Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 434

analysis to assess the rates of methyl chloroform breakdown. In a study that analyzed groundwater data
collected over a 10-year period from a methyl chloroform release site, methyl chloroform exhibited i rst-
order kinetics and a half-life of 2.9 years at 15°C for the abiotic transformation to 1,1-dichloroethylene
and acetic acid (Wing, 1997). This reaction is described further in Section 9.2.2.
Methyl chloroform is less soluble and has a higher afi nity for soil organic matter than 1,1-
dichloroethylene; therefore, its migration is relatively retarded. Because 1,1-dichloroethylene is less
prone to degradation, ratios of 1,1-dichloroethylene to methyl chloroform should increase with
distance from the source zone. The superimposed effect of retardation may complicate the interpre-
tation of methyl chloroform degradation. To determine the degree to which retardation hampers
interpretation of degradation rates, McNab and Narasimhan (1994) modeled data from the Lawrence
Livermore National Laboratory (LLNL) site by using a transport code that accounts for sequential
decay chains. The modeling effort led to the conclusion that the effect of degradation of methyl
chloroform to 1,1-dichloroethylene overwhelmed the effect of retardation at the LLNL site (McNab
and Narasimhan, 1994). This conclusion suggests that sorption, dispersion, and matrix diffusion
play only a minor role in the ratios of methyl chloroform to 1,1-dichloroethylene and so permit
their use to interpret degradation rates. Their modeled methyl chloroform abiotic degradation rate
(half-life of two years) agreed well with literature values. It may therefore be unnecessary to use
1,4-dioxane as a control on migration rates when using ratios to interpret abiotic degradation of
methyl chloroform; however, 1,4-dioxane may still be a useful tracer for interpreting rates of reduc-
tive dechlorination of methyl chloroform to 1,1-dichloroethane.

9.2.2 U SING R ATIOS TO A PPROXIMATE THE D ATE OF A R ELEASE
Ratios of 1,1-dichloroethylene to methyl chloroform can be used to infer the timing of a release
when the release was episodic in nature. “Dating” a plume applies knowledge of the degradation
half-life for the chemical and the source-zone concentration to estimate when the chemical was
released into the subsurface (Morrison, 2006). The ratio approach to determining the timeframe of
a methyl chloroform release requires knowledge of (1) the rate of chemical hydrolysis of methyl
chloroform to acetic acid and (2) the rate of the elimination reaction to form 1,1-dichloroethylene
based on laboratory data or i eld measurements. Several assumptions are also required (Gauthier
and Murphy, 2003): *

• The rate of hydrolysis is constant over time.

• Because the hydrolysis of methyl chloroform exhibits i rst-order kinetics and is relatively

independent of pH between 4 and 9, its rate depends only on temperature. Assuming a
constant rate of hydrolysis implies an assumption of constant groundwater temperature.
Biodegradation of methyl chloroform and 1,1-dichloroethylene is either unimportant or can •

be adequately modeled.
Chromatographic separation effects such as differing volatilization rates or sorption rates •

do not skew the ratio of 1,1-dichloroethylene to methyl chloroform, that is, they are either
negligible or can be modeled.

The rate of the elimination reaction is i rst order; therefore, the Arrhenius equation can be used
to describe the temperature dependence of the rate of methyl chloroform degradation (Gauthier and
Murphy, 2003). The general form of the Arrhenius equation is shown as follows:

k = A e E a RT , (9.2)

* The ratio approach to establishing the timeframe of a release described here summarizes Gauthier and Murphy's (2003)
original article in the Journal of Environmental Forensics , to which the reader is referred for a more complete treatment
of the topic: “Age Dating Groundwater Plumes Based on the Ratio of 1,1-Dichloroethylene to 1,1,1-Trichloroethane: An
Uncertainty Analysis,” Thomas D. Gauthier and Brian L. Murphy, Exponent, Tampa, FL, USA. Used with permission.