Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 172

1.0E + 02

TCA
DCA
CA

1.0E + 01

1,4-dioxane, 15 mg/L

TCA MCL
in CA = 200 mg/L

1.0E + 00

1.0E - 01

1.0E - 02

1,1-DCA CA MCL 5 mg/L

1 , 4 - diox a ne C A NL 3 m g / L

0

0 250 500 750 1000 1250 1500 1750

Plume centerline distance (feet)

FIGURE 3.10 BIOCHLOR-modeled transport of chlorinated ethanes and 1,4-dioxane, 10-year release sce-
nario. BIOCHLOR parameters include a 10-year continuous source release; source concentrations: 100 mg/L
TCA, 15 mg/L 1,4-dioxane. California regulatory and advisory standards are shown for comparison.
NL = notii cation level; MCL = maximum contaminant level (see Chapter 6 for explanation of NL and MCL).

Dispersion is a term inclusive of the physical processes that cause a plume to shear. Fixed values
for longitudinal dispersivity, horizontal transverse dispersivity, and vertical transverse dispersivity
( Table 3.23 ) were used in all model runs. The BIOCHLOR model was run several times by using
different source durations with an initial aqueous concentration of 100 mg/L methyl chloroform and
zero initial concentrations of degradation products. Separate trials were performed for 1,4-dioxane
at initial concentrations of 3 and 15 mg/L. The 3 mg/L scenario represents virgin methyl chloro-
form released to groundwater, whereas the 15 mg/L scenario is intended to represent the release of
vapor degreasing still bottoms enriched with 1,4-dioxane because of partitioning in the vapor
degreasing process, as described in Chapter 1.
BIOCHLOR only accounts for subsurface movement of dissolved-phase solvents; dense non-
aqueous-phase liquids and vapor transport are not considered. The initial concentration of methyl
chloroform modeled, 100 mg/L, is less than 10% of the overall solubility of methyl chloroform.
1,4-Dioxane persists over a longer distance from the source than the chlorinated ethanes owing to
its ini nite solubility, lower sorption, which is incorporated in the lower retardation factor ( R f )
reported by Jackson and Dwarakanath (1999), and an assumed lack of biodegradation. Calculated
migration using BIOCHLOR shows that 1,4-dioxane concentrations will remain higher than regula-
tory thresholds over greater distances. Figure 3.10 illustrates the distance from the source at which
the contaminant concentration reaches the regulatory level as a function of the source lifetime (see
Table 3.23). Figure 3.11 shows the distances that 1,4-dioxane is likely to migrate when released
under different duration scenarios and different source concentrations.

3.5 DIFFUSIVE TRANSPORT OF 1,4-DIOXANE AND STORAGE IN
FINE-GRAINED SOILS
Transport in i ne-grained soils composed predominantly of clay and silt plays an important role in
the overall subsurface fate on groundwater contaminants. In clays found in aquitards and in the
engineered clay liners formerly used as the primary liner for landi lls and waste lagoons, contami-
nant migration by advective transport is very slow. Where the aquitards or liners have low hydraulic