Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 175

TABLE 3.25
Characteristics of the Sarnia Clay in 1,4-Dioxane Diffusion Study

Property Value

Specii c gravity 2.73

Saturation 100%

Dry density 1.68 g/cm 3

Calcite/dolomite ratio 0.42

Moisture content 23%

Porosity 39%

Organic carbon content, f oc 0.58

Cation-exchange capacity 10 meq per 100 g dry weight

Minerals in < 0.074 mm fraction

Calcite and dolomite 34%

Quartz and feldspars 15%

Illite 25%

Chlorite 24%

Smectite 2%

Grain-size distribution

45%

Clay (<0.002 mm)

Silt (0.074-0.002 mm) 43%

Sand (2.0-0.074 mm) 10%

2%

Gravel (>2.0 mm)

Source: Barone, F.S., et al., Journal of Contaminant Hydrology 10(3): 225-250.

Distribution coefi cients ( K d ) were estimated from laboratory diffusion tests in plug samples of a
saturated and undisturbed clayey soil using i ve organic compounds, each tested alone at 300 mg/L
(Barone et al., 1992). A K d value of 0.17 mL/g was estimated for 1,4-dioxane, on the basis of a mea-
sured diffusion coefi cient of 4 × 10 −6 cm 2 /s in the clayey soil. The characteristics of the soil tested
are listed in Table 3.25. The results of the soil plug diffusion tests are listed in Table 3.26 .
The high retardation factor, 2.19, for 1,4-dioxane calculated in Table 3.26 is twice the 1.1 value
obtained by Priddle and Jackson (1991). The higher retardation factor calculated by using data from
Barone et al. (1992) rel ects the low hydraulic conductivity of the Sarnia clay used for diffusion and
distribution coefi cient testing.
The ratio of the measured effective diffusion coefi cient for a water-borne contaminant in soil to
the calculated diffusion coefi cient of that contaminant through pure water represents the tortuosity
of the soil. As a compound diffuses in a porous medium, the rate of diffusion is reduced by the
increased length of the molecule's path as it travels around solid particles in the soil. The length of
the path traveled by a diffusing molecule relative to the straight-path distance is called the tortuosity
factor,

τ (Bear, 1972). Tortuosity can be derived from diffusivity as follows:

L
LD

D

P

t= =

e

ªf ,

(3.43)

e w

where L is the length of a straight path through the porous media, L e is the length of a tortuous
path around solid grains, D e is the effective diffusion coefi cient of the water-borne contaminant,
and D w is the calculated diffusion coefi cient of the contaminant in pure water (Bear, 1972;
Hoffman et al., 1998). The last expression, ϕ

P , provides an empirical way to obtain tortuosity,