Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 141

organic matter and is referenced as the solubility of the organic compound into the soil organic mat-
ter, which is generally proportional to the inverse of aqueous solubility (Chiou and Kile, 1994).
Estimated values of K oc are often used in environmental fate assessment because measurement
of K oc is expensive and values are generally not transferable to other settings. Most published values
for K oc are estimated by using a variety of QSARs. More than 200 different correlations for estimating
K oc have been published. The correlations used to estimate K oc are based on compound solubilities,
octanol/water partition coefi cients, molecular factors, topological indices, solvation energy rela-
tionships, and others (Reinhard and Drefahl, 1999). Table 3.11 lists values for K oc estimated by using
the PCKOCWIN algorithm as well as values published in various chemical fate and transport prop-
erty compilations.
The variation among predicted values of K oc listed in Table 3.11 underscores the uncertainty
inherent to estimation of retardation due to sorption onto soil organic matter. Calculated values of
K oc can provide an order of magnitude estimate of retardation when combined with measured values
of the weight fraction of soil organic matter in aquifer solids. However, measured values of K oc must
address organic matter variability due to aquifer heterogeneity. In general, less soil organic matter
is present in higher-conductivity strata, and sorption to organic matter does not substantially con-
tribute to retardation in sands and gravels. K oc values in clays and silts are similar, but K oc values for
sands are generally 25% lower than for clays (McCarty and Reinhard, 1981).
When reviewing literature values of K oc , it is important to distinguish whether experiments used
whole soils or grain-size segregates, given the strong inl uence of particle size. Fine-grained, low-
conductivity sediments such as clays and silts are richer in organic matter and play a greater role in
retarding contaminant migration. The type of soil organic matter also makes a difference. Organic
matter in consolidated sediments that has been altered by compression and elevated temperature
(i.e., diagenesis) may behave differently than unaltered soil organic matter (Patterson et al., 1985).
PCKOCWIN (a soil adsorption coefi cient program) estimates K oc for organic compounds.
Earlier methods for estimating K oc relied upon relationships with the octanol/water partition coef-
i cient. PCKOCWIN uses a molecular connectivity index (MCI) to predict K oc values for hydropho-
bic organic compounds, but the MCI approach does not reliably estimate K oc values for hydrophilic
polar aprotic compounds such as 1,4-dioxane. * PCKOCWIN uses a series of group contribution
factors to predict K oc for polar compounds (USEPA, 2007a). The group contribution method outper-
forms the traditional estimation methods based on octanol/water partition coefi cients and water
solubility (Meylan et al., 1992). A comparison of PCKOCWIN estimates with experimentally deter-
mined distribution coefi cients shows that (1) the MCI-based prediction model provides unreliable
and inconsistent K oc values, for both polar and nonpolar compounds, and (2) the divergence between
experimental and calculated distribution coefi cients becomes larger with increasing molecular size
(Niederer et al., 2006b). K oc values generated by QSAR models should be used with caution. The
similarity of the compounds in the calibration data set for the model and the compound of interest
should be coni rmed (Niederer and Goss, 2007).
3.3.2.5 Predicting K oc from K ow , the Octanol/Water Partition Coeffi cient
The octanol/water partition coefi cient, K ow , is often used to predict the potential for a compound to
adsorb to the aquifer matrix. K ow is the ratio of the concentration of a chemical in octanol and in
water at equilibrium and at a specii ed temperature. Octanol is an organic solvent that is used as a
surrogate for natural organic matter. The relationship between the mass-specii c distribution coef-
i cient, K d , and the octanol/water partition coefi cient, K ow , has been i rmly established for nonpolar
organic compounds present in concentrations at less than half their solubility limit and in aquifers
whose organic carbon fraction is 0.1% or greater (McCarty and Reinhard, 1981; Schwarzenbach and
Westall, 1981). The total organic carbon content of the aquifer matrix has less bearing on the poten-
tial for sorption in compounds with low octanol/water partition coefi cients (Suthersan, 2002).
* A polar aprotic solvent is one that does not contain an O-H or N-H bond.