Sampling and Laboratory Analysis for Solvent Stabilizers - Environmental Investigation and Remediation - page 191

on a wider range of organic compounds than is included on the standard method list. For example,
the typical laboratory report for Environmental Protection Agency (EPA) Method 8260 (USEPA,
2006c), “Volatile Organic Compounds by Gas Chromatography/Mass Spectrometry (GC/MS),”
includes far fewer than the 94 compounds listed in Method 8260; often only 33 compounds, or an
extended list of 62 compounds, will be reported by commercial laboratories. The target compounds
in EPA Method 8260 analyses at commercial laboratories most often exclude 1,4-dioxane. The
compounds reported can vary from laboratory to laboratory and project to project. Project manag-
ers may specify which target analytes to include for each method requested (Simmons, 1997). The
choices of sample preparation, chromatographic column, detector, calibration standards, and mass
spectrometer settings all affect the ability of a particular analysis to detect and quantify compounds.
In order to favor detection of chlorinated solvents at low concentrations, other compounds are
typically excluded from reporting.
For laboratory methods using GC-MS, the chemist has the option of looking at nontarget peaks
and making tentative identii cations by comparison with a library of mass spectra. Tentatively iden-
tii ed compounds (TICs) are usually reported with an estimated concentration, because the instru-
ment is not calibrated for TICs. The USEPA's Contract Laboratory Program (CLP) requires that
laboratories report up to 20 TICs per sample. Some laboratories have i xed prices for reporting the
10 or 20 most commonly encountered nontarget compounds (Simmons, 1997).
A variety of ancillary benei ts can be obtained from knowing the full complement of anthropo-
genic compounds in groundwater samples at solvent-release sites. As discussed in Chapter 3, the
chlorinated solvents targeted in an investigation are often recalcitrant and retarded relative to
groundwater l ow rates. Hydrophilic compounds introduced as solvent stabilizers can be helpful for
mapping the full extent of a plume, as they are retarded to a much smaller extent and therefore
behave like a conservative tracer. Knowing the migration pathways traversed by the more mobile
contaminants can be useful for predicting the likely direction of migration of less mobile contami-
nants released at the same time. Further, parameters of plume dispersion such as dispersivity can be
inferred from the distribution of the most mobile contaminant. Knowledge of these migration
parameters aids modeling site history and remedial alternatives. Tracking molar ratios of chlori-
nated solvents (which biodegrade slowly under certain ambient conditions) to 1,4-dioxane (which is
not expected to biodegrade under ambient conditions) can also be instructive for analysis of moni-
tored natural attenuation or enhanced biodegradation as a remediation strategy for chlorinated
solvents.
The consequences of not analyzing for a wider variety of analytes and not requesting that TICs
be reported at solvent-release sites may include the following: (1) The site characterization may be
incomplete (see Chapter 8); (2) the capture zone and treatment system design may be ineffective in
protecting drinking water (see Chapter 8); (3) the risks to drinking water consumers may not be
fully addressed (see Chapter 6); (4) adjudication of source allocation in legal disputes may be
erroneous (see Chapter 9).
Unfortunately, 1,4-dioxane is not a benign tracer. At sufi ciently high concentrations, it can cause
health effects when consumed, as explained in Chapter 5. Therefore, an expanded analysis of
samples for 1,4-dioxane at solvent-release sites should be considered.
In several conference presentations, Christian Daughton of the USEPA summarized the inherent
i ltering process that results from following the paradigm of restricting analyses by method analyte
lists. Daughton has noted that nearly 28 million organic and inorganic substances had been docu-
mented as of April 2006 (as indexed by the American Chemical Society's Chemical Abstracts
Service in their CAS Registry); 10 million of these are commercially available. Only 240,000 of
these compounds are subjected to any form of regulation. Of these 240,000, a much smaller number
of chemicals are subjected to routine laboratory analysis of drinking water or environmental sam-
ples. A still smaller number of chemicals have been subjected to sufi cient study to provide reliable
information about health effects (Daughton, 2005).