SOIL GAS VOLATILE ORGANIC COMPOUND CONCENTRATION
CONTOURS FOR LOCATING VADOSE ZONE NONAQUEOUS PHASE
DAVID J. WILSON
Eckenfelder Inc., 227 French Landing Drive, Nashville, Tennessee 37228, U.S.A.
(Received 12 April 1996; accepted 11 September 1996)
Abstract. Diffusion of VOC from cylindrical vadose zone domains contaminated with distributed
nonaqueous phase liquid is examined in the steady state approximation. The effects of ﬁrst-order
biodegradation and of an extended underlying pool of LNAPL ﬂoating on the water table below the
cylindrical domain are explored by means of an orthogonal functions approach. The distribution of
VOC in the vadose zone above a ﬂoating circular LNAPL pool of ﬁnite size is explored by a numerical
method. The results provide information useful in estimating the spacing and depth at which soil gas
samples should be taken in order to detect the presence of such source areas. Comparison of model
results with soil gas VOC measurements at various depths also provides insight into the extent to
which natural biodegradation of the VOCs is taking place.
Key words: modeling, soil gas surveys, vadose zone, volatile organic compounds
The use of soil gas surveys as a tool for the assessment of hazardous waste sites
involvingvolatileorganic compounds (VOCs) is now a standard procedure. Pankow
and Cherry (1996) have reviewed the technique, and its general principles were
reviewed by Marrin and Thompson (1987) and Marrin and Kerfoot (1988); these
authors also presented several case histories. The technique is not without its
limitations, asdiscussedbyMarrin(1988);for instance,it is not effectivein locating
contaminant plumes below the water table, as demonstrated by Rivett (1995) in
ﬁeld experiments. Despite its limitations, however, it often yields quite valuable
information and its convenience and low cost make it a very useful screening tool.
See also McCarthy and Johnson (1992).
If a portion of the vadose zone is contaminated with residual nonaqueous phase
liquid (NAPL), there is a ‘halo’ of elevated VOC concentrations in soil gas in
the vadose zone surrounding this domain which, because of its greater extent, is
generallyeasier to locate than the NAPL-contaminated domain itself. If the Henry’s
constant for the VOC is large enough that appreciable concentrations will exist in
the gas phase, if sorption on the solid phase is not too great, and if the soil is
not too heterogeneous, the diffusion process by which the VOC ‘halo’ is formed
can be mathematically modeled. These results can then be used, together with
soil gas measurements, to locate the NAPL source areas in the vadose zone. Such
information is quite useful in siting soil vapor extraction wells.
Environmental Monitoring and Assessment 48: 73–100, 1997.
1997 Kluwer Academic Publishers. Printed in the Netherlands.
INTERLINIE - DATA: pc3/emas4801/emas271: PIPSnr.: 121203 SPACKAP
emas271.tex; 13/10/1997; 10:27; v.7; p.1