Formatted Title
Recharge at PFAS-Impacted Sites: The Other Half of the Mass Discharge Story
Background/Objectives
Understanding the soil-to-groundwater contaminant mass discharge (Md) of per- and polyfluoroalkyl substances (PFAS) leaching from unsaturated zone source areas to underlying groundwater is critical to understanding the nature of PFAS source zones and their risks to groundwater. To determine the PFAS mass discharge from the unsaturated zone, two variables are multiplied together: i) a representative concentration of PFAS in porewater in the unsaturated source zone (units of mass per volume); and ii) the groundwater recharge rate through the unsaturated source zone (units of volume per area per time). There is extensive ongoing research on how best to obtain representative concentrations of PFAS in porewater in the unsaturated zone, including soil partitioning and leaching methods, modeling, and direct measurements using field-deployed suction lysimeters. However, there has been much less attention paid to the best method to obtain representative groundwater recharge rates through the unsaturated zone, which is required to estimate mass discharge from the unsaturated zone.
Approach/Activities
This study integrated the remediation field’s current knowledge about the general spatial and temporal characteristics of different PFAS unsaturated source zones. Based on an extensive compilation of over 35 recharge estimation methods from the literature, a tiered system for recharge estimation was developed and included factors such as cost, complexity, and certainty to assist researchers and practitioners in determining what methods or techniques may be most appropriate for a range of sites from simple, low risk sites (Tier 1) to highly complex, higher-risk sites (Tier 3). Fifteen existing recharge methods with particular applicability to PFAS source zones were compiled and categorized into these three Tiers, with practical examples provided for practitioners. Field examples and preliminary data from an ongoing Environmental Security Technology Certification Program (ESTCP) project, with data collection beginning in November 2023, will provide site-specific examples of the mass discharge estimation methods.
Results/Lessons Learned
Once the mass discharge for an unsaturated PFAS source zone has been estimated, two key comparisons can be made to efficiently prioritize remediation resources:
- The vertical PFAS mass discharge from the unsaturated zone can be compared to the horizontal PFAS mass discharge in groundwater to determine the relative contribution of different PFAS source compartments; and
- The PFAS mass discharge from an unsaturated source zone at a specific site can be compared to the PFAS mass discharge from other source zones to determine the relative strength of different PFAS source zones.
Given the vast number of sites that are currently or will be in the remedial investigation phase, the financial implications of site investigation and remediation at PFAS-impacted sites, as well as the anticipated need to meet very low regulatory criteria, the use of mass discharge from unsaturated PFAS source zones may be a critical component to consider and provides a flexible framework to manage and prioritize PFAS sites, especially in the current uncertain and rapidly evolving regulatory framework. As such, a tiered approach to estimating PFAS mass discharge may be a critical tool for practitioners, remediation managers, and regulators.