Formatted Title
Using Passive Flux Meters to Design In Situ Remediation and Overcome Limitations of Other Characterization Methods: Case Studies for Two Sites
Background/Objectives
High-resolution site characterization (HRSC) methods such as membrane interface probe/ hydraulic profiling tool (MiHPT), electrical conductivity testing, and cone penetrometer testing are now employed as a standard best practice on environmental cleanup sites to precisely delineate and characterize volatile organic compounds (VOCs) and unconsolidated lithologic units. However, site geology and depth limitations prevent use of these technologies at many sites.
Passive flux meter (PFM) sampling was performed in existing monitoring wells at two sites where standard HRSC methods, including MiHPT, could not be used due to difficult drilling conditions. The sampling took advantage of the existing robust well network that included longer-screened wells (up to 30 foot long). The PFM results provided new insights into contaminant flux and groundwater flow velocity that were used to optimize remedy design, with low cost and minimal site disruption. The testing demonstrated a novel approach to obtain high resolution data that are superior in several ways to data obtained via standard HRSC methods.
Approach/Activities
PFM testing was implemented at two sites for developing or refining an in situ groundwater remediation design. At Site 1, PFM testing was performed in six wells for 1,2-dichloroethane, ethylene dibromide, and trichloroethene (TCE) at multiple depths in each well, ranging from approximately 40 to 70 feet. At Site 2, PFM testing was performed for TCE, cis-1,2-dichloroethene, and vinyl chloride in eight wells at multiple depths per well, ranging from 69 to 116 feet. PFM sample intervals at both sites ranged from 1 to 3 feet long, allowing high-resolution vertical profiling of VOC flux and Darcy velocity.
The PFMs were lowered into the saturated, screened portions of groundwater monitoring wells where they passively intercepted VOC flux within the natural groundwater flow system. VOCs in groundwater adsorbed to granular-activated carbon (GAC) in the PFMs allowing quantification of mass flux in groundwater. In addition, alcohol tracers pre-loaded in the GAC leached out proportionately to groundwater velocity allowing quantification of Darcy velocity. After 2 to 3 weeks, the PFMs were retrieved from the wells, sampled, and analyzed.
Results/Lessons Learned
The VOC fingerprint and magnitude obtained from the PFMs were consistent with nearby groundwater samples collected from wells and HydroPunch® borings. The PFM results allowed precise depth-discrete identification of VOC flux and high Darcy velocity zones that proved to be extremely valuable to design effective, injection-based remedies at both sites. The PFM results also demonstrated low VOC mass flux in several wells where injection pilot studies were historically performed, providing another line of evidence to demonstrate remediation effectiveness.
This presentation will include VOC-mass-flux and Darcy-velocity depth profiles with comparisons to other elements of the existing conceptual site model (CSM), including geologic logs, groundwater sample results, and traditional groundwater velocity calculations. The presentation will summarize the unique advantages PFMs provided to refine the CSM, optimize injection-based remedy design, and validate performance of previous remediation efforts.