A detailed study of a 34-year aged dense non-aqueous phase liquid (DNAPL) release (51,300 L of parachlorobenzotrifluoride (commonly referred to as 4-CBTF)) was conducted in a surficial clayey aquitard in a physiographic region of southwestern Ontario known as the St. Clair Clay Plain. Prior to this study, little was known about the environmental fate of 4-CBTF. This study offered a unique opportunity to characterize and evaluate source evolution of the aged 4-CBTF DNAPL where the release conditions are well documented (known timing and contaminant volume, reasonable estimates of contaminant mass removal during preliminary remediation, etc.). Evaluation of 4-CBTF DNAPL fate was informed by insights obtained from previous studies conducted within the regional aquitard, including hydrogeologic conditions, transport properties, and DNAPL behaviour (as characterized during historic experimental releases of other DNAPL compounds). With a low solubility and high octanol water partitioning coefficient, 4-CBTF DNAPL persistence remains evident over three decades post-release, and plume mobility is strongly controlled by diffusion from fractures into the matrix in the shallow active flow zone and underlying unfractured zones.