weeks) to establish an in-situ, time-
transferred directly to a vial with hermetic seals,
integrated amount for a unknown volume of soil.
that then has to be punctured by a needle in order
Furthermore, because this application is usually
to perform a VOC analysis. Furthermore, Hewitt
designated for screening purposes, concentration
and Lukash (1996) recommend that these soil
determinations are not rigorous (i.e., single point
samples be collected within a few minutes of ex-
calibration). Grab samples on the other hand rep-
posing a fresh surface and transferred to the col-
resent a single point in time and are obtained in
lection/analysis vial with minimal disruption to
an intrusive fashion, by collecting a discrete quan-
the native soil structure.
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