Aqueous Film Forming Foams (AFFF) containing per- and poly-fluorinated alkyl substances (PFAS) have been used historically for fire suppression and training at the Williamtown RAAF Base. PFAS chemicals are highly soluble and do not degrade, remaining in the environment for long periods.
RAAF Williamtown is situated on the highly permeable Tomago Sand Beds aquifer which adjoins the Stockton Sand Beds aquifer. Both aquifers are tapped by municipal and private bores and groundwater discharges to waterways used for recreational and commercial fishing.
As part of AECOM’s Environmental Site Investigation, HydroSimulations built a regional-scale MODFLOW-SURFACT groundwater flow and transport model to verify conceptualisation of the aquifer system and predict the fate of PFAS originating from the base. The model was calibrated to replicate groundwater levels and match the conceptual distribution of PFAS based on recent groundwater samples, identifying key sources and uncertainties in PFAS distribution. MODPATH was used to identify the ultimate groundwater discharge locations and estimate the PFAS travel time in groundwater.
A key finding was that the simulation of PFAS transport through groundwater alone was not able to replicate the current PFAS concentration distribution observed downgradient of the Base. Subsequent simulations including PFAS movement through surface water drainage networks improved the modelled distribution against laboratory reported concentrations, indicating that overland flow plays a role in PFAS transport from the base. There has been concern for the potential contamination of the adjacent Stockton Sands aquifer, however model results indicate a hydraulic barrier exists at Tilligerry Creek, caused by convergent groundwater flow from north and south of the creek upwelling against estuarine clays and discharging at the creek. The upwelling effect will continue as long as groundwater levels in the Stockton Sands are maintained, and tidal gates at surface water drains allow discharge of surface water during low tides.