RN-24 : Environmental isotopes for hydrological investigations

Abstract

In this report it is proposed to review the status of applications of environmental isotopes in hydrological investigations. These include Tritium H³ Carbon-14(C14), naturally occurring isotopes and st isotopes and stable isotopes Deuterium(D) and oxygen-18(0 18 ) In some cases measurements of environmental isotopes of only few samples may provide important hydrological information which could be obtained only with difficulty or not at all by other methods. For example, the detection of tritium in ground water proves that ground water was formed after 1953-54 and origin of different waters is given by variation in stable isotopic contents. Applications of environmental isotopes are useful for dating ground water by using tritium and Carbon-14, evaluation of ground water recharge by measuring environmental tritium profile in soil water and studying interconnections between surface and subsurface waters. Other applications of environmental isotopes employ stable isotopes of Deuterium and oxygen-18 to identify source of recharge, to study effect of local surface bodies on the ground water and lake evaporation studies.

Variation in the concentration of stable isotopes in rainwater is due to evaporation and condensation processes. Depletion in stable isotope composition is also observed with decreasing temperature and with increasing latitude. The effect of latitude, altitude and distance from sea on environmental isotopes is briefly discussed in this report.

Stable isotopic composition of water is measured using Mass spectrometerand environmental tritium is measured by Liquid scintillation spectrometerand also by Proportional counters. Electrolytic enrichment is compulsory for counting very low activities. The measurement of these environmental isotopes is a vast subject and is not discussed in this report. Attempts have been made to present a review of environmental isotopes in hydrological investigation covering briefly application in many fields and water balance studies of lakes and reservoir in particular.