Determining groundwater recharge for quantifying PFAS mass discharge from unsaturated source zones

Soil-to-groundwater contaminant mass discharge (M-d) is the authoritative metric defining source strength at sites impacted by per- and polyfluoroalkyl substances (PFAS) and is increasingly being reported. Accurate estimates of groundwater recharge at representative spatial scales, however, is critical to quantitatively estimating M-d, which to date has received comparatively little attention relative to PFAS-specific partitioning and retention processes within unsaturated zone source areas despite a plethora of available literature. The objective of this review is to summarize the concept of M-d as it applies to PFAS-impacted sites, present standardized terminology, and collate published literature on groundwater recharge for a practitioner-level summary of practical methods. Critically, the primary aim is to reduce uncertainty in recharge (and, thus, M-d) estimates commensurate to a given site management application (i.e., the data quality objective process). Therefore, we propose a tiered system of increasing cost, complexity, and certainty depending on specific project requirements and site conditions and recommend applicable recharge methods for each tier based on these factors. Ultimately, a framework is presented for the assessment of PFAS-impacted source areas based on the concept of M-d.