Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 432

TABLE 9.7
Gas Chromatographic Measurements of Solvents,
Stabilizers, and Impurities

Fraction in Solution (wt%)

Solvents with Stabilizers
(S) and Impurities (I) Before Use In Sump Distilled

Trichloroethylene 96.24 98.89 95.41

1,2-Butylene oxide (S) 0.5 0.22 0.78

Methyl chloroform (I) 0.022 0.015 0.089

1,1,2-Trichloroethane (I) 0.099 0.15 0.046

Unknowns 3.14 0.72 3.67

Methyl chloroform 96.25 96.34 96.21

sec -Butanol (S) 0.97 0.80 1.12

1,4-Dioxane (S) 0.91 1.50 0.80

1,3-Dioxolane (S) 0.84 0.75 0.95

Nitromethane (S) 0.57 0.34 0.77

1,2-Butylene oxide (S) 0.32 0.25 0.38

1,2-Dichloroethane (I) 0.12 0.13 0.11

Trichloroethylene (I) 0.0087 0.013 0.0074

Cyclohexane (S) 0.0049 0.0046 0.0088

Nitroethane (S) 0.0047 0.0063 0.0027

Source: After Holt, R.D., 1990, Physical properties of contaminated TCE and
1,1,1-trichloroethane. Technical Communications. Kansas City, Missouri:
Allied Signal Aerospace Company.

9.2 RATIO ANALYSIS
At many large-scale solvent release sites, passive remediation approaches such as monitored natural
attenuation (MNA) are favored where risk can be managed and where processes that degrade or
destroy contaminants can be demonstrated. The MNA approach is often a cost-effective and pre-
ferred remedial strategy where the contaminated property has a single owner or where long-term
passive remediation will not interfere with benei cial land and groundwater uses.
To demonstrate MNA, remediation consultants must prove that natural processes, whether
abiotic or microbially mediated, are transforming the solvents and associated chemicals into break-
down products that are less harmful, thereby decreasing risk and reducing contaminant mass. This
demonstration is often attempted by plotting changes in ratios of the molar concentrations of the
contaminant and its breakdown products with distance from the source. The ratio can serve as an
indicator of the degree to which the breakdown reaction or microbial process has progressed.
However, the original contaminant and its breakdown product may behave differently in the subsur-
face, leading to different rates of migration. For example, the molar ratio in a down-gradient moni-
toring well may rel ect differences in the rate of adsorption due to the organic carbon partition
coefi cients for these compounds (TCE has an average K oc value of 101, while cis -1,2-dichloroethylene
has an estimated K oc value of 240). Consequently, the molar ratios of these two compounds may
rel ect both the rate of biotransformation and differences in rates of migration. Because the more
mobile compound is likely to migrate farther down-gradient, the ratio of the more mobile compound
to the less mobile compound will decrease in monitoring wells located near the source zone and
increase farther from the source zone. Similarly, the ratio of the compound more resistant to degra-
dation to the compound more prone to degradation will increase with time, especially when the
compound more prone to degradation produces the more resistant compound as a daughter product