Abstract
Stable noble gas isotopes are excellent groundwater tracers. Radioactive noble gases are emerging new tools in the study of groundwater circulation dynamics. Among these, the 85Kr and 81Kr, and 39Ar have advanced very fast in recent years and exhibit strong potential in the reconstruction of the history of groundwater recharge and evolution in sedimentary basins at different scales. Here, we report the findings in groundwater circulation dynamics as relative to intensive water-rock interactions, heat transfer and He gas flux in Guanzhong Basin located in Xi’an, the geographical centre of China, which is a rift basin created by collision between the Eurasia and Indian plates, with active neotectonic activities. The recent technological breakthrough in noble gas isotope measurements, i.e. the atomic trap trace analysis (ATTA) techniques on Kr and Ar gas radionuclei, has revolutionized groundwater dating. Noble gas samples from shallow and deep wells to 3000 m depth have been collected to study isotope variations to reconstruct the history of groundwater recharge and understand the water-rock interaction processes. Stable isotopes of water show strong water-rock interaction in the formation, creating a strong positive O-isotope shift up to 10 ‰, a phenomenon that is rarely seen in a fairly low temperature environment. Analysis of 85Kr and 81Kr show groundwater ages up to 1.3 million years old along both North-South and a West-East cross sections, which offers strong evidence about the slow moving flow, strong water-rock interaction, rich geothermal resources as well as He gas resources.
Highlights
The utilization of geothermal water from natural hot springs in Guanzhong Basin, Sha’anxi Province, China, began more than 1000 years ago in the Tang dynasty (618 to 907 A.D.)
According to samples 1, 2, 4 and 5, the oldest 81Kr age was identified to occur in the centre of Guanzhong Basin, which can be considered as groundwater retention area with a relatively closed subsurface environment and a significantly low rate of groundwater renewal
The 81Kr ages yield obvious evidence of groundwater recharge from both sides, which has confirmed our previous understanding regarding the geothermal waters in Xi’an which is recharged by precipitation from the Qinling Mountains in the southern side of the basin and the recharge of Xianyang geothermal field from both sides of the Guanzhong Basin [4]
Summary
The utilization of geothermal water from natural hot springs in Guanzhong Basin, Sha’anxi Province, China, began more than 1000 years ago in the Tang dynasty (618 to 907 A.D.). The well known Huaqingchi hot spring in Lintong city located about 20 km to the east of Xi’an city was an imperial thermal spa. Groundwater circulation condition is crucial to the efficiency of geothermal resources development. Previous studies have showed that geothermal water in Xi’an city is low in 14C activity, but close to the dating limit of 14C method [1]. The 36Cl dating method revealed a residence time of a few hundred thousand years up to one million years for the Tertiary thermal water aquifer [2]. We have studied the groundwater age using advanced ATTA technique which helped to interpret the hydrological conditions and geochemical evolution of the groundwater in the deep aquifer
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