Remediation of hydrocarbon contaminated soil and groundwater is a complex process because it can involve volatilization, adsorption, dissolution and migration, biotransformation and biodegradation. Proof of biodegradation of hydrocarbon contamination is difficult and expensive to obtain. The fate of the contamination is a crucial component of the intrinsic remediation protocol. Although all measurement technologies used to measure and demonstrate remediation of hydrocarbon contamination have inherent drawbacks, hydrocarbon loss is still the best and most reliable indicator that remediation has occurred. A link is required between hydrocarbon loss and biological activity. Carbon is composed primarily of two stable isotopes: 12C and 13C with abundances of approximately 98.89% and 1.11% respectively. Due to their mass differences isotopically different molecules participate in reactions at slightly different rates. Because biological processes prefer the lighter isotope, the residual hydrocarbon after microbial attack will contain a relative enrichment of the heavier isotope, 13C. Hence, the abundance ratio, 13C/12C, will change as biological processes consume the hydrocarbon contamination. Relative enrichment of13C in the residual hydrocarbon was measured as benzene was utilized by a mixed microbial culture. This may be used as a technique to monitor hydrocarbon biodegradation and ongoing site remediation.
Natural or intrinsic remediation is viewed as the combined effect of naturally occurring destructive and nondestructive processes which reduce a contaminant's mobility, mass and associated risks. Non-destructive mechanisms include sorption, dilution and volatilization. Destructive attenuation processes include abiotic and biotic transformation and oxidation, and mineralization via aerobic and anaerobic mechanisms.
Interest in intrinsic remediation has grown over the last few years. This is due mainly to a desire for a less expensive alternative to implementing active remediation at the large number of contaminated sites in the U.S.A. and in Canada. Initially intrinsic remediation was viewed as applicable only to remote sites with little public interaction or access. It is now utilized much more extensively in the U.S.A where even the highly environmentally conscious state of California recognizes it as a viable remediation option.
Some of the first evidence of intrinsic remediation was compiled by McKee and coworkers in the early 70's showing a correlation between hydrocarbon contamination and microbial numbers in groundwater8. Since then interest has grown in intrinsic remediation as a viable process. The US EPA, Rice University, Stanford University, the US Geological Survey and Canada's University of Waterloo are among those who have investigated the phenomenon. The US Air Force has aggressively pursued this remediation option and has issued a technical protocol13 for its application. Recently a book entitled "Intrinsic Remediation" was published in 19955. It contained papers on this topic presented at the Third International In-Situ and On-Site Bioreclamation Symposium held in 1995. The Second Annual International Symposium on Intrinsic Bioremediation was held in December, 1996. The Third Annual Conference entitled "Natural Attenuation '97" is scheduled for December, 1997 in Arizona.
Intrinsic remediation is being slowly applied in Canada. There are no federal guidelines as yet and some provinces are cautiously assessing its utilization.