Regulatory Policy Implications of 1,4-Dioxane - Environmental Investigation and Remediation - page 461

with discrete interval groundwater sample recovery). The transect must be oriented orthogonal to
the primary groundwater l ow direction. The resulting geologic and contaminant concentration data
can then be integrated to estimate the rate of groundwater l ow and contaminant mass migrating
across the grid cells in an imaginary plane dei ned by the contaminant concentrations at each verti-
cal interval in the transect, yielding the estimated mass discharge from the release site. The estimate
is improved where two or more orthogonal sampling transects are installed, allowing calculation of
M d at different points along the length of a dissolved plume (Einarson and Mackay, 2001).
M d can serve as the objective function for the optimization of site remediation. Ideally, the dis-
charger will control the source zone so that M d 0, and downgradient supply wells are not impacted.

At 1,4-dioxane sites, it is often the case that the releases are old enough such that 1,4-dioxane migra-
tion has already progressed well beyond the point from which source control would be useful.
For two supply wells down-gradient of plumes, in which all attributes of the plumes, hydrogeo-
logy, and wells are identical except for the pumping rates, the well with the higher continuous pump-
ing rate will have lower concentrations, because contaminant concentrations in continuously pumped
supply wells are inversely proportional to pumping rates (Einarson and Mackay, 2001). Therefore,
smaller supply wells and especially private wells are more vulnerable to contamination, because
there is less dilution by blending with clean water. Wells that pump less have a smaller chance of
capturing a plume of contamination, but if the plume does intersect the well capture zone, there will
be less dilution and therefore higher concentrations. The mass discharge approach is therefore use-
ful to anticipate impacts to private wells and small water supply wells.
Water utilities are very unlikely to perform mass discharge investigations by installing transects
of multilevel wells due to the high cost and because the responsibility is assigned to the discharger.
Because an important goal of groundwater cleanup projects is to protect against exposure to chemi-
cals through consumption of drinking water, dischargers should conduct mass discharge investiga-
tions where other plume attributes such as length, duration, and concentrations suggest that the
mass of contaminant present is sufi cient to impact down-gradient supply wells. High-resolution,
multilevel monitoring well transects to investigate mass discharge from contaminant release sites
yield benei ts to both the remediation team and to the operator of the down-gradient well. For
example, mass discharge investigations can be used to establish whether natural attenuation is
occurring or to estimate the rate at which contaminant transformation products are being formed
(Einarson and Mackay, 2001).

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10.2.5 M OVING B EYOND C HECKLIST C OMPLIANCE
It is not an accident that drinking water quality in the United States is among the world's best. The
American drinking water regulatory framework is highly developed and nearly eliminates the risk
of ingesting harmful chemicals in drinking water. State and federal regulatory agency staffs are
effective at ensuring that the regulations are enforced, and local government and private water util-
ity operators regularly deliver outstanding performance to ensure that American drinking water
remains safe. In the spirit of continuous improvement, there is nevertheless an opportunity to exam-
ine features of the current regulatory framework and ask whether some adjustments could bring
about additional safeguards against drinking water contamination.
As discussed in Section 4.1 (The Flawed Paradigm of Analyte Lists), when cleanup site investiga-
tions or drinking water testing programs rely on prescribed USEPA method lists of analytes without
checking for unidentii ed peaks in the gas chromatogram, it is likely that the presence of other pos-
sibly signii cant contaminants will be missed. Often, the analyses are performed by staff chemists in
the water utilities' laboratory. Many of these chemists hold advanced university degrees and are
capable of conducting research projects and analytical method development, yet they are relegated to
routine analyses using standard method lists to achieve regulatory compliance, without exploring the
complete chemistry of the samples they handle. Of course there are also exceptions; for example, see
the OCWD case study of method development for 1,4-dioxane analysis in Chapter 4.