Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 427

TABLE 9.5
Comparison of Effective Solubility and Corresponding Groundwater
Concentrations of Trichloroethylene and Perchloroethylene as a Manufacturing
Impurity in Trichloroethylene Vapor Degreasing Waste

Parameter Trichloroethylene Perchloroethylene Ratio

Molecular weight (Da) 131.4 165.8 —

Aqueous solubility (mg/L) 1100 150 7.33

Mass fraction in still bottoms a 99.75% 0.25% 400

Mole fraction in still bottoms a 0.998 0.001982 503.5

Effective solubility (mg/L) 1097.8 0.2974 3692

Probable groundwater concentration (μg/L) 15,000 4 —

Source: Lane, V. and Smith, J., 2006, Fact or i ction? The source of perchloroethylene contamination in ground-
water is a manufacturing impurity in chlorinated solvents. Presented to the National Groundwater
Association Ground Water and Environmental Law Conference, July 7, 2006. Chicago, Illinois:
National Groundwater Association. http://www.ngwa.org (accessed August 2, 2006).
Notes: This analysis ignores volatilization that may occur at the surface of the spill site or in the unsaturated
zone, differential sorption in transit to the saturated zone, diffusion, biodegradation, and other fate
processes that would alter the probable groundwater concentration. The 15 ppm concentration of
trichloroethylene in groundwater is an assumed value found at more highly contaminated trichloro-
ethylene release sites; the 4 ppb concentration of perchloroethylene is calculated from the trichloro-
ethylene value.
a After assumed 500-fold concentration.

other solvent wastes that are the source of the many chlorinated solvent plumes and the subject of
ongoing cleanup. The resulting high effective solubility of 1,4-dioxane explains the elevated con-
centrations at which it is sometimes encountered at methyl chloroform release sites. Table 9.6 also
proi les the potential for other solvent stabilizers to become concentrated in waste and preferentially
dissolve into groundwater. For example, 1,3-dioxolane is rarely analyzed in groundwater samples;
yet it may be present at some dichloromethane release sites. However, where 1,3-dioxolane was a
stabilizer of methyl chloroform, the small boiling-point difference (only 4°C) does not favor enrich-
ment: a starting formulation of 2 wt% 1,3-dioxolane in methyl chloroform will have an ending
1,3-dioxolane concentration of 4.5 wt% after seven weeks and an effective solubility only seven
times higher than methyl chloroform. This small increase could still be sufi cient to produce detec-
tions useful as a marker chemical, but it is unlikely that a 1,3-dioxolane plume would develop at a
methyl chloroform release site.
The numerous assumptions necessary to estimate partitioning and calculate stabilizer concentra-
tions will not provide a technically rigorous or legally defensible analysis of the likely origin of
solvents, as indicated by detections of stabilizers in groundwater. The most reliable evidence for
stabilizer enrichment in vapor degreasing is laboratory analyses done in the context of industrial
engineering studies. The data from Bohnert and Carey (1991) and Tarrer et al. (1989, 1993) are
based on such studies, as are the data from Holt (1990), presented in Table 9.7 . The Holt study mea-
sured a 65% increase in 1,4-dioxane concentration in a vapor degreasing sump, which corroborates
the analyses by Tarrer et al. in which a 68% increase in 1,4-dioxane in the sump was measured. The
decrease in 1,4-dioxane concentration in solvent recovered by distillation (the last column in Table
9.7) is due once again to the boiling-point difference; 1,4-dioxane remains behind in the distillation
process, as discussed in Section 9.1.1 .