Soil-Vapor Versus Discrete Soil Sample Measurements
for VOCs in the Near-Surface Vadose Zone
Feasibility Study
ALAN D. HEWITT
INTRODUCTION
adjacent saturated zone (Hughes et al. 1992, Con-
ant et al. 1996). In their experiments, Hughes et
Using a soil-gas survey to investigate subsur-
al. (1992) gave special attention to the soil-gas
face volatile organic compound (VOC) contami-
sampling system to minimize its effect on the in-
nation has been a common practice for well over
situ conditions during installation and soil vapor
a decade. Traditionally, this technique establishes
removal. This has not been one of the top priori-
the presence and relative distribution of VOCs in
ties for the soil-gas surveys used to detect VOCs
the near-surface vadose zone on temporal and spa-
leaking from underground storage tanks or to
tial scales. To make this method of site charac-
delineate concentration gradients above ground-
terization even more attractive, it is often coupled
water plumes. The soil-vapor sampling probe
with an on-site method of analysis, allowing data
developed by Hughes et al. (1992) had internal
acquisition to approach real time. Indeed, the
volumes of 530 mL, whereas most conventional
low cost and speed of soil-gas surveys are often
systems usually exceed 100 mL. The intent behind
touted as their greatest advantages.
designing a probe with a smaller dead volume
However, the data only qualify for screening
was to allow discrete locations in the subsurface
because a relationship between concentrations
to be measured instead of taking samples that are
obtained from discrete grab samples and those
averaged over large, unspecified areas and
from soil-gas surveys has yet to be established,
times. Using the approach taken by Hughes et al.
despite numerous attempts (e.g., Smith et al.
(1992), this study developed a soil-vapor probe
1990, Sextro 1996, Minnich et al. 1997). Even with
with a micro (1.2 mL) internal volume.
this handicap, soil-vapor surveys have excelled
To compare soil-vapor and discrete soil matrix
for locating hot spots of groundwater contami-
samples, however, also requires that the grab
nation (Marrin and Thompson 1987, Marrin and
samples collected for analysis be representative
Kerfoot 1988) or residual product in the overbur-
of the in-situ conditions. Several studies have
den (Spittler et al. 1985). In both cases, the soil-
shown that procedures requiring multiple hand-
vapor measurements provide guidance for plac-
ling operations often result in underestimating
ing monitoring wells or obtaining soil borings, or
soil VOC concentration by one to three orders of
both. Thereafter, the collection and analysis of
magnitude (e.g., Urban et al. 1989, Illias and Jae-
discrete aqueous or soil samples are used to aid
ger 1993, Hewitt et al. 1995, Liikala et al. 1996).
risk assessment decisions, to establish both the
As an example of how fast the VOC concentra-
horizontal and vertical profiles of contamination,
tion can change, a bulk sample (in a 7.5-cm-long
and to monitor the progress of remediation.
7.5-cm-wide core barrel liner), consisting of a
More recently, soil-gas sampling has been
silty-sand matrix experiencing minimal disrup-
applied to the study of the fate and transport of
tion, can lose greater than 90% of these analytes
VOCs in the vadose zone. One of the major con-
in less than 40 minutes (Hewitt and Lukash
cerns of this effort was to establish whether or
1996).
not vapors originating from a residual contami-
In recognition of the problems with past sam-
nation source could affect water quality in the
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