Tritium hydrologic studies in four closed‐basin lakes in the Great Basin, U.S.A.1

Abstract

Four closed‐basin lakes in the Great Basin (Lake Abert, Oregon; Mono Lake, California; Pyramid Lake and Walker Lake, Nevada) were analyzed for tritium (3H) in water samples collected in 1969, 1976, 1980, and 1981. In each case the tritium concentration increased to a maximum in the mid‐to late 1960s in response to fallout produced by nuclear weapons testing and then slowly decreased soon after the 1966 moratorium. The highest level attained was a function of the water residence time of the lake (i.e. shorter residence time corresponding to higher maximum tritium concentration), and the rate of decrease was slowest for lakes with longer water residence times. The residence time of tritium is less than the water residence time and depends strongly on the evaporative exchange of tritium with the atmosphere.A tritium hydrologic model was constructed in an attempt to constrain hydrologic properties such as evaporation rate, relative humidity, and the degree of homogeneity of the water column with respect to tritium. If evaporation rates determined from hydrologic balance are within 20% of the true rates, then the tritium input must be between 50 and 90% enriched over that calculated from data on tritium fallout in Portland rain. This enrichment is considerably reduced if the surface waters are on the average depleted by about 10% in tritium relative to the whole lake. Calculated mean relative humidities lie between 0.45 (Mono Lake) and 0.25 (Walker Lake). Calculated exchange velocities in the gas phase are between 17 and 27 m h−1, which corresponds to a mean wind speed of about 4 m s−1.