Leaking underground storage tanks are estimated to be the largest source of soil and groundwater contamination in North America by hydrocarbon compounds. Some of the more insidious groundwater contaminants are the chlorinated organics manufactured as solvents. The most common solvents include perchloroethylene (PCE, Cl₂C=CCl₂), trichloroethylene (TCE, ClCH=Cl₂C) and 1,1,1-Trichloroethane (TCA, CH₃CCl₃), used in a variety of industries. TCE, used in dry cleaning, is perhaps the most ubiquitous. With densities >1, these DNAPLs (dense, non-aqueous phase liquids) sink below the water table, leaving a residual pure phase that contributes to chronic contamination for decades. PCE and TCA have solubilities of 200 and 480 mg/L, respectively. The solubility for TCE is 1100 mg/L, compared to the maximum acceptable concentration (MAC) for drinking water of 0.05 mg/L (Environment Canada, 1995). A developing field of environmental isotope geochemistry is that of fingerprinting contaminants in groundwater. Such forensic geochemistry can identify perpetrators of pollution, and may offer insights into transport processes and fate. The isotopes used are ¹³C and ³⁷Cl.

 
Bartholomew et al. (1954) pioneered chlorine isotope research on organic compounds, showing that Cl–C bonding favoured the ³⁷Cl isotope. Fractionation of ³⁷Cl during manufacturing of chlorocarbons was demonstrated by Tanaka and Rye (1991). Variations in manufacturing processes and source materials then affect their ³⁷Cl and ¹³C contents (Warmerdam et al., 1995). This serves to distinguish products from different manufacturers (Fig. 6-20). Noteworthy is the strong variation in both d¹³C and d³⁷Cl for a single compound, reflecting differences in manufacturing processes. Measurement of these isotopes on solvents extracted from groundwaters at spill sites offers a tool to assign responsibility, and the cost of cleanup.
 

Fig. 6-20 The isotopic composition of chloro-organic solvents from four different manufacturers. One s standard deviations shown for each sample, based on 2 to 10 analyses (modified from Warmerdam et al., 1995).
 

It is yet to be established whether any isotope effects accompany dissolution of the pure phase. A more ambitious area of research is to use isotopes to observe degradation of these contaminants (either by biodegradation or reductive dehalogenation) in groundwaters.

Remediation of leaked and spilled products by extractive technologies (pump and treat or by soil removal) are expensive and often ineffective. Biodegradation, on the other hand, is an in situ technology that can operate passively or by addition of nutrients and air to accelerate the bacterial oxidation of the contaminant. Loss of the contaminant can also occur by simple volatilization, but this can contravene air quality regulations and signals an inefficient biodegradation system. The efficiency of biodegradation can be tested by measuring CO₂ and d¹³C_CO2 in the soil gas.

Controlled biodegradation of gas-condensate (C₅ to C₃₀ hydrocarbons) in laboratory experiments showed a steady increase in CO₂ from less than 3% to over 12% of the gas phase, accompanied by a decrease in O₂ from 18% to 11% (Aravena et al., 1996). Throughout, the d¹³C of the CO₂ remained within 1‰ of the bulk value for the condensate. Two field studies (Fig. 6-21) show an increase in the CO₂ content of the soil atmosphere as the contaminants are degraded. The d¹³C_CO2 decreases with the exponential increase in P_CO2, approaching asymptotically the d¹³C value of the contaminant. The isotope effects during degradation may include selective oxidation of certain components of the hydrocarbon by bacteria, rather than the quantitative degradation of the compounds (Stahl, 1980).

Fig. 6-21 Biodegradation of solvent and jet fuel in contaminated soils. Open symbols represent uncontaminated soils associated with contaminated soils (filled symbols). Data sources: TCE — Suchomel et al. (1990); Jet fuel — Aggerwal and Hinchee (1991).