Application of environmental isotope to groundwater study of the Qinling Railway tunnel

Abstract

A comparison of isotopic data of groundwater with those of surface water and rainfall indicates that groundwater in the Qinling Railway tunnel, which is the longest single-track tunnel under construction in China, originates from precipitation. However, measurements of deuterium and oxygen-18 in the fissure water in the adjacent drift have shown that these two isotopes are obviously depleted relative to the spring water in the area. This permits the groundwater being classified into two types: shallow weathering fissure water and deep structural fissure water, both with different features of circulation, and makes it possible to determine mixing ratios of the water inflows into the drift. The reason for the deep water depletion in the two heavy isotopes with respect to the shallow water is believed to be the difference of their recharge elevations, above 2200 m and below 1800 m for the deep and shallow water, respectively. Unlike the deep water, with slow motion and a circulation depth of over 1000 m, the shallow water is in an active circulation zone and moves usually in a depth of 40–50m. Calculation of isotope mass balance indicates that the relative contribution of the shallow water to the inflow at main locations or in sections of the drift ranges between 25.1% and 67.3%, indicating that the shallow water is also a significant component of the water inflow.