Simulated groundwater flow paths and travel times near Joint Base Lewis–McChord, Washington Active

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Problem: 

PFAS are a broad class of anthropogenic chemicals used in industrial and consumer products for their stain- and water-resistant properties. One of the most common sources of PFAS at military installations, including at JBLM, is aqueous film-forming foam used as a fire suppressant. PFAS degrade very slowly in the environment and PFAS exposure has been linked to adverse health effects in humans. PFAS can enter groundwater and travel through the subsurface, so understanding the groundwater flow system is a critical part of combating PFAS contamination.

The current understanding of the hydrogeology in the JBLM area suggests that groundwater under JBLM generally flows to the northwest, toward off-base public-water supplies and eventually into the Puget Sound; however, given the large number of potential source areas and the complexity of the subsurface, a higher-resolution understanding of groundwater-flow paths originating within JBLM is required to inform management decisions and support remediation efforts. For example, additional groundwater-monitoring wells may be needed to better delineate contaminant plumes, and particle-tracking simulations of groundwater flow could be used to ensure that wells are located to maximize the utility of potential data collected from the wells.

Objectives: 

The objective of this study is to use a recently completed groundwater-flow model—developed by the U.S. Geological Survey (USGS) for the southeastern Puget Sound area—to simulate particle tracking in groundwater to estimate flow paths and travel times for hypothetical water particles starting from within potential PFAS source areas and known contamination areas at JBLM. 

Relevance and Benefits: 

This study will provide foundational information to the U.S. Army that will inform and direct current and future subsurface-remediation efforts near JBLM The efficient remediation of current and future subsurface contaminants helps to ensure that on- and off-base drinking-water supplies remain potable for all water users. 

Approach: 

A groundwater-flow model of the southeastern Puget Sound area developed by the USGS will be used to simulate forward particle tracking from potential contaminant source areas within JBLM. The groundwater model simulates (1) transient monthly hydrologic conditions from January 2005 to December 2015 (11 years), and (2) steady-state flow that represents average hydrologic conditions for the 11-year period. Particle tracking will be completed using MODPATH, a post-processing program based on the USGS MODFLOW groundwater-flow model. For each potential contaminant source area, particle tracking will be executed in two modes: steady state and transient. The steady-state simulations will show the farthest potential travel paths of particles at full equilibrium (not time specific), whereas the transient simulations will show path lines and particle ending points at the end of 11 years. If particles simulated in transient mode have not traveled as far as the equivalent particles for steady state, this will indicate that particle ending points for travel times greater than 11 years will be somewhere along the path lines for the steady-state simulation.