Per- and polyfluoroalkyl substances (PFAS) are often referred to as “forever chemicals” because of their extreme persistence in the environment and the human body, and are increasingly linked to cancer, endocrine disruption, and immune suppression. Although regulatory standards are emerging, reported PFAS concentrations often vary widely across datasets, raising questions about how sampling conditions influence measured values. This project investigates how time, depth, and environmental factors affect PFAS concentrations within a single water source: the Occoquan Reservoir in Northern Virginia. I expect to find that PFAS concentrations fluctuate significantly depending on sampling context, even when overall contamination levels remain stable. By identifying how methodological and environmental factors shape reported values, this project contributes to improving sampling protocols and strengthening confidence inPFAS testing. Because regulatory decisions and public trust rely on accurate water quality data, understanding the variability behind PFAS measurements is critical for environmental transparency and public health protection.