 Complete the following phrase:  Transient tracers change over time, such as radioactive material Tritium and Cesium-137 and chemical concentrations CFCs and SF6, which are used to date water masses and can also track mixing. In the mid-1900s, Nuclear weapons testing and chemical production released tons of compounds that are not naturally found in the environment. While extremely unfortunate, scientists were able to use the concentrations of anthropogenic compounds and half-lives of radioactive material to determine how old a water body is. The 