Standard protocol for demonstrating natural attenuation of soluble hydrocarbons in groundwater calls for measuring the concentration of total petroleum hydrocarbons (TPH) and electron acceptors (e.g., dissolved oxygen, nitrate, etc.). However, where groundwater discharges to nearby surface water, obtaining acceptance of natural attenuation as a remedial option may require proving that TPH reduction is by biodegradation rather than dilution and sorption. In such cases, forensic analysis of chromatograms for organic indicator compounds as described by Kaplan et al. (1996) can provide clear evidence of biodegradation. In one example, a diesel range product in contact with groundwater and discharging to the Pacific Ocean 250 m away generated a 220 m long TPH plume toward the ocean. TPH ranged from 22 to 17 mg/L within 60 m of the source, dropping rapidly to, but persisting at ~1 mg/L between 60 and 120 m downgradient. Forensic analysis showed that even though TPH decreased only 5 mg/L in the first 60 m, hydrocarbons (non-polar compounds) decreased from 71% to 3% of total TPH and n-alkanes, C1-naphthalenes and phenanthrenes were eliminated, thus indicating progressive hydrocarbon biodegradation. Using the groundwater velocity (0.4 m/day), biodegradation of hydrocarbons to polar compounds and complete attenuation of TPH occurred in ~76 and 553 days, respectively. Forensic analysis is frequently used to discriminate among contaminant sources, but this method is also a valuable supplemental tool to demonstrate natural attenuation.