Formatted Title

Development of In Situ Stabilization Performance Criteria Using Groundwater Modeling

Background/Objectives

In situ solidification/stabilization (ISS) is a remedial technology commonly used to treat subsurface dense nonaqueous phase liquid impacted source materials such as heavy-end petroleum hydrocarbons and other contaminants common to industrial facilities. ISS has proven effective at reducing contaminant flux from source-area materials by decreasing hydraulic conductivity and contaminant leachability. ISS can also be enhanced by the incorporation of amendments that facilitate contaminant adsorption and/or chemical or biological contaminant destruction. The success of ISS remedies is influenced by a number of factors including the selection of ISS performance criteria, installation of the ISS amendments, and site conditions. Implementing a rigorous quality assurance (QA) program when applying ISS allows teams to document that the ISS application methods and reagents are achieving the desired subsurface conditions. The QA program also provides an opportunity for project teams to identify areas where the design’s performance criteria are not met and implement corrective measures before equipment is demobilized and the project is complete. Hydraulic conductivity (K) and unconfined compressive strength (UCS) are two common performance criteria used to evaluate ISS performance in the field as leachability results such as Synthetic Precipitation Leaching Procedure and Leaching Environmental Assessment Framework require extended periods of time to produce results. Acceptable limits of K and UCS are established prior to remedy implementation based on the results of laboratory bench-scale treatability testing. However, test results for laboratory-prepared treatability samples can vary from what is observed or measured in the field during actual ISS implementation due to differences in mixing methods and the heterogeneity of in situ subsurface materials. Therefore, development of performance criteria that are achievable in the field, but also representative of effective ISS implementation and achievement of remedial objectives is key to project success. The performance standards applied to a project must facilitate a meaningful reduction in contaminant flux from the site, while being achievable using commercially available ISS methods. If this balance cannot be achieved, then ISS may not be the most appropriate remediation technology for the site. This presentation will provide attendees with an understanding of how the selection of ISS performance criteria affect remedy performance and cost; insight into the importance of selecting performance criteria that are attainable in the field; and training on how groundwater modeling can be used to better understand how changes in performance criteria affect remedial performance.

Approach/Activities

While performing a 23,000 cubic yard ISS project, bench-scale treatability testing was used to develop K and UCS ISS performance criteria. Pilot testing was then performed at the beginning of the ISS field effort to evaluate ISS performance and volumetric expansion under site-specific conditions, and to optimize the water to grout ratios to be used in full-scale application. When initial pilot study results did not demonstrate achievement of the performance criteria established during bench-scale testing, the project team was forced to regroup and further evaluate the performance of the pilot study results relative to remedial action objectives established by the regulatory agency, and present alternative paths forward. Prior to considering costly alterations to the overall project design, a MODFLOW groundwater model was used to demonstrate that the hydraulic conductivities achieved during the ISS pilot study, while orders of magnitude above what was observed during bench-scale testing, did not significantly diminish reductions in groundwater contaminant flux reductions that could be achieved by the ISS remedy. 

Results/Lessons Learned

The modeling effort included in this case study showed that the newly proposed performance criteria would result in a 99.93% reduction in groundwater flux through the treatment area as opposed to a 99.99% reduction using the original criteria. This difference in flux was determined to have a negligible impact on the overall performance of the remedy and was accepted by the regulatory agency. This same approach can be used to model a variety of post-ISS conditions at a project site to make informed decisions about how changes to performance criteria may influence the overall effectiveness of a remedy and its ability to be protective of human health and the environment.