Tritium concentrations of precipitation, total rainout, and rainfall amounts for 1964 and 1965 are listed for 15 stations in the conterminous United States, Alaska, and Puerto Rico. These data show that the major increase in tritium rainout observed in 1963 was followed by • decline of about 27% in 1964 compared with 1963, and an additional decline of about 54% in 1965 compared with 1964. Rainout patterns are characterized by increases in tritium rainout with northern latitudes and with distance from the oceans. The total tritium rainout, being a function of both concentration and amount of precipitation, is more random than average tritium concentration. Seasonal, annual, and even ten-year tritium pulses have provided a valuable tag for tracing water in many kinds of hydrological and meteorological investigations. The tritium data now available make it possible to estimate the tritium input to the hydrologic environment at any part of the United States. (Key words: Radioactivity; precipitation; quality of water; tracers) STEWART AND FARNSWORTH To document tritium input to surface and ground waters, the U.S. Geological Survey established a network in 1958 for collecting and testing rainwater samples for tritium activity. Since about January 1963, the network has included 15 stations, collecting bi-weekly samples for tritium analysis. U.S. Geological Survey tritium data from 1958-1963 are published in reports by Thatcher [1962], Thatcher and Ho#man [1963], and Stewart and Ho#man [1966]. This report covers 1964-1965 tritium rainout data with some reference to earlier data for comparison purposes. The primary purpose of this report is to document tritium rainout data, to discuss general observations, and to point out important hydrologic implications. Subsequent reports will list tritium data for 20 United States rivers and will give more detail on the meteorological factors causing observed rainout patterns as well as trends ]ess general than those included here. THE NETWORK To provide bases for natural tritium input to the hydrologic systems of the Nation and yet hold the sample testing volume to a manageab!e level, stations were selected where they could be representative of a large segment of the surrounding country. Where possible, the stations were located in cities where U.S. Geological Survey personnel could collect the samples. In some cases, however, it was advantageous to locate the stations at regular U.S. Weather Bureau stations. The Washington, D.C., and Boston, Massachusetts, stations are operated by U.S. Weather Bureau personnel. Other stations, operated by the Geological Survey, include:
