Guidance Document for Applying the Pesticide Root Zone Model in Screening-Level PFAS Leaching Assessments
This document provides guidance on how to use the Pesticide Root Zone Model (PRZM) as a screening-level tool to assess the potential for per- and polyfluoroalkyl substances (PFAS) to leach into groundwater from land applied residuals, including industrial solids and municipal wastewater biosolids. For one individual PFAS compound at a time, this modeling framework can be used to identify adequate management strategies that balance the mass loading rate of PFAS in land applied residuals and maintain safe PFAS chemical concentrations in groundwater at levels below drinking water levels of concern. The document was prepared by NCASI, in collaboration with Stone Environmental and the National Association of Clean Water Agencies (NACWA).
Descriptions of PRZM inputs and outputs are presented and discussed in the specific context of simulating potential leaching of PFAS from land applied residuals. Step-by-step examples show how to implement PRZM simulations representing the most vulnerable groundwater scenarios developed by U.S. Environmental Protection Agency (US EPA), as well as customized scenarios that may better reflect local conditions (climate, soil, and groundwater conditions). These examples demonstrate how to assess leaching to groundwater based on conservative assumptions of PFAS chemical and physical properties as well as more typical properties and environmental conditions. A procedure for calculating a dilution attenuation factor (ratio of chemical mass applied over its concentration in the groundwater) is described which can be used to determine a maximum allowable PFAS application mass rate (per unit area) for any specified drinking water level of concern (DWLOC). An example comparison of PRZM modeling simulation results with field data is presented and demonstrates the reasonable accuracy of the modeling approach and the level of conservatism compared to measured groundwater concentrations. This provides confidence that the PRZM modeling approach is appropriate as a screening level tool for assessing potential levels of PFAS chemicals in groundwater resulting from land applied residuals.
The screening level modeling approach presented here, as well as the parameter selection guidance and options for refinement to local conditions, are designed to be used in an initial analysis of potential PFAS leaching to groundwater from land applied residuals. The standard groundwater leaching scenarios from the US EPA are designed to represent “worst-case” conditions nationally relative to potential chemical contamination of groundwater, and thus serve as effective scenarios to conservatively identify whether PFAS leaching to groundwater could be a concern. Use of this PRZM screening-level modeling approach will allow regulators and other stakeholders to efficiently evaluate PFAS groundwater contamination potential and determine whether a more comprehensive and rigorous modeling and/or field investigation is warranted.
Appendix A of this report (added October 2021) examines the suitability of using the SESOIL/AT123D modeling approach to study possible leaching of PFAS into the groundwater from land applied residuals as compared to the use of the PRZM modeling framework.