Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 163

where R F is the retardation factor . For compounds that are strongly sorbing, u p u 0
and the pollutant concentration front is slowed down considerably. For compounds that
are nonsorbing (such as chloride ions), u 0 = u p and no retardation is seen. McCarty
et al. (1992) report the results of an experiment in which the retardation of various halo-
genated compounds present in a reclaimed municipal wastewater was injected into an
aquifer in Palo Alto, CA. The fractional breakthrough in an observation well downfield
was obtained for three adsorbing pollutants (chloroform, bromoform, and chloroben-
zene) and a nonsorbing tracer (chloride ion). The results are shown in Figure 4.11. The
field-measured retardation factors were 6 for chloroform and bromoform, and 33 for
chlorobenzene. Clearly the greater the hydrophobicity of the pollutant, the slower its
movement in the aquifer. Retardation is an important process in groundwater for two
main reasons. Firstly, if an aquifer were to become polluted with compounds of dif-
fering hydrophobicity, they would tend to appear in a down-gradient well at different
times in accordance with their retardation factors. This would make the concentrations
and nature of water at the observation well quite distinct from the original contami-
nation, and hence identification of the pollution source will be difficult. Secondly, the
retardation factor will give us an idea of how much material is on the solid phase and
how much is in free water, and therefore develops appropriate remediation alternatives
for the restoration of both the groundwater and the aerial extent of the contaminated
aquifer.

TABLE 4.12
Correlations between log K oc and log K ow for Various Compounds
of Environmental Significance

log K oc = a + b log K ow

r 2 Compound Class B a References

Pesticides 0.544 1.377 0.74 Kenaga and Goring (1980)

Aromatics (PAHs) 0.937 − 0.006 0.95 Lyman et al. (1982)

Aromatics (PAHs ) 1.00 − 0.21 1.00 Karickhoff et al. (1979)

Herbicides 0.94 0.02 — Lyman et al. (1982)

Insecticides, fungicides 1.029 − 0.18 0.91 Rao and Davidson (1980)

Phenyl ureas and carbamates 0.524 0.855 0.84 Briggs (1973)

Chlorinated phenols 0.82 0.02 0.98 Schellenberg et al. (1984)

Chlorobenzenes, PCBs 0.904 − 0.779 0.989 Chiou et al. (1983)

PAHs 1.00 − 0.317 0.98 Means et al. (1980)

Polychlorinated biphenyls 0.72 0.49 0.96 Schwarzenbach and Westall

(1981)

Note: K oc is in L/kg or cm 3 /g.