Estimating the turnover time of groundwater reservoirs by the helium- 3/tritium method in the era of declining atmospheric tritium levels: opportunities and limitations in the time bracket 1990-2000

During the period of high atmospheric tritium levels, tritium was used successfully to estimate the residence time of groundwater systems. With the gradual decline of atmospheric tritium, the age determination has become ambiguous, since one cannot distinguish between young waters and older ones in which the high tritium values are decayed. However, measurement of both tritium and of its decay product, ³He, extends the usefulness of the method. The possibilities and limitations in this application of the "dating method' up to the turn of this century are discussed for a range of flow and mixing conditions within an aquifer. The practical problem of loss of helium during recharge and discharge of the aquifer is also considered. -Authors