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| Figure 1. Elements of a Conceptual Site Model |
Development of a conceptual site model (CSM) is the first step in assessing the potential remedies that may be suitable for a site contaminated with chlorinated solvents (Figure 1). Characterization of the nature of the release, the resulting contaminant plume, and site hydrogeology, as well as an exposure pathway analysis, are required to determine the level of risk posed by the contaminant release and to select and to design an appropriate remedy. The physical and chemical characteristics of chlorinated aliphatic hydrocarbons (CAHs), whether in a DNAPL or aqueous phase, affect the fate and transport of these contaminants and are also taken into account when developing the CSM.
Enhanced in situ anaerobic bioremediation has emerged in recent years as a viable and cost-effective remediation strategy for CAHs in groundwater. Advantages include complete mineralization of the contaminants in situ with little impact on infrastructure, and relatively low cost compared to more active engineered remedial systems (e.g., groundwater extraction, air sparging, permeable reactive iron barriers, or chemical oxidation).
Enhanced in situ anaerobic bioremediation as a remediation technology may not be appropriate at all sites due to the complexity of chlorinated solvent contaminant plumes (e.g., DNAPL source areas) and potential site-specific limitations (e.g., difficult hydrogeologic conditions), particularly as a sole remedy. However, it is clear from the "success" stories experienced by AFCEE Technology Transfer that the technology holds great promise when properly applied. Figure 2 illustrates the steps involved in pursuing site closure using enhanced in situ anaerobic bioremediation.
Development of a CSM, remedial objectives, and preliminary screening are the first steps in evaluating the potential for applying enhanced in situ anaerobic bioremediation. Additional site characterization, laboratory microcosm studies, or small-scale field tests may be required as pre-design steps before a field-scale system can be designed and a cost calculated for comparison to other remedial technologies. If a determination is made to proceed with enhanced in situ anaerobic bioremediation, site-specific factors will continue to influence the design of the remedial system and the interpretation of performance results.
Enhanced in situ anaerobic bioremediation may be appropriate at sites where:
- Site-specific data indicate that the contaminants present (including any toxic degradation products) can be readily degraded by native microbial populations under anaerobic conditions.
- Subsurface conditions (e.g., aquifer permeability) are conducive to adequate emplacement and distribution of a substrate and creation of an in situ reactive zone conducive to anaerobic degradation of the targeted contaminants.
- A cost/benefit analysis indicates that the technology is cost-effective relative to other remedial measures (e.g., monitored natural attenuation [MNA], air sparging, groundwater extraction, permeable reactive iron barriers, or chemical oxidation).
Conditions that may preclude the use of enhanced in situ anaerobic bioremediation include the following:
- Sites with impacted receptors, or short travel time or distance to potential discharge and/or exposure points.
- Difficult hydrogeologic conditions that may preclude cost-effective delivery of amendments, such as low permeability or a high degree of aquifer heterogeneity.
- Natural geochemical conditions (e.g., unusually low or high pH) that inhibit the growth and development of dechlorinating bacteria.
- Enhanced in situ bioremediation has yet to be proven effective for DNAPL source areas.
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| Figure 2. Enhanced Anaerobic Bioremediation Roadmap |
There are many considerations to take into account when selecting and designing an enhanced in situ anaerobic bioremediation system. The intent of the roadmap is to aid in appropriate and successful implementation of enhanced in situ anaerobic bioremediation, while identifying "red flags" and avoiding "road blocks" that may limit success or lead to failure to achieve remedial goals.
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