injection of radioactive tracers along with the fracturing fluid is sometimes used to determine the injection profile and location of created fractures. radiotracers are selected to have the readily detectable radiation, appropriate chemical properties, and a half life and toxicity level that will minimize initial and residual contamination. radioactive isotopes chemically bonded to glass (sand) and/or resin beads may also be injected to track fractures. for example, plastic pellets coated with 10 gbq of ag-110mm may be added to the proppant, or sand may be labelled with ir-192, so that the proppant's progress can be monitored. radiotracers such as tc-99m and i-131 are also used to measure flow rates. the nuclear regulatory commission publishes guidelines which list a wide range of radioactive materials in solid, liquid and gaseous forms that may be used as tracers and limit the amount that may be used per injection and per well of each radionuclide.