Field tracer tests and parallel laboratory experiments were performed to establish tracer performance and solute transport velocity in a contaminant plume arising from an unleaded fuel spill in the Chalk aquifer of the southern UK. An initial forced gradient test between injection and pumped wells 24 m apart largely failed for practical reasons; subsequently other down-gradient wells were periodically monitored for tracer breakthrough under natural gradient conditions over the next 10 months, giving tracer velocities of 37–51 m a−1, consistent with plume development and implying significant retardation of the tracer by fracture–matrix diffusion. Amino-G Acid concentrations fell rapidly during the field tests and laboratory experiments, and indicated that this tracer was susceptible to biodegradation under model plume conditions. Sodium Fluorescein was more strongly sorbed to aquifer materials than Amino-G Acid and exhibited some suppression of fluorescence in the presence of contaminants, but was resistant to biodegradation. Elevated background fluorescence produced an effective detection limit for Sodium Fluorescein of 1.8 μg l−1 in the most contaminated part of the plume.