Indirect potable reuse (IPR) projects – groundwater replenishment and surface water augmentation – continue to rise to diversify water supply portfolios and maximize future water production. The Los Angeles Department of Water and Power (LADWP) evaluated the potential for use of advanced treated water for IPR through infiltration at the Tujunga Spreading Grounds. A tracer test is a necessary regulatory step to determine the underground retention time of water during transit from spreading to capture by the adjacent Tujunga production wells. This project reviewed potential tracer test options, groundwater modeling to estimate the retention time, assessment of field-testing conditions, and preparation of a draft tracer test execution plan. Facilitated Research Partner: LADWP. Published in 2022.