Estimated value

The age of groundwater is also important as well as source and flow path for sustainable use of groundwater resources especially in arid and semi-arid regions. The age or residence time of the groundwater means a time between the groundwater recharge and present. Tritium, 3H is a effective tracer for age dating of the groundwater, because the 3H concentration of precipitation has been decreasing with a extraordinary peak in 1962 caused by nuclear tests performed on the northern hemisphere in 1960s (Fig. 1). Fig. 2 shows a spatial distribution of 3H concentration in the groundwater sampled in 2002 and 2003 in the Kherlen River basin. The 3H concentration of groundwater is relatively high in the upper stream region, whereas that is relatively low in down stream How old is groundwater in Kherlen River basin