Theme: 10. Characterization, Fate and Transport

Topic Area: 10a. Advanced Investigation Tools and Techniques

Formatted Title:

Assessment, Planning, and Execution of Passive Methods to Measure Mass Flux and Groundwater Velocity

Background/Objectives:

Background/Objectives. A common approach to in situ remediation involves the injection of specialized amendments into an aquifer to construct a permeable reactive barrier (PRB) capable of intercepting and treating a contaminant plume, preventing downgradient contaminant migration. The chemical and physical interaction between an in situ remedy and a contaminant is only made possible when both constituents make contact, and the dose of remedial amendments is sufficient. PRB construction can be challenging due to a number of factors such as varying geology along the treatment zone, depth of plume, contaminant concentration, contaminant mass flux, and groundwater velocity. Properly accounting for these heterogeneities when designing and constructing a PRB are critical to the success of this remedial approach.

A particularly useful approach for characterizing site hydrogeology, contaminant distribution, and transport is through the deployment of passive flux devices into existing monitoring well networks. These devices are capable of collecting contaminant and groundwater mass flux simultaneously and through discrete depth sampling, the vertical variations present in the aquifer can be readily identified. To maximize the quality and utility of the data collected by passive means, careful consideration of the number of devices, location, and sampling layout, must be made prior to deployment.

Approach/Activities:

Approach/Activities. Knowing the mass flux is critical for engineering a successful PRB, whether it be for PFAS or any other contaminant class. Mass flux determines the placement of an amendment, can reduce the number of reapplications, and provides insight into economically dosing the remedy to achieve project goals and meet budgets. Accordingly, passive in-well methods for measuring mass flux have been used at an increasing rate to quantify both groundwater and contaminant flux. Data from these devices are then used for designing and confirming in situ remediation approaches. Several dozen passive sampling devices were deployed across 25 sites in 2023 before commencing remedial activities. The sites were primarily potential chlorinated solvent or PFAS plumes located across the United States. Data on the variations within these sites, eventual outcome on the recommended remedial design and when possible, outcomes of the treatment are being collected. The results of this analysis are being used to refine the deployment of passive flux devices and data interpretation.

Results/Lessons Learned:

Results/Lessons Learned. Best practices of evaluating a site to determine the number of passive sampling devices that should be used as well as placement of devices will be discussed. Guidelines will be provided to determine the deployment timeframe based on estimated groundwater velocity and contaminant groundwater concentrations. Several case studies will be presented showing how the use of passive sampling data can accurately characterize a plume to ensure appropriate product placement for in situ remediation which in turn can reduce reapplication.

Joshua Moreno

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Assessment, Planning, and Execution of Passive Methods to Measure Mass Flux and Groundwater Velocity