Regulatory Policy Implications of 1,4-Dioxane - Environmental Investigation and Remediation - page 460

interval underlying a coni ning layer, the recharge location from which the well ultimately draws
water may be located a considerable distance upgradient from the well. Delineating capture zones
can be complex because the zones are three-dimensional and occupy aquifers with a range of
hydraulic conductivities. Variable pumping rates, geologic controls over groundwater movement,
vertical l ow within the well, interference from the operation of other nearby wells, variable sources
of recharge such as rainfall runoff or dam releases, and vertical conduits facilitating l ow across
aquitards all add to the complexity of assessing water supply well capture zones (Einarson and
Mackay, 2001). In addition to conventional i nite difference and i nite element modeling, analytic
element modeling is often used to simulate the three-dimensional water supply well capture zone to
delineate wellhead protection areas (e.g., WhAEM, USEPA's Wellhead Analytic Element Model).
When the 1,4-dioxane release site lies entirely within the delineated capture zone of a water sup-
ply well, the mass discharge of 1,4-dioxane from the release site will constrain the maximum pos-
sible concentration of 1,4-dioxane that could be detected in groundwater pumped from a supply
well. Contaminants arriving at the well screen are often substantially diluted in supply wells because
they typically occupy a narrow radial l ow tube, whereas the remaining radial l ow tubes are usually
occupied by clean water. Moreover, many municipal supply wells are screened in multiple intervals.
Contaminants often arrive at the shallowest screened interval, while clean water is concurrently
pumped from deeper screened intervals. The combined radial and vertical contributions of clean
water to the well result in substantial dilution of contaminants entering the well. There are probably
instances in which plumes of 1,4-dioxane have arrived at and entered water supply wells but have
not yet been detected because the concentrations are below current detection limits due to in-well
blending with clean water (or because 1,4-dioxane is not analyzed for insamples from the well).
Antidegradation policies prohibit discharges that degrade water quality, regardless of whether a
water supply well becomes contaminated. The antidegradation approach protects all current and
future benei cial uses of the groundwater resource to preserve the highest quality groundwater.
Therefore, it is generally unacceptable to manage a contaminated site cleanup based on whether a
supply well will become contaminated, or to use dilution in a supply well as a contamination mitiga-
tion measure. Nevertheless, determining the likelihood that a well will become contaminated from
a known release within the well capture zone can be useful to help water utilities adjust monitoring
priorities and operations. The analysis of mass discharge from release sites provides a simple method
to estimate the maximum contam inant concentrations that can be expected to be detected in ground-
water samples from the well (Einarson and Mackay, 2001).
The mass discharge of contaminant migrating from a release site located within a well capture
zone should equal the contaminant mass discharge from the well if there is a continuous source that
releases the contaminant at a constant rate with conservative transport, that is, contaminant retarda-
tion from sorption and diffusion are negligible and the contaminant is not transformed abiotically or
through biodegradation (Einarson and Mackay, 2001). The maximum concentration of the contami-
nant that could occur in a downgradient supply well is obtained from the following relationship:

M d

____

C SW =

Q SW ,

(10.1)

where C SW is the maximum concentration of contaminant in water extracted from the supply well
(mass/volume), M d is the mass discharge from the release site (mass/time), and Q SW is the pumping
rate from a supply well (volume/time) (Einarson and Mackay, 2001). *
The total mass of contaminant leaving the release site per unit of time (e.g., grams per day) can
be evaluated by performing detailed plume proi ling in a transect of multilevel monitoring wells or
a one-time investigation using direct-push grab-sampling methods (e.g., cone penetrometer testing

* The details for applying the mass discharge approach to estimating the maximum anticipated concentration of a contami-
nant in a supply well are provided in the online supplemental material to Einarson and Mackay's 2001 article, “Predicting
Impacts of Groundwater Contamination,” Environmental Science and Technology 35(3): 66A-73A.