Mass flux: The smarter way to cut PFAS risk

Per- and Polyfluoroalkyl Substances (PFAS) management is a moving target. Regulations shift, science evolves and public scrutiny intensifies. Traditional approaches focus on contaminant concentrations, but these numbers rarely tell the full story. PFAS compounds persist, migrate and resist natural breakdown, making them difficult to track and even harder to control.

Mass flux assessment changes the game. Instead of chasing compliance thresholds, it measures what matters - how much PFAS mass moves, where it goes and how quickly it travels through the environment. This approach identifies the sources and pathways that contribute most to overall risk, allowing interventions to be focused where they count.

For specialists, regulators and communities, mass flux provides a consistent and defensible way to demonstrate progress. It turns PFAS management from a reactive compliance exercise into a proactive strategy built on measurable reduction in risk.

PFAS are everywhere. More than 4,000 man-made chemicals, used since the 1940s, have found their way into soil, water and even the North Pole.

They are synthetic compounds found in everything from firefighting foam to non-stick cookware and water-proof fabrics. As outlined in the GHD Navigating PFAS Webinar Series - hosted by geochemist Mailyng Aviles - PFAS are valued in industrial and consumer applications for their unique water, oil and heat-resistant properties.

Their carbon-fluorine bonds are strong and the size of fluoride ion limits access to attack the bond, making them highly resistant to degradation and earning them the label “forever chemicals.” The most studied compounds - perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) - have been linked to a range of environmental and human health concerns. Dietary intake is the primary exposure route for most people, as these compounds enter food chains through contaminated soil and water.

Managing PFAS is complex. Criteria for safe concentrations keep dropping, making compliance a moving target. In Australia, drinking water limits have fallen from 70 nanograms per litre to just 8 for PFOS, after the US EPA adjusted their criteria. While natural systems can slowly depurate PFAS, the key driver of risk remains the mass entering the environment.

Mass flux reframes the challenge. Instead of just measuring PFAS concentrations at a compliance point in a regulatory setting where the compliance criteria are evolving, it asks how much is moving, where it is going and how fast. Mass flux is the amount of contaminant passing through a unit area over time, expressed in grams per square metre per day. It helps to understand the rate of contaminant transport at specific locations. Mass discharge is the total mass flowing through a cross-section, measured in grams over a defined unit of time such as grams per day. It is important to understand both parameters as one provides information on the speed of response actions required and the other at how much mass has to be dealt with over the long term respectively.

In simple terms, mass flux (J) equals groundwater flux (Q) times contaminant concentration (C): J = Q × C. Mass discharge is mass flux multiplied by the cross-sectional area.

Understanding these concepts is the first step to tracking PFAS, targeting interventions and demonstrating real progress.

Regulators keep tightening the rules. Communities want proof that risks are being reduced, not just shifted. PFAS concentration targets change year by year, but mass flux offers a stable, science-based measure that helps produce measurable results.

By tracking how much PFAS moves through soil and water, site teams can identify high-risk zones and focus resources where they will have the greatest impact. This approach supports more targeted remediation, clearer communication and stronger alignment with regulatory and community expectations.

For both environmental receptors and human health, it is the mass entering the system that drives outcomes. Reduce that flux and concentrations in people and nature begin to fall. Mass flux turns PFAS management into a measurable reduction in risk - a shift from chasing compliance numbers to delivering tangible results.

A robust conceptual site model is the backbone of PFAS management. It maps sources, pathways and receptors, showing how contamination moves and where it accumulates. Mass flux brings this model to life, turning abstract risks into measurable flows.

Start by defining each source and estimating its mass flux. Identify the main pathways - surface water, groundwater or sediment - and rank them by mass discharge. These ranking reveals which routes deliver the most PFAS to sensitive areas, guiding where to act first.

Accurate monitoring is critical. Controlled hydraulic environments, such as V-notch weirs, typically provide more reliable data than methods to collect data in complex uncontrolled field conditions. The rule is simple: Monitor where you can collect high-quality data, then apply those parameters at the point of interest. Match the setup to the events you need to capture - storms, baseflow or post-storm pulses - and use live data connections to maintain accuracy when conditions make access difficult.

Baseflow, stormflow and post-storm drainage each play a role in PFAS transport. It is tempting to assume storm events dominate, but baseflow can quietly move more mass over time. Concentrations and flux can vary within a single storm, so automatic samplers and continuous gauging are essential to capture the full picture.

Target controls at the sources and pathways that matter most. Continuous data collection helps track progress, refine interventions and demonstrate real reductions in PFAS movement.

Turning mass flux assessment into practical decisions means targeting interventions where they will remove the most mass, fastest. Begin with source controls: immobilisation using granular activated carbon performs well at moderate concentrations, while thermal treatment remains the most effective option for small volumes of highly contaminated soil.

For pathway controls, strategies such as surface water interception and controlled discharge monitoring can quickly reduce the mass leaving a site. With respect to groundwater here are options to intercept flow through hydraulic controls or in-situ treatment. These measures often provide early wins, demonstrating visible progress to regulators and communities.

Adopt a tiered strategy that reflects real site conditions and concentration ranges. Focus on the sources and pathways driving the highest flux, then use continuous monitoring to measure performance and refine your approach over time. This progression - from assessing movement to actively reducing it - shifts PFAS management from reactive compliance chasing to measurable, outcome-driven certainty.

Mass flux delivers the greatest value when it is embedded in every stage of site assessment and remediation planning. The process is straightforward, but precision at each step makes the difference between data and insight. Define sources clearly, rank pathways by impact and design monitoring systems that capture real movement through the environment. Use the results to focus interventions and track performance over time.

A practical roadmap for specialists and site teams:

1. Define sources and estimate source mass flux.
2. Identify and rank pathways by mass discharge.
3. Design monitoring using controlled hydraulics and continuous data capture.
4. Characterise the flow regime, including baseflow and post-storm pulses.
5. Implement targeted controls at the most significant sources and pathways.
6. Track reductions in mass flux and adjust the strategy as data evolves.
7. Measure success by reductions in mass to receptor, captured or immobilised mass, and lagging improvements in receptor concentrations.

Integrating these steps into the conceptual site model turns PFAS management into an iterative process of continuous improvement which is transparent, measurable, and defensible.

When integrated into everyday site practice, mass flux and mass discharge characterisation transforms PFAS management from data collection into decision making.

Mass flux reframes the conversation - from chasing compliance targets to achieving measurable reductions in risk. Early adoption brings less uncertainty, lower costs and faster progress toward credible, defensible results. Focus on what moves, not just what is present and you will cut through complexity to deliver real impact.