Preliminary Evaluation of Geothermal Reservoir in Central Nepal Using Geochemical Data

Abstract

The oxygen-18/16 and deuterium ratios of water samples collected from 36 hot springs have been determined in the central region. The tritium activity of 14 of them shows that the thermal waters where outflow temperatures are up to 71°C are characterized by two features, results of stable isotope measurements fit the so-called world meteoric line and all the springs tested have tritium activity between 7 and 68 T.U. The author has combined the results of other geochemical data of the thermal water in the area with the results of the above isotope study to undertake preliminary estimation of the reservoir size.
It the tritium content is appreciable and variable with time, it means that an appreciable amount of water younger than 40 years is present and the variations imply a short circulation time of the order of a few years. Another possibility is that waters from two different sources are present: a mixture of old tritium-free water and a young water containing tritium. If the tritium content is appreciable and constant in time, the younger water is well mixed in the aquifer with old water and the size of the reservoir masks any fluctuations in recharge.
However, it has been found that the waters at Sadhu Khola, Tatopani-Mustang and Mayangdi lying in this region have relatively high chloride content suggesting that the waters are fairly mature. Variation in oxygen and hydrogen isotopes show the high possibility that waters from two different sources are mixed. Since tritium is present in appreciable quantity, younger water is well mixed in the aquifer with old water and the size of the reservoir masked any fluctuations in recharge. Hence there is possibly a large geothermal reservoir in the Sadhu Khola – Jomsom area in the central region of Nepal.