Understanding risk to protect a critical water system
Per- and polyfluoroalkyl substances (PFAS) can pose a concern in drinking water supplies. For large, complex water systems, understanding potential PFAS risks early is critical to proactively protecting public health and maintaining regulatory confidence. Reservoirs often play a role in managing water supplied to US cities. We were engaged to evaluate a terminal balancing reservoir for a major US city that receives most of its water from large regional watersheds but that also captures a contribution from its own local multiple square mile catchment. Although routine monitoring has not identified PFAS at the reservoir outflow, the utility wanted a clear picture of potential PFAS sources within the catchment and an assessment of whether PFAS sources nearby could create compliance or operational challenges. Long story short: the water for the city is currently not at risk based upon current and future planned operations.
Our system‑wide watershed assessment was based on an inventory of presumptive and known PFAS sources, and an analysis of water quality and flow data. Because of the uncertainty in federal and state PFAS drinking water regulations, this project reviewed any PFAS—not only those with federal and state limits–as having the potential to raise a concern and our team remained attentive even to very low probability sites and low concentrations, regardless of regulatory status, for PFAS in drinking water.
Building a clear, evidence‑based picture of potential PFAS exposure
We completed an environmental assessment of PFAS in the reservoir’s drainage basin using a structured, desktop‑based approach that leveraged existing field measurements in and around the reservoir. The objective was to cast a wide net for discovery of known or presumptive PFAS sources, to identify the location of these potential sources within the watershed, understand migration potential into the reservoir, and determine the likelihood of a realistic threat to PFAS regulatory compliance.
We reviewed available PFAS monitoring data from the reservoir, its tributaries, and key system interconnections. Using state and federal databases, we identified known and potential PFAS sources within the catchment area, which spans thousands of acres, and mapped them using geographic information systems to determine their relationship to the watershed and surface water and groundwater flow boundaries. In addition to all environmental sites such as military sites or airports already identified by the state regulatory agency, presumptive sources based on business types (e.g., metals platers, furniture manufacturers) were also included for evaluation. All of these potential sources were ranked according to relative likelihood of contributing PFAS for purposes of screening current and future concerns to the watershed.
To move beyond source identification, our team developed a PFAS loading methodology. Some PFAS sample data existed at sites and within the watershed as part of active environmental monitoring. We used existing flow data and measured concentrations at key locations to estimate the relative potential contribution of inflows to the reservoir. The analysis evaluated scenarios that could influence PFAS concentrations at the reservoir outflow and assess the likelihood of exceeding current or future regulatory thresholds.
The results were documented through a series of technical deliverables, culminating in a final summary that focused on operational considerations, regulatory context, analytical methods, and future monitoring needs. The end conclusion was that none of the existing or presumptive sites were likely to have a detectable measurable contribution to the reservoir based on current and known potential future operations.
Managing complexity across scale, regulation, and uncertainty
The complexity of this work was driven by scale and uncertainty rather than physical design. The reservoir serves one of the largest municipal water systems in the United States, with multiple source waters, interconnections, and operational controls. Understanding PFAS risk required a system‑level perspective moving beyond a standard narrowly focused, site‑specific PFAS evaluation.
Regulatory uncertainty added another layer of complexity. PFAS standards and compliance timelines continue to change, with new federal rules, proposed revisions, and evolving state requirements. The assessment needed to remain relevant under a range of potential regulatory futures, without assuming outcomes that are outside the client’s control.
Finally, the work required translating complex science — including analytical methods, modeling assumptions, and regulatory frameworks — into findings that could support informed decision‑making by the client and stakeholders.
Supporting confident decisions without unnecessary treatment investment
Based on available data and modeling, the assessment found that the current risk of PFAS concentrations exceeding federal regulatory limits at the reservoir outflow is low. PFAS were not detected at the outflow, and the relative contribution from the local catchment is small compared to inflows from large regional watersheds feeding water to the reservoir.
The work confirmed that no operational changes or treatment actions are currently warranted. It also outlined practical, widely accepted measures that water providers typically consider to minimize future PFAS non‑compliance, should conditions change.
Beyond the immediate findings, the assessment provided a defensible framework for understanding PFAS risk across a complex system. It equipped the client with clear documentation to support regulatory discussions, respond to public inquiries, and prioritize future monitoring and source‑control efforts over costly end‑of‑line treatment.
A foundation for long‑term water supply resilience
PFAS regulation and science will continue to evolve. This assessment establishes a clear baseline against which future monitoring results, regulatory changes, and emerging technologies can be evaluated.
By focusing on source identification, relative loading, and system context, the work supports a long‑term strategy centered on source water protection and targeted response rather than reactive treatment. That approach helps preserve operational flexibility, manage costs, and maintain confidence in the resilience of this major city’s water supply over time.