The bioremediation discipline is a multidisciplinary field led by microbiologists and supported by a range of other professionals. Working closely with remediation engineers, microbiologists have developed bioremediation to a point now demonstrated to be effective in environmental cleanups throughout the country and overseas. Many such projects have claimed successes, including bioremediation projects, but many successes are a result of volatilation, not to bioremediation per se.
The primary need in bioremediation projects is to address hydrogeology issues, which will characterize the subsurface for use in designing effective bioremediation programs. Laboratory chemical analyses are used to characterize subsurface hydrochemical conditions, and a familiarization with biological processes in hostile, subsurface environments is important to resolve the numerous, complex issues involved in bioremediation.
Gasoline spills (including MTBE, TBA, etc.) are common and can cause water contamination issues as hydrocarbons dissolve in ground water and travel offsite in the aquifer. Hydrocarbons naturally degrade in the subsurface due to microbial-mediated reactions. However, the reaction rates are slow because electron acceptors, like oxygen, are quickly depleted in contaminated ground water and are slowly recharged. Contaminated ground water has significant hydrocarbon concentrations but depleted electron acceptors, whereas the overlying unsaturated zone contains oxygen but lower hydrocarbon concentrations. Early response and intervention is the key to minimizing extent and costs of remedial action for gasoline and its components and is essential to protecting public health and the environment.
When attempting to clean up such spills, timely and comprehensive source control and associated hydrogeological investigations are needed once a release is detected. This includes:
- Immediate control and cessation of the release,
- Repair or removal of the release source (tank, pipe, flange, pump, etc.),
- Removal / recovery of free product in both the saturated / unsaturated zones, and
- Removal of residual free product from the subsurface soils.
Any remedial action initiated before the source is controlled is ineffective and has the potential of expanding the scope of the remedial action as uncontrolled sources continue to migrate in the subsurface. Physical action, like excavation, is the usual approach to source control of small releases. Contaminated soils removed by excavation can be treated by disposal (asphalt batching, daily landfill cover), or physical (thermal desorption), or biological (biopiles) treatment. For larger releases, however, alternatives include free-phase LNAPL recovery, barrier installation, and hydraulic control of the ground-water plume. A variety of single and multi-phase extraction techniques moves both liquid and gas phases to the surface for treatment. At the surface, dewatering and subsequent recycling treats the higher concentrations of recovered material. Direct onsite thermal catalytic processes destroy lower concentrations and absorbents like Granular Activated Charcoal (GAC) polish the air or water before discharge.
From Technology to Techniques
Two major objectives of site remediation include destruction of residual or dissolved gasoline constituents or their removal from the impacted area. Destruction can range from total mineralization/oxidation or reduction to inorganic components or transformation to some unlisted form. Chemical reaction, biological means, or thermal processes accomplish destruction. This is a brief description of the myriad of innovative techniques developed to refine and enhance the implementation of four basic technologies that have evolved for the active remediation of gasoline released to the subsurface:
- Subsurface ventilation,
- Pump-and-treat technology,
- In-situ chemical oxidation, and
- In-situ bioremediation.
Specific techniques refining these technologies were developed to protect human health and the environment, reduce risk, reduce treatment time, reduce costs, and achieve lower decontamination objectives, among others. Table 1-1 lists the four major technologies and related range of associated techniques that refine and enhance them. In many cases, the techniques expand the matrix (i.e. soil, water, and air) capability of the technology. For example, under subsurface ventilation, the combination of air sparging with soil-vapor extraction expands the capability of SVE to remove gasoline vapors from both the saturated and the vadose zones. In contrast to basic technology, specific techniques exploit unique characteristics of the site as well as those of the contaminant to expedite remediation.