Environmental Isotope Study on Recharge and Groundwater Residence Time in a covered Ordovician Carbonate Rock

This paper applies environmental isotope techniques to interpret recharge mechanisms and estimate transit time of a covered karst aquifer in semiarid NW China. The aquifer is of major importance to the water supply of the region, in particular in meeting the increasing demand on water resources and in sustainability assessment in the future. Deuterium, oxygen-18 and tritium of 459 samples were analyzed from precipitation, surface water and groundwater during one hydrological year. Based on the isotope analysis, the Ordovician carbonate recharge to the aquifer is mainly from direct infiltration of atmospheric water in carbonate outcrops in Sandao and the Dacha valley. Stable isotopes show that karst groundwater in the Dacha valley seems to have recharged in a catchment area with a mean elevation of 1685–1854 m. The distinct independent isotope composition of water in the limestone aquifer in the Dacha valley from that in Anguo suggests no interconnection between them. However, the temporal evolution of the stable isotopes concentrations, the tritium contents and the hydrochemistry show the existence of an interconnection between Dacha and Sando Ordovician limestone aquifers. The isotope compositions also show that the groundwater in the Ordovician limestone is a mixture of old groundwater with modern recharge from local infiltration. The maximum percentage of modern water is 71%, which was usually found at the intensively fractured areas. Adopting a model with exponent and piston time distribution, the mean turnover time of groundwater in the Ordovician carbonate rocks was evaluated to be 36 years. The size of the groundwater reservoir is estimated to be 0.134 billion m³ and the mean storage coefficient is 0.0073.