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Natural production of the radionuclide chlorine-36 (Cl-36) has provided a valuable tracer for groundwater studies. The nuclear industry, especially the testing of thermonuclear weapons, has also produced large amounts of Cl-36 that can be detected in many samples of Groundwater. In order to be most useful in hydrologic studies. the natural production prior to 1952 should be distinguished from more recent artificial sources. The object of this study was to reconstruct the probable preanthropogenic levels of Cl-36 in groundwater in the United States. Although significant local variations exist, they are superimposed on a broad regional pattern of Cl-36/Cl ratios in the United States. Owing to the influence of atmospherically transported ocean salt, natural ratios of Cl-36/total Cl are lowest near the coast and increase to a maximum in the central Rocky Mountains of the United States.