Among the most prevalent problems today in the environmental arena is the contamination of soils and groundwater with petroleum compounds. EPA estimates that of the approximately seven million underground storage tanks (USTs) in the United States, the majority are leaking some form of petroleum. Another more limited but nevertheless important source of petroleum contamination involves transportation related releases. Even though stricter regulation, as well as the use of higher integrity USTs, will undoubtedly reduce future releases, significant problems exist today due to previous releases from aging, deteriorated or non-conforming USTs. The purpose of this article is to discuss generally how bioremediation works and the advantages of bioremediation over conventional remediation methods.

because of the large number of petroleum contaminated sites requiring cleanup and the cost involved with the conventional approach to excavation and landfilling, a need exists to develop new remedial technologies for such sites. One technology that has been increasingly refined and developed is bioremediation. Bioremediation uses microorganisms, primarily bacteria and fungi, to detoxify environmental pollutants and transform them into simpler, less toxic compounds. Microorganisms degrade pollutants for approximately 100 years in the treatment of municipal wastewater. These biological treatment systems are "living" systems that rely on mixed biological cultures to break down waste organic compounds and remove organic matter from the solution. Municipal wastewater provides the biological food, growth nutrients and innoculum, while the treatment unit provides a controlled environment for the desired biological process.

The application of the use of microorganisms to remediate soils and groundwater is a relatively new approach. However, based on recent research and applications of bioremediation, it appears that bioremediation will become an increasingly popular remedial alternative. In general terms, bioremediation can be classified as either ex situ or in situ. Ex situ bioremediation involves the physical removal of the contaminated media to another location for treatment. In contrast, in situ involves the treatment of the contaminated media in place. Regardless of the method chosen, all bioremediation projects require the use of the specific microorganisms under the environmental conditions which will facilitate the biodegradation of the contaminants.

Many factors influence the proper choice of a bioengineered remediation system. These include the type and extent of contamination, site characteristics, cleanup goals and economics. Therefore, it is not possible to apply a single remedial approach to every situation. Prior to implementing a bioremediation project, a multidisciplinary approach to characterizing the hydrogeologic, geologic chemical, and microbiological characteristics should be undertaken. Key microbiological considerations include carbon and energy source, electron acceptor availability, temperature, nutrients available to the microbes, soil or groundwater pH, and soil moisture content.

Petroleum and petroleum products such as gasoline, fuel oils and diesel fuels are complex mixtures of organic compounds. Gasoline, for example, contains over 100 different substances which can hinder biodegradation because no single microbe can degrade all the individual substances. Therefore, during the site characterization, an evaluation of the indigenous microorganisms present to perform biodegradation of the petroleum may be required. From this information, the overall effectiveness of the remediation can be evaluated to determine if additional microbes are necessary to achieve the cleanup goals.

Once the petroleum compound enters into the soil or groundwater, natural biodegradation will occur by the indigenous microbes. However, several limiting factors slow the rate of biodegradation, allowing the petroleum to persist and continue to migrate thus causing further contamination. Two of the most important limiting factors are the availability of oxygen and nutrients for use by the microbes. Without ample supplies of oxygen and nutrients, the microbes will eventually deplete the existing supplies, thus limiting the amount of contamination removal that can occur.

The most common bioremediation method utilizes the indigenous microorganisms to degrade the petroleum. Other methods include the addition of microorganisms from another area capable of degrading the particular contamination, or in some cases genetically engineered microbes designed to degrade the compound of interest. When use of the indigenous microbes is selected, and after all preliminary investigation work is complete, it is generally possible to stimulate the microbes to begin using the petroleum compounds as a food source at a much greater rate by compensating for limiting parameters; for example, by introducing additional oxygen or nutrients.

Even simple steps such as the addition of fertilizer will provide nutrients required by the microbes. This will allow the microbes to multiply at a rapid rate while at the same time consuming the petroleum compounds. It has been shown that by increasing the metabolic rates of the petroleum degrading microorganisms with the addition of fertilizers, the removal rates of the petroleum compounds can be increased by as much as 10 times the normal rate.

With respect to leaking USTs which have contaminated both soil and groundwater with aromatic hydrocarbons, bioremediation can be used to clean up these sites when other conventional technologies often cannot. For example, when USTs are located beneath a building, thus precluding excavation of the contaminated material, in situ bioremediation can be used without disturbance of the structure. A second example would be releases affecting a railroad bed and track. Conventional remediation would involve excavating the contaminated soils, causing delays and loss of revenue. In situ bioremediation would allow the treatment of the impacted soils without any interruption of rail service, while realizing a significant overall cost savings.

Other than keeping site disruption at a minimum, there are other advantages of bioremediation over conventional excavation and landfilling. The microbial processes permanently destroy the contaminants, eliminating long term liability exposure that may persist by using nondestructive treatment methods. By remediating in situ, liabilities and costs associated with the transportation of the excavated soils also are eliminated. Biological systems in general are often less expensive to implement, have no moving parts and require no conventional energy source, thereby reducing overall remediation costs. Moreover, bioremediation can be coupled with other treatment techniques into a treatment train.

As with any treatment technology, bioremediation also has some limitations. Although bioremediation is effective for petroleum compounds, it is not applicable to highly chlorinated compounds and metals. Another drawback is that the breakdown products of bioremediation may be more toxic than the original target compound. For example, trichloroethylene (TCE) can be broken down into vinyl chloride, a known carcinogen. Table 1 shows some common types of compounds and their susceptibility to bioremediation. Further, there is a need for long-term, extensive monitoring of the bioremediation project that would not be required for excavation and disposal techniques. Monitoring would include visiting the site location on a routine schedule to assess the status of the biodegradation and ensure that all necessary constituents required by the microbes are available.

Another limitation which greatly influences whether bioremediation will be the technology of choice is the time factor involved in achieving the cleanup goals. Depending on the circumstances of the remediation project, it could take years to complete. In addition, there are regulatory restraints. Bioremediation has not been accepted by all state regulatory agencies because there is a perception that it is an unproved technology. As a result, some agencies are reluctant to approve the technique for remediations.

Despite these limitations, the many advantages of bioengineering petroleum-contaminated soils and groundwater will cause continued growth of this technology as a remedial option. Bioremediation offers the ability to remediate the contaminant in place, has the potential to completely break down contaminants to innocuous end products and is very cost-effective. As more bioremediation projects are completed successfully, the technology will undoubtedly gain acceptance as a routine treatment method for remediating petroleum-contaminated soils and groundwater. Where the bioremediation has gained regulatory approval, this technology has been shown to reduce costs, avoid interruption of uses of affected sites and provide an environmentally sound cleanup.