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

Appendix D
uncontaminated organic matter-rich strata or other major sinks the dilution approach
taken here may underestimate attenuation. Such situations can be identified at the
site-specific objectives stage.
Provision of an arithmetic base for calculation requires other more explicit and detailed assumptions (see
Section 2.0)
2.0 Approach
2.1 Background
Many models exist that describe the transport dynamics of contaminants in soils and groundwaters (as
examples, see Rao and Jessup 1983, Jury and Godhrati 1989, Korfiatis et al. 1991, Piver and
Lindstrom 1991 and Toride et al. 1993). Most models are theoretically rigorous, mechanism-based
descriptions that focus on advective and dispersive processes but sometimes incorporate
biodegradation and fate of derived products. Mathematically, all involve complex partial differential
equations that can be solved only under restrictive boundary and continuity conditions. Numerical
solutions supported by sophisticated computer codes are usually needed. These models have been
developed for research purposes or for application to specific field sites and are scientifically sound.
The above approaches could form a basis for the estimation of a generic dilution factor. However,
mechanistic models have several properties that make them inappropriate for this application. First,
they are extremely parameter intensive. It is common for mechanistic models to require 20 or more
parameters. Many of these parameters are not readily available and can be measured or estimated only
with difficulty. Second, all descriptions are temporally-dependent whereas the generic protection sought
for groundwater is not -- see assumption 4 in Section 1.0 of this Appendix. Third, the mathematical
descriptions are abstract and complex, making them poorly accessible to many contaminated sites
stakeholders.
For guideline development, a more general approach is desired that is simple, practical, sensitive and
effective. An appropriate model should:
• be clearly documented and easily understood,
• require parameters that can be derived from readily available sources,
• include only the most influential parameters,
• apply to Canadian conditions, and
• be tuned to the appropriate temporal and spatial scales.
A first step in attaining these properties is to identify a minimum set of key parameters.
Key Parameters
144