Chemical Preservation of
Volatile Organic Compounds in
Soil Subsamples
ALAN D. HEWITT
were consistent with the published degradation
INTRODUCTION
half-lives extrapolated from aqueous aerobic sys-
Soil samples collected during hazardous waste
tems: 4 to 22 days for Ben and Tol and 4 weeks to 1
site investigations for determining the presence
year for TDCE and TCE (Printup 1991). Thus, even
and concentration of volatile organic compounds
though the breakdown products were not identi-
(VOCs) can be stored, according to current proto-
fied, the results for aqueous media strongly sug-
cols, for up to 14 days at 4C prior to analysis (U.S.
gest that the losses of Ben and Tol were due to bio-
Environmental Protection Agency 1986). Lab-
degradation (Hewitt 1994c).
oratory studies addressing this holding period
For this reason, when soils subsamples are held
have shown large losses of VOCs when soil sub-
up to 14 days prior to analysis, preservation mea-
samples were held under these conditions (Jack-
sures other than, or in addition to, refrigeration
son et al. 1991, Maskarinec et al. 1992, King 1993).
are necessary to inhibit the biological degradation
In addition, studies associated with site investiga-
of some VOCs. Two logical methods of chemical
tions using bulk sample collection and handling
preservation are the use of acids and solvents.
protocols have shown that the length of storage is
Acidification to a pH of 2 is already a recommend-
an important variable affecting the VOC concen-
ed practice for water samples collected for the an-
trations (Hewitt 1994a,b). In general, the losses ob-
alysis of VOCs, while solvents are often used to
extract these compounds from soils, particularly
when concentrations are expected to exceed 1 g/g
analyte vapor pressure and were independent of
analyte chemistry, i.e. halogenated recalcitrant
(U.S. Environmental Protection Agency 1986).
compounds were lost as fast as readily degradable
For this study methanol (MeOH) and sodium
hydrocarbons (Suflita 1989, Russell et al. 1992). For
bisulfate (NaHSO4) were the solvent and acid
these reasons the observed reductions in VOC con-
tested. Methanol is often the solvent of choice for
centrations can be attributed to volatilization.
the extraction and dilution of VOCs, while the so-
To my knowledge there has only been one study
dium salt of sulfuric acid has been successfully
that has clearly demonstrated that VOC concen-
used to inhibit biodegradation of VOCs in labora-
trations in soil are subject to loss mechanisms in-
dependent of volatilization when refrigerated at
ples (Maskarinec et al. 1990). In addition to using
4C and stored for 14 days (Hewitt 1994c). This
study demonstrated that under aerobic conditions,
experiment was performed to determine if el-
concentrations of trans-1,2-dichloroethylene
evated concentrations of the VOCs could suppress
(TDCE) and trichloroethylene (TCE) tend to remain
biological activity.
This study, then, consisted of four experiments.
stable, while benzene (Ben) and toluene (Tol) lev-
The first experiment varied the analyte concentra-
els rapidly decrease, sometimes to below detec-
tion while following the protocols that successfully
tion. The rate of loss of these two hydrocarbons
was even greater when subsamples were held at
room temperature (22C). The observed losses
matic compounds (Hewitt 1994c). After the fortifi-