An evaluation of the source of volatile organic compounds in groundwater samples was performed at a landfill in southern California. The ³H (tritium) content of the water in leachate and water from the gas-collection system (condensed water and entrained water droplets) and the δ ¹³C and ¹⁴C content of the inorganic carbon in landfill gas CO₂, leachate, and gas-collection system water were used to characterize the dissolved inorganic carbon (DIC) inside the landfill, while the same parameters were monitored in groundwater samples from affected monitoring wells and an unaffected well. Tritium levels from leachate and gas-collection system condensate ranged from approximately 2000 TU to over 4000 TU, orders of magnitude higher than unaffected groundwater. The average ¹⁴C content of DIC in the landfill pore-water samples was 121 pMC and the ¹⁴C content of unaffected groundwater DIC was 93 pMC, while the ¹⁴C content of the dissolved inorganic carbon in groundwater with VOC detections ranged from 105 to 119 pMC. The δ ¹³C of DIC in pore water was consistently above 0‰ and the δ ¹³C of unaffected groundwater DIC was −20.3‰, while the δ ¹³C of DIC in affected groundwater samples was increased from −17.3 to −13.2‰. The increases in both δ ¹³C and ¹⁴C in landfill gas-impacted groundwater DIC generally correlated with the number of volatile organic compounds detected and their concentrations. Based on the tritium and DIC ¹⁴C levels in leachate and water from the gas-collection system compared to those of unaffected water, significant increases in the tritium content of the water would be expected to accompany VOC detections and increases in δ ¹³C and ¹⁴C caused by landfill water. The results rule out landfill water as the VOC source, leaving landfill gas as the source. The identities and concentrations of the specific VOCs in affected groundwater samples varied among wells as well as between two leachate samples, ruling out the use of a VOC “fingerprint” for leachate or landfill gas to be compared to groundwater VOC concentrations.