Per- and polyfluoroalkyl substances (PFAS) pose a growing threat to industries faced with assessing and potentially remediating sites impacted by these emerging contaminants. Although the U.S. Environmental Protection Agency (EPA) has established drinking water health advisories for certain PFAS, enforceable standards have yet to be enacted at the federal level. However, several states have promulgated enforceable regulations for drinking water, surface water, soil and/or groundwater.
When it comes to addressing complex contaminated groundwater sites, subsurface conditions are the greatest source of uncertainty. The role of the conceptual site model (CSM) is critical for successful PFAS site characterization and remediation.
The CSM is a comprehensive illustration and/or database of site conditions, contaminant extent and distribution, and processes affecting contaminant fate and transport. CSMs for complex sites must accurately reflect geological heterogeneities that form the primary controls on contaminant transport and mass storage. The CSM is refined as new information becomes available and is used throughout the project life cycle to identify data gaps, guide site investigations and inform remediation strategy decision-making.
The CSM should be founded on sound geologic interpretation and incorporate subsurface processes critical to PFAS fate, transport and remediation performance. In these cases, CSMs can help avoid costly site characterization scope creep and poorly conceived remediation strategies. A process-based CSM can address specific site management challenges associated with PFAS, including:
- The propensity of PFAS to form expansive groundwater plumes.
- The potential for multiple contamination sources at many PFAS-impacted sites.
- The extremely low (part per trillion) concentration standards being promulgated for these compounds.
- Unique electrostatic forces affecting the mobility and transport of PFAS.
- Changing contaminant transport behavior along migration pathways due to PFAS precursor transformation.
Environmental Sequence Stratigraphy (ESS) is an advanced geologic interpretation method used to map interconnected sand channels and other hydrostratigraphic units in three dimensions. This technique, which has been adopted by the EPA as a best practice for developing CSMs, will be appropriate for many PFAS sites characterized by large plumes with multiple potential sources. CMSs enhanced by ESS and other tools result in more predictable and cost-effective remediation strategies.
An accurate CSM is needed to plan and execute PFAS groundwater investigations. Even a limited amount of data can be used to develop a CSM that results in a more cost-effective and technically productive site characterization effort. In addition, a sound understanding of aquifer and contaminant properties is needed to conduct exposure pathway identification, forensic evaluation and remediation alternatives evaluations.