Gas exchange across the air-water interface is a critical process that maintains adequate dissolved oxygen (DO) in the water column to support life. Oxygen reaeration rates can be accurately measured using deliberate gas tracers, like sulfur hexafluoride (SF 6

California guidelines for indirect potable recycled wastewater reuse projects currently require groundwater tracers to demonstrate subsurface residence time for pathogenic microorganism control. Residence times over 6 months from infiltration to drinking water extraction are required. Two prospective tracers were evaluated in this case study: boron-10 (as 10 B-enriched boric acid) and heat (with recharging water ∼10°C warmer than native groundwater). Bromide (Br − ) was also released as a control. 10 B is attractive as a deliberate tracer because (1) reasonably accurate and affordable measurements can be made on an inductively coupled plasma mass spectrometer (ICP-MS) system, and (2) isotopic tracers require significantly less mass to tag an equivalent water volume than concentration-based salt tracers like Br − . 10 B and Br − tracer breakthroughs were observed at seven of nine monitoring wells, although at one well the detection of 10 B was barely observable and may have resulted from a slight change in source water composition. 10 B arrived 25% later than Br − on average, showing retardation through exchange with clay surfaces. Heat flow, requiring no artificial input, was interpreted from temperature changes recorded hourly at well loggers. Residence times to all wells were successfully determined from temperature changes with the longest flow path of 6 months. This implies that dilution of Br − and 10 B is a limitation the geochemical tracer experiment.