Hydrogen isotope biogeochemistry reactors in 1988, but nuclear weapons testing in the 1950s added large spikes of radioactive elements to the atmosphere, especially radiocarbon and tritium. This complicated measurements for geologists using radiometric dating of carbon. However, some oceanographers benefited from the tritium increase, utilizing the signal in the water to trace physical mixing of water masses. In biogeochemistry, scientists focused primarily on the stable isotope of deuterium as a tracer for environmental processes, especially the water cycle. The American geochemist Harmon Craig, once a graduate student of Urey's, discovered the relationship between rainwater's hydrogen and oxygen isotope ratios. The linear correlation between
Tritium 65 nuclear sites in the US. In one case, leaking water contained of tritium per litre, which is 375 times the EPA limit for drinking water. The US Nuclear Regulatory Commission states that in normal operation in 2003, 56 pressurized water reactors released of tritium (maximum: 2,080 Ci; minimum: 0.1 Ci; average: 725 Ci) and 24 boiling water reactors released (maximum: 174 Ci; minimum: 0 Ci; average: 27.7 Ci), in liquid effluents. According to the U.S. Environmental Protection Agency, self-illuminating exit signs improperly disposed in municipal landfills have been recently found out to contaminate waterways. The legal limits for tritium