Closing the PFAS Mass Balance to WRRFs and the Watershed

Background PFAS are a contaminant of major concern for Water Resource Recovery Facilities (WRRFs) due to increasing concern from the public and regulators about the impacts of these compounds in discharges from the WRRF. However, WRRFs are not designed to destroy or remove PFAS. Though technologies are emerging for addressing PFAS in biosolids and effluent, these are extremely costly and infeasible for most WRRFs. Source control remains the most feasible and cost-effective strategy for controlling PFAS at WRRFs. However, the source control mechanisms differ greatly depending on if the source is industrial, commercial, or domestic. Therefore, understanding the loadings from the site-specific dominant sources to a WRRF is important to drive cost-effective pretreatment programs for PFAS. Effluent discharges from WRRFs and industrial point sources are known sources of PFAS to surface waters. However, there are frequently many other non-point sources of PFAS to rivers which are less understood, such as urban and agricultural runoff and baseflows. The importance of the different sources may vary seasonally in response to precipitation and irrigation. Understanding both the point and non-point sources of PFAS to rivers and creeks and how each of these is affected by seasonal factors is important to inform integrated planning efforts to address PFAS in a watershed. Objectives Clean Water Services' (CWS) objective was to understand the dominant sources of PFAS to each of its WRRFs and the dominant source(s) of PFAS to the rivers and creeks in the Tualatin River Watershed to help guide its pretreatment, stormwater, and treatment programs in addressing PFAS. This was done by gathering PFAS concentration and flow data from the industries, commercial sectors, and domestic sectors for each WRRF and from each major creek and river in the watershed under various conditions. PFAS concentration and flow data were combined to calculate PFAS loading from the various sources and mass balances for each WRRF and the river. Methods and Status Since 2019, CWS has been conducting regular PFAS monitoring at the WRRFs, the collection system, and industries to identify and address sources of PFAS. CWS identified several key industries with high concentrations and worked with them to create PFAS Management Plans to decrease their PFAS discharges. Since then, CWS has collected hundreds of samples and greatly expanded the locations and types of matrices sampled to help close data gaps identified in previous studies. Several rounds of samples were collected from nine manholes with uniform, single land-use sewersheds (e.g. single family residential, commercial, etc) to predict loadings from these sectors. Several rounds of PFAS samples were also collected from 14 rivers and creeks around the Tualatin River Watershed during both the wet and dry seasons to capture a range of subbasin types (undeveloped, agricultural, urban) and locations upstream and downstream of effluent discharges and urban creek confluences. As of the writing of this abstract in January 2025, the mass balances for each of these have been created and have already been used in making data-driven decisions for source control. Additional samples are being collected to further refine these mass balances under more conditions and to further track down urban sources of PFAS to urban creeks prior to the submittal of the WEFTEC Proceedings paper in July 2024. Findings and Significance PFAS mass balances for each of the WRRFs are shown in Figure 1. Source control efforts since 2019 have been very effective in addressing the largest sources of PFAS to the Rock Creek and Durham WRRFs. While there is still a sizable contribution from industries in the Rock Creek sewershed, the other three WRRFs are now currently dominated by domestic and commercial contributions. Therefore, the source control strategy for these WRRFs will need to incorporate more outreach and education to have a meaningful effect on PFAS concentrations, and not just regulation of industries in the pretreatment program. Urban creeks with no effluent contributions were found to have PFAS concentrations similar to effluent even in the dry season when the creeks are made up mostly of baseflow. This represents a potentially major overlooked non-point source of PFAS to rivers and creeks. PFAS concentrations in the Tualatin River were found to be highly correlated to upstream urbanization and somewhat correlated to upstream agricultural areas. A PFAS mass balance for the lower Tualatin River is shown in Figure 2 for June 2023. This figure illustrates that effluent from the Rock Creek WRRF is the dominant source of PFAS mass to the Tualatin River in the dry season when the effluent makes up roughly 1/3 of the flow in the Tualatin River and the urban creeks had very low flows. However, this likely does not hold true for the wet season when the urban creek flows increase, and it also not likely to be true for rivers that are not effluent-dominated. Although PFAS were detected at much higher concentrations in the Tualatin River downstream of effluent discharges and urban areas, PFAS concentrations of all samples collected in the Tualatin River were below EPA's MCLs for all compounds. All samples for urban creeks, however, exceeded those MCLs. Therefore, additional focus on PFAS in urban stormwater is needed, and efforts are underway to track the source(s) in urban areas.