California guidelines for recycled water reuse currently give preference to groundwater tracers to determine underground retention time, with a target retention time of >6 months from infiltration to drinking water extraction. Two prospective tracers were evaluated here: boron-10 (10B, the lesser abundant boron stable isotope), and heat (with recharging water warmed at sewage treatment plants and by solar radiation in surface-spreading basins). Bromide (Br-), a conservative deliberate salt tracer with limitations to low-volume settings, was released as a control. Tracer injection occurred when 3 kg of 10B-enriched boric acid and 36 kg of NaBr were added to the research test basin at San Gabriel Spreading Grounds, part of the Montebello Forebay, Los Angeles County, CA, USA. Recycled wastewater was piped directly to this basin at a rate of 2200 m3/day for ~two continuous months. 10B and Br- tracer breakthroughs were observed at seven monitoring wells ranging from directly beneath the basin floor to 150 m downgradient. Travel times ranged from 0.7 to 40 days. Arrival of Br- to each well was always coincident or preceded 10B arrival, reflecting retardation of 10B presumably due to exchange with clay surfaces. 10B/Br - travel time ratios determined from the center of mass ranged from 1 to 1.4. Travel times were successfully determined to all wells. Peak matching of diurnal variations was only possible at the nearest well (<1 day travel time). Basic analysis of temperature measurements, even in the absence of peak matching, yielded a reliable estimate of the underground residence time. Heat has great potential as an intrinsic tracer for managed recharge applications. Based on average tracer travel times, a vertical flow line directly below the test basin had an average vertical velocity of 3.3 m/day, equivalent to a recharge rate of 0.8 m/day.