Distribution of tritium in precipitation and surface water in California

Tritium is an important tool in hydrological research. As a radioactive isotope and part of the water molecule, it can be used in tracer studies and to provide an estimate of water residence time. The spatial distribution of cosmogenic tritium in precipitation is affected by geomagnetic latitude and distance from the ocean, but was overwhelmed by tritium from atmospheric nuclear weapons testing in the 1950’s and 1960’s. The purpose of this study was to determine the current tritium input from precipitation and the distribution of tritium in surface water across California. This study presents a data set with a higher spatial resolution sample population than previously performed in California. Precipitation was sampled directly when possible. Greater spatial coverage was accomplished by sampling surface waters (mainly major rivers) as a proxy for precipitation. Data were analyzed using geospatial software and compared to the tritium distribution predicted from previous models. The arithmetic mean tritium in California precipitation and surface water was found to be 9.4±3.6 pCi/L. Mean tritium in precipitation alone was found to be 11.1±4.8 pCi/L, within two standard deviations of the estimated pre-bomb pulse background concentration of 6.0 pCi/L. Mean concentrations in spatial groups ranged between 8.0±3.5 pCi/L in the San Francisco Bay Area and southern coast to 11.7±2.9 pCi/L in the Sierra Nevada Mountains, indicating a trend of increasing tritium concentration with inland distance. Repeat sampling indicated that Sierra Nevada water exhibited seasonal variation in tritium concentration, increasing between January and May due to Spring Leak effect. Waters collected from urbanized areas around the San Francisco Bay were found to be significantly altered by anthropogenic tritium. The relationship between tritium and electrical conductivity was found to depend upon water origin and anthropogenic activity, such as agricultural use. Stable isotopes of oxygen and hydrogen in the water molecule were also analyzed since they are reliable indicators of water source area and can also be used to determine whether the water has experienced evaporation. In the California data set, stable isotopes proved useful for distinguishing possible evaporative effects from small additions of anthropogenic tritium.