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Radionuclides associated with hydraulic fracturing are man-made radionuclides which are used in the hydraulic fracturing process as radioactive tracers or naturally occurring radionuclides released by this process. Injection of man-made radioactive tracers, along with the other substances in hydraulic-fracturing fluid, is often used to determine the injection profile and location of fractures created by hydraulic fracturing. In addition, hydraulic fracturing releases naturally occurring radioactive materials (NORM) from shale deposits, and these substances return to the surface with wastewater. Concerns have been expressed over the potential health and environmental impact of these materials.

Naturally occurring radionuclides
There are naturally occurring radioactive materials (NORM), for example radium, radon, uranium, and thorium,  in shale deposits. Brine co-produced and brought to the surface along with the oil and gas sometimes contains naturally occurring radioactive materials; brine from many shale gas wells, contains these radioactive materials. When NORM is concentrated or exposed by human activities, such as hydraulic fracturing, it is classified as TENORM (technologically enhanced naturally occurring radioactive material). Naturally occurring radionuclides are of more concern than man-made radionuclides used in fracture monitoring because the former have longer half lives and so remain in the environment longer.

Injected radionuclides
Injection of a wide range of radioactive tracers in solid, liquid or gaseous forms, is often used to determine the injection profile and location of fractures created by hydraulic fracturing. While running the well casing, small radioactive sources are inserted to act as depth markers. The logs indicate whether the logging tool reaches the defined depths. These sources each contain about 50 kBq of Cobalt-60. They are inserted into holes in the casing collars or screw threads at casing joints. Tags are attached to the perforation gun during well completions. When hot ionized gas perforates the casing, the radioactive material contaminates the perforations and the fluid passing through them, and the material is distributed with the fluid. A logging tool is used to detect the path of the radioactive material to determine whether charges have all been detonated at the correct depths and whether the process has been successful. The radioactive material may later be brought to the surface with the flowback, but the concentrations are diluted by the time they reach the plant and equipment.

It is recommended that the radiotracer is chosen to have readily detectable radiation, appropriate chemical properties, and a half life and toxicity level that will minimize initial and residual contamination. Operators are to ensure that licensed material will be used, transported, stored, and disposed of in such a way that members of the public will not receive more than 1 mSv (100 mrem) in one year, and the dose in any unrestricted area will not exceed 0.02 mSv (2 mrem) in any one hour. They are supposed to secure stored licensed material from access, removal, or use by unauthorized personnel and control and maintain constant surveillance of licensed material when in use and not in storage. Federal and state nuclear regulatory agencies keep records of the radionuclides used.

As of 2003 the isotopes Antimony-124, argon-41, cobalt-60, iodine-131, iridium-192, lanthanum-140, manganese-56, scandium-46, sodium-24, silver-110m, technetium-99m, and xenon-133 were most commonly used by the oil and gas industry because they are easily identified and measured. Bromine-82, Carbon-14, hydrogen-3, iodine-125 are also used.

Examples of amounts used are:

Concerns about environmental impact
In 2010 the Pennsylvania Department of Environmental Protection (DEP) limited surface water discharges from new treatment plants to 250 mg/l chloride; the chloride limitation was designed to preclude most of the potential for radium contamination. Existing water treatment plants were "grandfathered," and still allow higher discharge concentrations. In 2011, the level of dissolved radium in hydraulic fracturing wastewater released upstream from drinking water intakes had been measured to be up to 18,035 pCi/L (667.3 Bq/l), and the gross alpha level measured to be up to 40,880 pCi/L (1,513 Bq/l). The New York Times reported that studies by the United States Environmental Protection Agency and a confidential study by the drilling industry concluded that radioactivity in drilling waste cannot be fully diluted in rivers and other waterways. Recycling the wastewater has been proposed as a solution but has its limitations. A recent Duke University study sampled water downstream from a Pennsylvania wastewater treatment facility from 2010 through Fall 2012 and found the creek sediment contained levels of radium 200 times background levels. The surface water had the same chemical signature as rocks in the Marcellus Shale formation. The facility denied processing Marcellus waste since 2011. In May 2013 the facility signed another agreement to not accept or discharge wastewater Marcellus Shale formations until it has installed technology to remove the radiation compounds, metals and salts. According to the Duke researches the ‘waste treatment solids/sludge’ have exceeded U.S. regulations for radium disposal to soil. The study by Duke University also found that radium has been ‘absorbed and accumulated on the sediments locally at the discharge’.

Potential health impact
The U.S. Environmental Protection Agency and regulators in North Dakota considers radioactive material in flowback a potential hazard to workers at hydraulic fracturing drilling and waste disposal sites and those living or working nearby if the correct procedures are not followed.

Regulation in the US
The NRC and approved state agencies regulate the use of injected radionuclides in hydraulic fracturing in the United States.

The US EPA sets radioactivity standards for drinking water. Federal and state regulators do not require sewage treatment plants that accept gas well wastewater to test for radioactivity. In Pennsylvania, where the hydraulic fracturing drilling boom began in 2008, most drinking-water intake plants downstream from those sewage treatment plants have not tested for radioactivity since before 2006. The EPA has asked the Pennsylvania Department of Environmental Protection to require community water systems in certain locations, and centralized wastewater treatment facilities to conduct testing for radionuclides. and although water suppliers are required to inform citizens of radon and other radionuclides levels in their water, this doesn't always happen.