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| Resource Library > Technology Transfer > Programs and Initiatives > Source Zone Treatment > Site Screening |
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Site Screening
At every site where a non-aqueous phase liquid (NAPL) has contaminated the local groundwater, there are two principal components to the problem: a subsurface source of residual and/or mobile DNAPL, and an associated dissolved-phase plume in groundwater. Most of the contaminant mass is in the source zone, although the plume usually occupies a much larger volume of the subsurface. The NAPL phase in the vadose and groundwater zones often contains sufficient chemical mass to cause dissolved plumes to persist for centuries (Pankow and Cherry, 1996). Unless essentially all of the NAPL mass (more than 99 percent) is removed from the source zone, permanent restoration of all groundwater to drinking-water quality will not be achievable (Freeze and McWhorter, 1997). However, reduction of NAPL mass to a degree less than 99 percent may reduce the size of, and contaminant concentrations in the groundwater plume and lessen long-term plume containment and monitoring costs.
Prior to initiating a source-zone treatment or removal action, it is necessary to determine whether source removal is warranted, and if warranted, the extent to which source zones should be addressed. Presented here is an overall strategy for NAPL source-zone remediation based on current technologies and regulatory trends. One should keep in mind that no remediation strategy can be applied universally.
Rational strategies for determining the need for, and degree to which NAPL source-zone reduction should be carried out, have been developed by the Texas National Resosurces Conservation Commission (in conjunction with the Texas Risk Reduction Program), among other entities. The general decision logic underlying this strategy is presented in a structured decision-process framework, supported by a decision-logic diagram and tables to assist in evaluating whether NAPL recovery is practical (which takes into account site-specific factors that will affect remedy performance), and identifying the likely net benefits associated with NAPL recovery. This strategy is based on five primary points:
- An accurate Conceptual Site Model (CSM), which includes a description of the characteristics, location, and extent of the source zone, is critical to understanding the possibilities and limitations of remediation, and for establishing site-specific cleanup goals. An accurate CSM can be developed only if thorough site characterization has been completed. The degree of remediation should be based on a realistic assessment of exposure potential (risk) for the source zone, and for contaminants migrating away from the source.
- The decision to remediate the NAPL source zone must be based on an understanding of NAPL accessibility, technology limitations, and the net benefits of only partial removal. If site-specific or NAPL-specific characteristics are such that removal or significant reduction of the source zone is unlikely, the site will be proposed for a Technical Impracticability (TI) waiver. The requirements for obtaining a TI waiver must be integrated into site characterization and pilot-testing activities.
- Unless a complete exposure pathway exists or an actual drinking-water supply is threatened, monitored natural attenuation and alternative treatment methods should be fully evaluated before an active source treatment or removal measure is selected as a remedy.
- Regardless of the remedial approach selected for a site, the remedy should be subject to an annual performance evaluation. Inefficient systems should be identified for detailed remedial process optimization (RPO) evaluations.
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