Development of a Protocol and a Screening Tool for Selection of DNAPL Source Area Remediation

Carmen Lebrón | Naval Facilities Engineering Command, Expeditionary Warfare Center

ER-200424

Objectives of the Demonstration

Multiple technologies have been developed and applied over the past few decades for remediation of chlorinated solvents in the subsurface. The remediation of solvents in the form of dense non-aqueous phase liquids (DNAPLs) is particularly challenging. Factors such as geology, geochemistry, hydrogeology, the composition and distribution of the DNAPL, as well as the presence and absence of other contaminants, play a role in technology selection and performance. To date, despite a multitude of reviews on several individual technologies, no comprehensive studies have been completed that illustrate which technologies generally work best under specific site conditions and desired remedial outcomes or goals.

The objective of this project was to develop a user-friendly screening tool that could be used by decision makers during the remedial technology selection or evaluation process to:

  • Evaluate potential technology performance at a particular site
  • Evaluate potential technology performance in different geological strata at a complex site
  • Aid in the selection of feasible technologies for a particular site based on desired performance metrics
  • Reduce the uncertainty of estimating and predicting remedial outcomes and implementation costs at DNAPL source zone sites.

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Technology Description

The DNAPL Technology Evaluation Screening Tool (DNAPL TEST) was developed using data from published literature cases and modeling simulations that were used to supplement existing data. Hence, the basis of the screening tool is a database of information derived from case studies of field implementation of various remedial technologies, supported by numerical modeling of targeted technologies to address data gaps, and laboratory studies to provide information on fundamental processes impacting technology performance.

DNAPL TEST can assist environmental remediation practitioners in evaluating and selecting appropriate remedial technologies given particular site conditions and performance goals. With the U.S. Department of Defense moving rapidly towards achieving Response Complete at 95% of Installation Restoration Program sites by 2021, the information and screening tool developed as part of this project also can be used to evaluate existing remedial systems. For those sites where remedies are not meeting established remedial action objectives (RAOs), the screening tool can assist in determining whether there is a realistic expectation of meeting the RAOs for a given site and technology. It also can provide an assessment of alternative technologies to consider that may offer a higher likelihood of success.

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Demonstration Results

Observations on technology performance can be made based on the modeling results and field case study data collection completed to date. Some of these are summarized below.

  • Reductions in Groundwater Concentrations: None of the site characteristic or technology implementation parameters that were evaluated as part of the statistical analysis were found to have a statistical correlation with reductions in groundwater concentrations; however, there does appear to be a relationship between the amount of DNAPL mass removed from the subsurface during treatment and reduction in groundwater concentration. This relationship appears to be independent of treatment technology.
  • DNAPL Mass Removal: Near complete mass removal has been achieved with all technologies with the exception of hydraulic displacement (sometimes referred to as waterflooding). In field studies, the highest DNAPL mass removal was observed in thermal treatment case studies (94% to 96%) and the median mass removed for anaerobic enhanced in situ bioremediation (EISB), in situ chemical oxidation (ISCO), surfactant enhanced aquifer remediation (SEAR), and co-solvent flushing ranged from 64% to 81%. If modeling cases are included, the range of percent DNAPL mass removal increases for each technology, but the median value decreases. This is likely due to the fact that treatment duration during the modeling was varied to evaluate sensitivity of remedial performance, rather than treatment being terminated as a result of achieving desired performance levels as is more typical for field applications.
  • Matrix Diffusion: Modeling results demonstrated that in fractured rock environments with an older DNAPL release, matrix diffusion (diffusion of DNAPL into lower permeability media) has a substantial influence on the distribution of DNAPL mass. If degradation of DNAPL within the lower permeability matrix is limited, back-diffusion of contaminant mass out of the matrix will sustain groundwater concentrations for long periods of time.
  • DNAPL Properties: The solubility of the DNAPL was observed to influence the resulting net benefit of implementing more aggressive DNAPL treatment technologies over other approaches that rely primarily on dissolution of the DNAPL as the DNAPL mass reduction mechanism (e.g., pump and treat [P&T]). For more soluble DNAPLs such as trichloroethene (TCE), dissolution of the DNAPL is a significant component of the DNAPL mass removal, and incorporating other degradation or mass removal mechanisms (e.g., oxidation, biodegradation, enhanced dissolution) may only result in relatively small incremental increases in DNAPL mass removal.
  • Precipitate Formation: Through the modeling sensitivity analysis, it was observed that the formation of a manganese dioxide rind (resulting in encapsulation of DNAPL pools and flow bypassing around DNAPL areas) significantly increased the time required to remove TCE DNAPL in ISCO applications using permanganate as the oxidant. This evaluation is specific to permanganate treatment and the corresponding manganese dioxide rind formation; however, it is anticipated that similar results may be observed with other technologies that result in the formation of a precipitate or result in permeability reductions. The influence of the precipitate formation on DNAPL treatment is anticipated to be particularly pronounced where the precipitate forms within close proximity of the DNAPL phase, as occurs when permanganate reacts with the DNAPL.

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Implementation Issues

DNAPL TEST has been designed to be updated in the future to reflect new data. As new field, laboratory, or modeling case studies become available, they can be added to the database. When sufficient new case studies have been added, the statistical analysis can be conducted again to refine statistical relationships. The costs to operate DNAPL TEST are very low. The time required to run an analysis is approximately 10-20 minutes.

This tool is most effectively used as a preliminary screening for technology selection or as a screening for possible performance limitations for a remedy in place. It cannot replace appropriate site-specific evaluations based on engineering judgment. DNAPL TEST cannot “predict” technology performance for a particular site but will provide the user with an anticipated range of performance and the ability to compare performance observed for multiple technologies. This information can be used as the basis for developing realistic remedial end goals, as well as developing a short list of potential technologies for a site.

For users interested in obtaining potential technology performance information for a specific site, a Site Specific Analysis will better focus the screening evaluation to include sites that are anticipated to have similar performance. Multiple Site Specific Analyses may be completed for the same site to focus on different areas with different site characteristics. It should be noted that the Site Specific Analysis does require a minimum level of knowledge of conditions at the user’s site, which may limit its usefulness for some sites. Guidance for estimating these parameters is provided within the tool.

For more general analyses of overall performance trends between parameters and for sites at which key site parameters are unquantified, the General Analysis will be a better option. Modifications to search parameters can easily be changed at any time during the screening process, allowing users to refine their analyses to better meet their needs.

Specific examples illustrating a General and a Site Specific Analysis are provided in the User’s Manual, accessible via the Database (DNAPL TEST Tool) link.

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Points of Contact

Principal Investigator

Ms. Carmen Lebrón

Naval Facilities Engineering Command, Expeditionary Warfare Center

Phone: 805-982-2907

Fax: 805-982-4304

Program Manager

Environmental Restoration

SERDP and ESTCP

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