PFAS concentrations in drinking water sources are not static. Seasonal variation driven by precipitation patterns, groundwater recharge, surface water dilution, and treatment plant operations can cause PFAS levels to fluctuate meaningfully throughout the year. Understanding this variation affects both testing strategy and filter sizing decisions.
Precipitation Effects on Surface Water
Surface water reservoirs and rivers used as drinking water sources often show lower PFAS concentrations during high-flow periods when dilution from precipitation is greatest, and higher concentrations during low-flow dry seasons when baseflow from contaminated groundwater constitutes a larger fraction of total flow. Communities drawing water from rivers or lakes may experience a 2- to 5-fold seasonal variation in PFAS concentration depending on their watershed’s contamination pattern.
Groundwater Variability
Groundwater PFAS concentrations are generally more stable than surface water, but they do respond to precipitation events that increase recharge. Heavy rainfall can temporarily flush PFAS from contaminated soil zones into underlying aquifers, causing concentration spikes in well water. In coastal areas, saltwater intrusion management that changes pumping patterns also affects PFAS concentrations in well fields.
Testing Recommendations
A single water test result represents a snapshot that may not reflect worst-case conditions. Testing during the expected low-flow, high-concentration season provides the most protective basis for filter system design. For well owners and communities monitoring their own supplies, quarterly or semi-annual testing captures seasonal patterns and ensures filter systems are sized for peak concentrations rather than average conditions.