JOINT BASE SAN ANTONIO-LACKLAND, Texas --
Two demonstration projects may hold the key to accelerating environmental cleanup efforts at multiple Air Force sites across the country.
Funded through the Air Force Civil Engineer Center's 2015 broad agency announcement, the projects substantiate technological methods for mapping underground contaminant pathways at some of the Air Force's most challenging sites.
At many complex cleanup sites, the ground transitions between layers of rock, gravel, sand and silt at different depths and locations. Contaminants pass quickly and easily through some ground types, such as gravel and fractures, but can be blocked from moving through intact rock and clay.
Accurately mapping the path a contaminant takes underground is both critical and challenging for Air Force remediation teams, who require the knowledge for time- and cost-effective cleanups.
Through analysis of these complex sites, the AFCEC critical process analysis team, comprised of environmental remediation experts, has found that inaccurate site characterization is a factor in approximately half the sites experiencing difficulty in achieving cleanup goals -- resulting in long-term liability for the Air Force.
"Similar to rivers, contaminants tend to prefer the path of least resistance," said Dr. Adria Bodour, who manages the AFCEC BAA program. "When a cleanup site is considered 'complex,' it's usually the result of not understanding the subsurface and, therefore, not being able to map out these contaminant pathways. Understanding the subsurface is critical in accelerating cleanups."
To combat this issue, AFCEC awarded funding for two projects aimed at improving characterization at these types of complex sites. Both teams are putting their technologies and techniques to the test at Air Force Plant 4 in Fort Worth, Texas, where long-term cleanup actions are ongoing.
The first, awarded to GSI Environmental, combines two technologies, hydraulic and geophysical tomography, to more accurately map how contaminants travel underground at different sites. GSI is conducting a two-phase onsite demonstration at the Hawks Creek Golf Course area of Air Force Plant 4.
In April 2016, GSI employed geophysical tomography to generate electrical signals at various ground depths and locations. Voltage and current are measured at other locations and depths nearby, much like an electrical CAT scan of the site. The results help determine the type of soil present at various locations, which translates into how easily and quickly contaminants can pass through these locations.
The GSI team plans to use hydraulic tomography during the second phase of the demonstration, which begins in July 2016. This technology uses a network of wells to determine groundwater flow paths. Water is pumped or injected at one well location, and the water pressure response is monitored at different depths in the initial well and surrounding wells.
"Using a computer model, we are able to use the data we collect at the multiple directions, distances, and depths to estimate what is occurring between the wells, with the end result being very much like a hydraulic CAT Scan of the site," said Dr. Bill Mok, principal investigator for the project at GSI Environmental.
Two academic pioneers in hydraulic and geophysical tomography are also collaborating on the project-- Dr. Jim Yeh at the University of Arizona and Dr. Walter Illman at the University of Waterloo.
The combined information from both technologies is expected to provide a more detailed and accurate map of the underground geology than either technology could provide independently.
The second project, awarded to Arcadis, seeks to build a better conceptual site model for the Air Force Plant 4 Chrome Pit 3 area using preexisting data gathered over many years of site investigations and sampling. Arcadis' approach, called stratigraphic flux, combines traditional geological interpretation with advanced high resolution site characterization methods to map the transport pathways.
"By mapping the concentrations of solvents in the water with relative permeability measurements, we are able to show where contaminants are migrating so that we can choose the optimal remedies," said Joseph Quinnan, Arcadis lead investigator. "Additionally, data gaps will be addressed through fieldwork to improve site characterization, support and expand modeling efforts, and validate employed methods and technologies."
The Arcadis team plans to collect necessary field data this month.
"There's a lot of information collected over the years that hasn't been looked at collectively," said Lynden Peters, Arcadis project manager. "We are taking this data, tying it together and cross-referencing it, to give us a comprehensive view and help us achieve quicker resolution on some of these trickier cleanup sites."
Integrated information from both projects is expected to provide the Air Force environmental remediation program with tools to assess complex sites and target remedial efforts to reduce Air Force liabilities.