A flow tracer is any fluid property used to track flow. The concentration of a chemical compound in the fluid can be used as a chemical tracer, and characteristics such as temperature are physical tracers. Tracers may be artificially introduced, like dye tracers, or they may be naturally occurring; radioactive tracers may be either. Conservative tracers remain constant following fluid parcels, whereas reactive tracers (such as compounds undergoing a mutual chemical reaction) grow or decay with time.