Environmental isotope study for estimating leakage and runoff of ground waters in the Xi'an area

Ninety-six water samples were collected in the Xi'an area and analysed for D, T, ¹⁸O and ¹⁴C. Stable isotope contents of water samples from three aquifers show differences between, and mixing among, the three aquifers; the mixing ratio was 37.4% of phreatic water in the first confined aquifer and 43.8% of first mixed water to the second mixed water in the second confined aquifer. The phreatic water comprised about 43.8% × 37.4% = 16.4% of the second mixed water in the second confined aquifer. After the tritium contents in precipitation in the Xi'an region had been reconstructed by a multiple cell linear regression method, the groundwater velocity in the first confined aquifer was calculated as 30-39 m year^-1 from an exponential flow model. A ¹⁴C piston flow model gave estimates of average flow velocity in the second confined aquifer of 1.14 year^-1. The runoffs in the two aquifers were also calculated.