Environmental Sequence Stratigraphy
Using Environmental Sequence Stratigraphy (ESS), Trihydro created conceptual site models (CSMs) for three Air Force installations to support long-term remediation decisions.
The installations have soil and groundwater impacts resulting primarily from per- and polyfluoroalkyl substances (PFAS) and trichloroethylene (TCE) and are regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).
Defining Contaminant Migration Pathways
To develop the CSMs, Trihydro used ESS methods to create frameworks for understanding groundwater flow, contaminant source areas, preferential migration pathways, and how surface water and groundwater systems interact at each installation.
ESS is an advanced geological approach for defining contaminant migration pathways, particularly in areas with complex geology and hydrogeology. It is centered on pattern recognition and helps identify depositional sequences and distinctive grain size distributions. ESS techniques can enhance CSMs by identifying preferential flow pathways that may or may not align with general potentiometric gradients.
Compiling Data
To compile the data sets supporting the CSMs, Trihydro conducted an extensive search of existing electronic data sets via the Environmental Restoration Program Information Management System and collected hard-copy reports and data from the Air Force public record, as well as other data sets and reports from local, state, and federal agencies.
Conducting Fieldwork
Trihydro then conducted fieldwork, which included collecting synoptic water level data at each installation to understand holistic groundwater flow conditions. From the data sets, Trihydro developed GIS databases for each installation to create the CSMs.
Through ESS techniques, portions of previously unidentified plumes were found, and the finalized CSMs helped the client make informed decisions regarding subsequent site characterization and remediation activities.
Below is a summary of CSM work and the insights they provided at each CERCLA-identified installation.
Ellsworth Air Force Base
Trihydro developed methods to synthesize decades of lithology data from over 1,100 boreholes with varying geologic interpretation at the Ellsworth Air Force Base (AFB). Trihydro looked at fine-scale stratigraphic patterns in the contaminant transport pathways to predict where additional remedial solutions should be located, find portions of previously unidentified plumes, and inform subsequent site characterization and/or remediation activities. Using simple potentiometric surface maps, the CSM showed how a PFAS groundwater plume diverged from the predicted direction. The project at Ellsworth AFB was completed under an accelerated schedule to support a follow-on contract for a PFAS remedial investigation.
FE Warren Air Force Base
The groundwater impacted by PFAS and TCE at the FE Warren AFB lies within a complex system of sediments, eroded terraces, and recent alluvial channels, all within the exposed Ogallala Formation, the region’s major potable water supply. By examining infrastructure and applying ESS techniques, the CSM showed the need for an evaluation of non-traditional contaminant pathways and how they can influence remedial strategies.
Trihydro synthesized decades of lithology data from over 675 boreholes with varying geologic interpretation. Our team used 3-D mapping of seasonal hydrologic gradients to show where contaminants could migrate vertically within a large water supply aquifer system. In addition, spatial-temporal evaluation of groundwater gradients showed how the different shallow aquifers interact. Typically, an upward hydraulic gradient helps mitigate downward migration of potential contaminants. However, ESS determined that groundwater vertical gradients reverse seasonally in parts of the installation and may enable dissolved contaminants to migrate downward into the Ogallala aquifer.
Air Force Plant Peter J. Kiewit and Sons
Air Force Plant (AFP) Peter J. Kiewit and Sons (PJKS) lies within a complex suite of near-vertical, upthrown sedimentary rock sequences. The CSM showed a complex relationship between the generally upward groundwater gradients and incised drainages that cut through the upthrown rock units. Trihydro synthesized decades of lithology data from over 540 boreholes with varying geologic interpretation. Trihydro also analyzed the complex structure of the hard rock units and erosional surfaces to identify preferential pathways.
An evaluation showed that portions of the drainages were discharge points for the shallow groundwater system and thus points of concern for potential TCE contaminant migration. In addition, an on-site test facility used large amounts of water to cool rocket test stands, and this water was, at times, discharged directly into unlined ditches where the impacted water could infiltrate the subsurface or discharge into natural drainages.
Trihydro accelerated select project deliverables to provide information for a follow-on contract supporting the US Environmental Protection Agency’s five-year review and a groundwater flow model at a Lockheed facility immediately downgradient of AFP PJKS.
