Remedial Design for Soil & Groundwater Contamination at
Air Force Plant 44

Using our previous experience with source-control and groundwater pump-and-treat remedial actions at the site, M&A served on a performance-based remediation (PBR) team to design optimization strategies to expedite cleanup. We are currently supporting the prime contractor to implement these strategies. Our field investigations, data analysis, and modeling efforts have helped ensure the program’s successful implementation.

Overview

U.S. Air Force Plant 44 (AFP-44) is part of the Tucson International Airport Area Superfund site, where VOCs and chromium were found in soils and groundwater in the early 1980s. A large-scale pump-and-treat remediation system began operating in 1987, along with soil vapor and dual-phase extraction programs at source zones and in situ methods for nonaqueous-phase VOCs. In the early 2000s, however, 1,4-dioxane was detected not only in groundwater but also in the treated water that was being re-injected for plume control. The pumping/injection strategy was temporarily modified until treatment for 1,4-dioxane was brought on line in 2009. The site is currently undergoing PBR, which includes actions to expedite achievement of remedial goals.

Services

M&A was originally hired to support monitoring and remediation activities. Our work included developing a model to evaluate remedial alternatives for containing both TCE and 1,4-dioxane and to project future concentrations at exposure points as part of a risk assessment. As part of the PBR, M&A prepares annual reports that include updated model results to help demonstrate that remedial actions are projected to achieve performance goals.

Groundwater Modeling

  • Updated and recalibrated the original AFP-44 model to simulate the effects of wellfield operation strategies on VOC capture and containment
  • Recalibrated the model and ran flow and particle-tracking simulations to re-evaluate wellfield operations after 1,4-dioxane was detected
  • Used the model to evaluate alternatives for containing VOCs while minimizing the off-site migration of 1,4-dioxane
  • Adapted the model to simulate the transport of 1,4-dioxane to assess the risk of future exposure and to evaluate the benefits of implementing a treatment program
  • As part of the PBR program, updated the conceptual site model and developed a geologic model using LeapFrog to incorporate new information about hydrogeologic conditions and transport processes
  • Adapted a site-wide, flow-and-transport model for annual use in projecting plume capture and containment as part of OES design and performance evaluation

Groundwater Remediation

  • Evaluated the ability of wellfield operation alternatives to achieve capture and containment objectives
  • Evaluated potential remedial enhancements — including in situ chemical oxidation and bioremediation — and recommended approaches to address mass removal near the source areas
  • Worked with the University of Arizona and a contractor to run pilot-scale tests for in situ chemical oxidation using potassium permanganate at two source areas
  • Pilot-tested various in situ oxidation, bioremediation, and chemical-reductive processes to address chromium and VOCs at other source areas
  • Provided field and analytical support for a pilot test of a groundwater remediation technology that entailed injecting and then recovering a solubility-enhancing reagent
  • Provided support for the design of full-scale in situ remedial actions to address recalcitrant VOC, 1,4-dioxane, and chromium in fine-grained sediments beneath historical source areas
  • Interpreted lithologic logs for injection wells to design in situ remediation programs using environmental hydrofracturing and biologic treatment
  • Recommended modifications to extraction wells to mitigate conduit flow and enhance mass removal and plume containment
  • Participated in regular agency and public meetings with the stakeholder group
  • Gave project-status and innovative technology presentations at annual Superfund Site technical exchange meetings

Contaminant Investigation

  • Designed programs to evaluate vertical yield and contaminant concentrations via spinner logging and depth sampling
  • Collected depth-specific soil and groundwater samples during drilling to characterize the distribution of contaminants
  • Developed a monitoring network to evaluate mass transport and containment at a property boundary in two critical aquifer zones