Due to low water demand, IRWD’s Coastal Zone reservoirs have long residence times that cause disinfectant degradation, leading to low chlorine residuals and increased microbial growth. IRWD installed submersible mixers to improve circulation and promote thorough mixing in the tanks, however, uneven mixing continued to be detected. The Clinic team evaluated mixing in the reservoirs using two approaches: a tracer study using fluoride, and modeling using Fluent Computational Fluid Dynamics Software.