Integrated DNAPL Site Strategy
Dense, nonaqueous-phase liquids (DNAPLs), which are chlorinated organic compounds, are one of the most widespread and high-risk types of subsurface contamination. Chlorinated organic solvents were produced on an industrial scale beginning in the 1930s. They were widely used for cleaning and degreasing in a number of industries, including chemical manufacturing, aerospace, semiconductors, electronics, commercial and retail dry cleaning, and pest control. The same properties that led to their use in industry also led to their widespread use at Defense and Energy department facilities. By the 1970s scientists began to recognize the exposure risk posed by chlorinated organics.
Chlorinated solvents share certain characteristics that combine to produce a lingering problem even when present in small amounts (liters). They are chemically stable; thus, they do not degrade under typical subsurface conditions. They have low solubility, causing them to persist for decades or centuries. They are immiscible with water, so they form separate "phases," and they have relatively high densities, so they migrate downward, polluting a large thickness of an aquifer. They are also very toxic.
Sites contaminated by chlorinated solvents present a daunting environmental challenge, especially at sites with dense nonaqueous phase liquid (DNAPL) still present. Restoring sites contaminated by chlorinated solvents to typical regulatory criteria (low parts-per-billion concentrations) within a generation (~20 years) has proven exceptionally difficult, although there have been successes. Site managers must recognize that complete restoration of many of these sites will require prolonged treatment and involve several remediation technologies. To make as much progress as possible requires a thorough understanding of the site, clear descriptions of achievable objectives, and use of more than one remedial technology. Making efficient progress will require an adaptive management approach, and may also require transitioning from one remedy to another as the optimum range of a technique is surpassed. Targeted monitoring should be used and re-evaluation should be done periodically.
This ITRC Integrated Dense Nonaqueous Phase Liquid Site Strategy (IDSS-1, 2011) technical and regulatory guidance document assists site managers in development of an integrated site remedial strategy. The IDSS highlights five important features
- A conceptual site model (CSM) that is based on reliable characterization and an understanding of the subsurface conditions that control contaminant transport, reactivity, and distribution
- Remedial objectives and performance metrics that are clear, concise, and measureable
- Treatment technologies applied to optimize performance and take advantage of potential synergistic effects
- Monitoring based on interim and final cleanup objectives, the selected treatment technology and approach, and remedial performance goals
- Reevaluating the strategy repeatedly and even modifying the approach when objectives are not being met or when alternative methods offer similar or better outcomes at lower cost
This IDSS guidance is intended for regulators, remedial project managers, and remediation engineers responsible for sites contaminated by chlorinated solvents. Because the subject matter is complex, this guidance assumes a functional understanding of the field and is targeted towards experienced users; however, novices will benefit through descriptions and references of the latest evolution of site characterization challenges; realistic planning of site restoration; evolving treatment techniques; and evaluating, monitoring and interpreting mass transport in the subsurface aqueous and vapor phases. While the primary focus of the document is on DNAPL sites, other types of contaminated sites (e.g. petroleum, mixed contaminants, etc.) can use the same fundamental process described in this guidance.
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