Previous uses of property can have impacts that spread to receptors located some distance away, calling for careful analysis of those potential effects before acting.
This was the case with a military base, where there was potential for impacts to the natural environment, as well as the general public, from the storage, testing and use of firefighting foams containing Per- and Polyfluoroalkyl Substances (collectively known as PFAS).
The site is hydraulically complex, with the main aquifer being connected to a network of ditches, some of which discharge to a water body of high natural significance. Furthermore, low topographic areas of the site flood during the spring, creating conditions for significant contaminant transport.
Careful work helps determine the scope of the PFAS problem
PFAS were suspected to have caused impacts to soil, groundwater, surface water and sediments. Several previous site investigations conducted at the site had confirmed the presence of some impacts in soil and groundwater, but during the document reviews, we found gaps in the data.
In order to fill the data gaps and formulate a risk management strategy for the site, our team developed a work program that included phased environmental site assessments and characterisation as well as a problem formulation to identify potential risks to human health and the environment.
Detailed investigation leads to a complete conceptual site model
Historical information was compiled and combined with the results of the gaps investigation to create a 3D conceptual site model (CSM) using 3D hydrogeological modelling software. This allowed our team to synthesize the available geological, hydrogeological, and contaminant data to support assessment and identification of the environmental concerns of the site, hydraulic connection between surface water and groundwater, and risks to receptors.
The CSM provided a better understanding of PFAS distribution in the different impacted media, the potential for transfer of PFAS between these media, and a good understanding of site-specific fate and transport properties of PFAS, especially in groundwater.
The CSM identified pathways and receptors and was used to support the assessment of potential risks to human and ecological receptors. The CSM also provided the technical criteria to identify potential remedial options based on the site-specific conditions.
The following technologies were selected in consultation with the client for the comparative feasibility assessment: excavation of contaminated soils and off-site disposal, pumping and treatment of groundwater, permeable reactive barriers for groundwater remediation, treatment of downstream surface water, and the reduction of PFAS runoff and recharge using capping.
The CSM provided the entry parameters to support preliminary design and cost estimates of the selected remedial options. The comparison of the different options was based on technical, environmental, social, and economic criteria.
The preferred option consisted of the implementation of surface soil capping to limit PFAS transfer to groundwater and eliminate pathways for human and ecological receptors. The installation of a permeable reactive barriers for the treatment of the PFAS plume migrating off site will be implemented if necessary.
This project illustrates several success factors in PFAS analysis and mitigation, particularly the value of having a three-dimensional conceptual site model to determine the pathways and receptors, and to provide information for choosing mitigation methods.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.