Sampling and On-Site Analytical
Methods for Volatiles in Soil
and Groundwater
Field Guidance Manual
ALAN D. HEWITT AND KAREN F. MYERS
INTRODUCTION
a time scale measured in minutes. Biodegradation
losses are also a function of several variables, e.g.,
In soil and groundwater, volatile organic com-
indigenous microbiological population, chemical
pounds (VOCs) coexist in equilibrium among two
properties of contaminant analytes, temperature,
to three physical phases, i.e., liquid, solid (sorbed),
and length of sample storage prior to analysis
and gaseous (Minnich 1993, Lewis 1994). The fairly
(Hewitt 1995). The potential for biodegradation
rapid shifts in equilibrium among these phases
losses is not as great as that for volatilization losses,
have often confounded efforts to establish repre-
because they are not as rapid. Indeed, most ex-
sentative environmental concentrations. For in-
perimental evidence has shown that it often takes
stance, when performing the collection, handling,
at least a couple of days before losses solely caused
or storage of soil samples prior to analysis, un-
by biodegradation are significant.
controlled losses of VOC can easily bias the quan-
titative estimates by one or more orders of magni-
tude (Siegrist and Jenssen 1990, Urban et al. 1989,
SAMPLING
Illias and Jeager 1993, Hewitt et al. 1995, Liikala et
al. 1996, Smith 1996). In extreme cases, losses from
Soil sampling
poor collection and handling techniques with re-
Soil sample collection and on-site preparation
spect to a porous medium can result in false nega-
and analysis require a timely and well-orches-
tives, e.g., the failure to identify VOCs when they
trated protocol for both surface and subsurface
are present. In general, the greatest sources of in-
characterization activities. For example, subsur-
determinate error associated with the character-
face investigations require that an intact bulk
ization of VOC concentrations are volatilization,
sample be brought to the surface so that
and to a lesser extent, biodegradation.
subsamples can be transferred to prepared
Volatilization losses occur whenever gaseous
container(s) for the chosen method(s) of chemical
molecules, which have diffusion coefficients up to
and geotechnical analysis. With respect to VOC
four orders of magnitude greater than liquid dif-
characterization, the two most common methods
fusion coefficients, are allowed to move freely. The
for preparing discrete soil subsamples for instru-
extent to which VOCs can be lost by this mecha-
mental analysis are methanol extraction and di-
nism depends on the vapor phase concentration
rect vapor partitioning (i.e., purge and trap or
(analyte vapor pressure), duration and extent (sur-
headspace). Choosing which (or both) methods of
face area) of exposure, and the matrix porosity
subsample preparation should be used depends
(Siegrist and Jenssen 1990). Siegrist and Jenssen
on the data quality objectives and instrumentation
(1990) and Hewitt and Lukash (1996) have shown
used for analysis (e.g., site-specific action levels
that significant volatilization losses often occur on
and detection limits for the sample preparation
Previous Page
