Lake Tahoe geochemical study. 1. Lake chemistry and tritium mixing study

Abstract

The study of vertical mixing in Lake Tahoe by temperature and oxygen measurements is limited to the top layer of the lake, since vertical gradients are extremely small at greater depth. Below 200 m the temperature gradient is about −1.6 ✕ 10−4 °C m−1, with little or no seasonal variation. A tritium profile was taken at a midlake station in 1973 together with samples for measurements on lake chemistry. Oxygen, nitrate, total inorganic carbon, and pH showed the effects of photosynthetic activity to a depth of about 80 m, well below the thermocline; average planktonic composition was calculated from these data. Silicate and carbonate alkalinity profiles were essentially uniform, showing that biological removal is negligible in comparison to the rate of vertical mixing.Essentially no variation in tritium concentration with depth was observed. Tritium measurements and input estimates in Crater Lake were used to calculate tritium concentration in Lake Tahoe between 1954 and 1973 and to estimate a ratio in tritium input of 32% precipitation to 68% vapor exchange. The tritium profile gives strong evidence for at least occasional complete mixing of Lake Tahoe. The last complete mixing may have occurred in March 1973 and at least one other such event took place between 1964 and 1968.