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

Appendix C
Derivation of the Partitioning Relationship for Protection of
Groundwater
Numerous studies have shown that sorption of organics by soils is highly correlated with the organic
matter content (e.g., Chiou et al. 1979, Hassett et al. 1980). Chiou (1989) presents evidence that the
linearity of sorption with organic contaminant concentration and correlation with soil organic matter
content reflect dissolution of the organic contaminant into the soil organic matter phase -- as opposed to
sorption to organic matter surfaces. Normally a Freundlich isotherm is fitted to the sorption data:
Cs = 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 non-ionic 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). The relationship between solution
concentrations and the sorbed concentration is described in equation 1. The relationship between total
soil concentration and soil solution concentration can be derived using a simple method based only on
mass ratio of water over soil considerations.
Consider a unit dry mass of soil, M s , at field capacity moisture content ? m defined by
? m = M w /M s [2]
where
M w = mass of water
M s = dry mass of soil
The soil is contaminated by a chemical to a concentration Y (mg kg -1 dry mass basis) and the chemical
is partitioned between solid and aqueous phases of the moist soil per:
By mass balance we have:
K d = C s /C w [3]
Y × M s = (C s × M s ) + (C w × M w ) [4]
139