Protocol for the Derivation of Environmental and Human ... - CCME

Appendix D
Experience with various vadose zone transport models has shown that soil parameters such as hydraulic
conductivity and porosity are good predictors of leaching behaviour only over very short time scales.
Regardless of soil texture seasonal and yearly leaching fluxes appear more responsive to the balance
between precipitation and evapotranspiration rates (Hutson and Wagenet 1991), which in turn are
strong determinants of recharge. Therefore, recharge is a critical parameter in transport and dilution of
solutes in groundwater. The other critical parameter in the vadose zone is the organic carbon content.
Together, these two parameters appear to be the principal factors affecting the contaminant flux to
groundwater over macroscopic time scales.
It is clear that the observed extent of dilution of contaminants in recharge water reaching the saturated
zone must depend on the "effective" mixing volume compared to the magnitude of recharge. The
effective mixing volume, in turn, must depend on the width, length and depth of the groundwater zone
affected by the contaminated site and the velocity of groundwater flow. Ignoring lateral dispersion, this
affected zone should be delineated by the length and width of the contaminated site itself and the
effective depth of mixing.
In addition to K oc , a generic preliminary assessment of contaminant dilution in groundwater will depend
on recharge, groundwater flow, length and width of the contaminated site, and effective depth of mixing
in the aquifer. The following sections describe the development of a screening model using these
parameters, applicable Canadian ranges for the parameters, and application of the screening model to
the dilution problem.
2.2 Development of Dilution Expression
A generic, remediated site is shown in Fig. D.1. The site is established on level or gently sloping land
typical of a former industrial use. Unconsolidated, dominantly inorganic, surficial geologic materials form
the parent material for the soil and underlying regolith. Groundwater resources include an unconfined
aquifer which supplies, or may supply human needs. Hydrologic fluxes include precipitation (P),
evapotranspiration (ET), recharge (R), and surface runoff (U). A non-polar organic contaminant is
present in the soil at a concentration equal to the PSQG HH . In the contaminated soil zone the
concentration of contaminant in the pore water is governed by equation 6 of Appendix C. The
contaminant flux to groundwater is mediated primarily by the recharge flux.
Dilution of an organic contaminant in recharge water may be described by considering the contaminant
fluxes in recharge and groundwater to an effective mixing depth as it leaves the contaminated site (see
Fig. D.2). The vertical contaminant flux through the vadose zone is given by:
Q cv = C sw × R × A [1]
where Q cv = vertical contaminant flux mg yr -1
C sw = concentration of contaminant in soil water (recharge), mg/m 3
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