TN-57 : Ground water recharge using tracer techniques

Abstract

Groundwater recharge is that amount of surface water which reaches the permanent water table either by direct contact in the riparian zone or by downward percolation through the overlying zone of aeration. It is this quantity which may in the long term be available for abstraction and which is therefore of prime importance in the assessment of any groundwater resource. Both natural and artificial tracers are being used in the country and abroad to study the soil moisture transport and estimate the direct recharge to the groundwater. The methods most commonly used are: (i ) Isotopic tracer injection method and (ii ) Environmental tracer method.

The injection of tritiated water below the active root zone and subsequent coring for samples in order to construct a profile of tritium concentration with depth provides one method for estimating recharge. Injections of the tracer are made at a number of locations about 10 cm apart along a line. In this way, the lateral spread of the tracer becomes of the order of a few decimetres owing to molecular diffusion. Subsequently cores are taken and cut into 10 cm sections for analysis in the laboratory. The core samples are weighed before and after extraction of the water by vacuum distillation at about 80°C, in order to obtain the soil water content of the individual cores. The accuracy of this technique is a function of the velocity of downward displacement, which in turn is dependent on the amount of recharge and the field capacity of the soil. Many examples of this technique have been described in this report.

The second method make use of environmental tritium for the estimation of groundwater recharge. The presence of tritium in groundwater indicated that atleast some of the water has been reached during the last few decades. During the early years of environmental tritium measurements, the converse conclusion, that absence of tritium implied that recharge was not occurring, failed to take into account that the unsaturated zone might be so deep that the recharging water had not yet reached the water table. Today the presence of tritium still provides at least qualitative information on recharge. In the early 1960s the concentration of tritium in precipitation reached a maximum after the moratorium on the atmospheric testing of thermonuclear devices'. This peak of tritium has been used for studying the infiltration of water through the unsaturated zone and for estimating the average recharge over the period from the peak in concentration in 1963 to the time of measurement of the tritium profile with depth.

The present status of groundwater recharge studies is reviewed in the light of techniques mentioned above.