Tritium profiles of pore water in the Chinese loess unsaturated zone: Implications for estimation of groundwater recharge

SummaryWithin the framework of the IAEA research contract No. 9402, two cores were taken in 1997 and 1998 in the loess unsaturated zone in Inner Mongolia and Shanxi province, respectively, where the tritium profiles of pore water from a test pit and a core taken in 1980’s had already been reported. It has been shown that even in the late 1990’s, the tritium profiles of pore water still showed clearly defined peaks, as those formerly reported. This is because both sampling sites were located at rather high latitudes of the Northern Hemisphere. This is also, thanks to the very special characteristics of the loess deposit, providing slow infiltration rate of pore water in the unsaturated zone. Average infiltration rate obtained from these tritium profiles was 0.30 m/a. The values of annual groundwater recharge were 47 and 68 mm/a for Inner Mongolia and Shanxi province, respectively. However, the tritium content of the groundwater aquifers near both sampling sites was higher than expected. Consequently, it can be concluded that the vertical infiltration in the soils of the semi-arid loess region, perhaps, is not the only mechanism of groundwater recharge. The unconfined aquifers may also be recharged by surface flow through deep valleys, or through vertical cleavages, which are generally developed in the loess deposit. Therefore, the values of groundwater recharge obtained by using tritium profiles in the loess unsaturated zone may be underestimated.