Forensic Applications for 1,4-Dioxane and Solvent Stabilizers - Environmental Investigation and Remediation - page 433

(McNab and Narasimhan, 1994). Determining rates of transformation from groundwater samples is
especially challenging—because the groundwater is moving, the solutes are subject to dispersion,
diffusion, and sometimes multiple pathways of biodegradation or abiotic degradation. Contaminant
migration is often retarded owing to sorption and matrix diffusion.
To discern the rates of biodegradation that are independent of variable migration rates among
parent and breakdown products, a conservative tracer can be useful to coni rm the rate of ground-
water l ow. Where 1,4-dioxane was released together with methyl chloroform, the 1,4-dioxane can
serve as a tracer. Molar concentration ratios of 1,4-dioxane to methyl chloroform and its elimination
reaction product, 1,1-dichloroethylene, can reveal patterns in the abiotic breakdown of methyl chlo-
roform. Similarly, ratios of 1,4-dioxane to methyl chloroform and its reductive dechlorination prod-
ucts, 1,1-dichloroethane and chloroethane, can proi le the rate of biologic transformation of methyl
chloroform. As a conservative tracer, 1,4-dioxane can demonstrate the degree to which physical
fates such as transverse dispersion or matrix diffusion may complicate the interpretation of molar
ratios to discern degradation rates. With sufi cient monitoring points, sampling for 1,4-dioxane can
reveal the degree to which subsurface heterogeneity at a site introduces uncertainty into the degra-
dation rates inferred from constituent ratios. This section examines the utility of ratio comparisons
to demonstrate MNA with 1,4-dioxane as a conservative tracer.
1,4-Dioxane has not been directly observed to undergo breakdown at i eld sites; * therefore, its
presence at a site may conl ict with the requirements for demonstrating MNA to the satisfaction of
the U.S. Environmental Protection Agency's (USEPA's) (1999) Directive on MNA. The USEPA
Directive notes that the fuel-oxygenate methyl tert -butyl ether (MTBE) migrates farther and threat-
ens down-gradient water supplies at the same sites where the migration of fuel constituents has
either reached steady state (i.e., plume stabilization) or diminished by natural attenuation. 1,4-
Dioxane released at methyl chloroform sites presents a similar dilemma. USEPA's Directive cautions
that compounds such as MTBE and 1,4-dioxane that are not readily degradable in the subsurface
and that may represent an actual or potential threat to the benei cial uses of groundwater should be
assessed when evaluating the appropriateness of MNA remedies (USEPA, 1999).

9.2.1 R ATIO A NALYSIS OF M ETHYL C HLOROFORM B REAKDOWN U SING 1,4-D IOXANE
When in use, methyl chloroform is usually stabilized against oxidative attack by and reaction with
metals and acids; once released to the subsurface, it breaks down fairly quickly due to hydrolysis
and biodegradation. Elimination reactions remove HCl from methyl chloroform to produce 1,1-di-
chloroethylene, and hydrolysis of methyl chloroform produces acetic acid. Reductive dechlorination
transfor ms methyl chlorofor m to 1,1-dichloroethane (Vogel and McCa r ty, 1987a, 1987b). The hydro-
lysis of methyl chloroform to acetic acid occurs about i ve times more rapidly than the elimination
reaction that produces 1,1-dichloroethylene (Vogel and McCarty, 1987b). Acetic acid (vinegar) is not
a contaminant of interest and is not usually monitored because it is quickly mineralized by soil
microbes. One can expect the detection of 1,1-dichloroethylene at concentrations of at least 5 μ g/L

where methyl chloroform has been in groundwater at concentrations greater than 120 μ g/L for more

than one year (Vogel and McCarty, 1987a).
The reported half-life of methyl chloroform in the elimination reaction has been variously cited as
ranging from 1.7 to 3.8 years at 20°C, whereas the yields of the abiotic elimination and hydrolysis reac-
tions of methyl chloroform range from 20% to 27% 1,1-dichloroethylene and 73% to 80% acetic acid,
respectively (literature survey summarized in Wing, 1997). For each molecule of 1,1-dichloroethylene
generated by the elimination of HCl from methyl chloroform, approximately three molecules of acetic
acid are concurrently generated by the hydrolysis of methyl chloroform (Wing, 1997). The fact that
1,1-dichloroethylene is itself resistant to both hydrolysis and biodegradation permits its use in ratio

* As this topic was going to press, Dr. Shaily Mahendra of UCLA reported evidence for biodegradation of 1,4-dioxane in
arctic soils; i ndings will be presented at the Battelle Chlorinated Solvents Conference in Monterey, CA, in May 2010.