Co-occurring chemicals of concern (COCs) in groundwater can pose particular challenges to characterization and remediation activities, which ultimately increases remedial cost. While scientific evidence supports degradation and potential destruction of some COCs when they occur as a single source, additional research is needed to understand the fate of COCs under conditions where other chemicals co-exist. A recent detailed review of 29 DoD sites illustrated that 59 percent of the sites had more than one COC in groundwater, often in commingling plumes. Identifying treatment synergies amongst the different COCs and treatment schemes has been an ongoing area of research under SERDP and ESTCP to develop more cost-effective treatment strategies particularly for those sites impacted by per-and-polyfluoroalkyl-substances and other COCs.