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

Part C, Section 5
GEOCHEM (Mattigod and Sposito 1979), MINTEQ (Brown and Allison 1987)}. A generic,
straightforward, and quantitative treatment of metal contaminant partitioning is not available at this time.
An alternative approach to the problem of metal mobility in soils has been to develop guidelines for soil
conditions that restrict movement. For most metals, soils of moderate pH and high redox potential
restrict movement. Waterlogged and/or strongly alkaline or acidic soils fail to attenuate different classes
of metals. This "restrictive range" approach will be used pending review and evaluation of the potential
for application of speciation models to guideline development.
Recommended soil conditions for application of generic guidelines for the common contaminant metals
are described in "Evaluation and Distribution of Master Variables Affecting Solubility of Contaminants in
Canadian Soils" (Alder et al. 1994). For certain metals, notably cadmium, cobalt, nickel, zinc, and
some oxyanions, significant deviations from the assumed conditions must be considered during the
development of site-specific objectives.
Organic Contaminants
In contrast to metals, partitioning of non-polar organic contaminants in soils depends mainly on the
organic matter content and can be described by a simple isotherm (Hamaker and Thompson 1972):
C s = K d × C w
1/n
[1]
where
C s = concentration in solid phase (mg/kg)
K d = distribution coefficient
C w = concentration in aqueous phase (mg/L)
n = empirical constant
For most nonionic organics n = 1 and sorption is a linear function of equilibrium solution concentration
up to 60% to 80% of its water solubility (Hassett and Banwart 1989).
Over a broad range of soil types the distribution coefficient is directly related to the organic matter
content:
K d = f oc × K oc [2]
where f oc = fraction of organic carbon in soil
K oc = sorption coefficient for soil organic carbon
A particular chemical's K oc can be measured or predicted by correlating with the water solubility (S) or
n-octanol/water partition coefficient, K ow .
The relationship between total soil concentration of an organic contaminant and its concentration in pore
water is (Appendix C):
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