Hongbing Tan

Isotopic constraints on the origin of groundwater in the Ordos Basin of northern China

Despite its extreme aridity, the Ordos Basin in northern China is rich in groundwater. Many artesian wells or springs with large fluxes are utilized for drinking, irrigation and industrial production. In a search for the origin of the groundwater, a detailed investigation of the stable isotopes of oxygen and hydrogen in the local precipitation, the river water, the springs, the well water, as well as the soil water extracted from six soil profiles in the Ordos Basin, was carried out. The data show that δD, δ18O and TDS values of the river water are similar to those of groundwater, while the TDS values of the soil water are about ten times greater than those of groundwater. Furthermore, the mean isotopic compositions of the local precipitation are significantly higher than those of river water and groundwater. Based on the chloride mass balance method, the estimated recharge rates range from 5.2 to 17.2xa0mm/year, with a mean value of 10.5xa0mm/year. The results show that the main source of recharge of the groundwater in the Ordos Basin is not the local precipitation, but must come from a region where the precipitation is characterized by much lower δD and δ18O values. In addition, the groundwater in the Ordos Basin contains a component of mantle-derived 3He and crust-derived 4He suggesting that the groundwater may partly derive from flows through basement faults beneath the Ordos Basin.

Isotopic and hydrochemical data to restrict the origin of the groundwater in the Badain Jaran Desert, Northern China

Despite its extreme aridity, the Badain Jaran Desert is rich in groundwater. In the southeastern part of the desert, it is characterized by coexistence of high megadunes and a great number of lakes. Deuterium and oxygen 18 isotope compositions as well as hydrochemistry of groundwater, lake water, soil water and river water were investigated in detail to gain an insight into their relationships and the origin of the ground-water. The results show that the groundwater and the lake water are genetically related, but unrelated to local precipitation and the leakage of Heine River at northern slope of Qilian mountain. δD and δ18O values of deep soil water (lower than 40 cm) and groundwater plot on the same evaporation line E11, which shows that they have the same recharge source. The point of intersection between E11 and LMWL suggests that the groundwater originates from the water resource which has a weighted mean value that is lighter by some 6‰ δ18O than the local precipitation in Badain Jaran Desert. 3H data of water samples shows that the groundwater in Badain Jaran Desert originates from the water recharged after the nuclear test. The deep fault zone underground maybe the water circulation channel based on Helium analysis of groundwater. The result has guiding significance to rational exploitation and utilization of the local groundwater.

New lakes in the Taklamakan Desert

[1]xa0At the beginning of this century, some new lakes unexpectedly appeared in the Taklamakan Desert – one of the largest and driest deserts in the world. The origin of the water in the new lakes, however, has been a matter of debate. We test the hypotheses by analyzing water samples collected from the new lakes, the Bosten Lake and the foothills of the mountains that surround the desert. Using the isotopic composition of these waters as tracers, together with data from remote sensing, local weather stations and stream gauges, we infer that the new lakes may originate from increased recharge of groundwater from meltwater in the nearby mountains. We further speculate, supported by first-order simulation of earthquake-induced discharge of groundwater from mountains, that the source of this increased recharge may be related to earthquake-induced discharge of groundwater from nearby Altyn and Kunlun Mountains.

Chemical and Sr isotopic compositions of rainwater on the Ordos Desert Plateau, Northwest China

The major ions and Sr concentrations and the Sr isotopic compositions of rainwater from four weather stations in the Ordos desert, Northwest China, were measured in this study. In the studied rainwater, Ca2+ was the most abundant cation with a volume-weighted mean (VWM) value of 387xa0µmol/L, and SO42− was the dominant anion, with a VWM value of 229xa0µmol/L. The Sr concentrations varied from 0.1 to 72xa0µmol/L, and the strontium isotopic ratios (87Sr/86Sr) range from 0.7098 to 0.7110, with an average of 0.7106, which are higher than that of seawater. The 87Sr/86Sr ratios showed the potential to trace sources of rainwater solutes when combined with other chemical composition data. The covariation between Mg/Ca vs. Ca/Na and Sr isotopic ratios vs. Ca/Sr in the rainwater suggested the mixture of at least three sources: soil dust derived from the local area and/or desert and loess areas in northwest China (87Sr/86Sr values of 0.7114), seawater (87Sr/86Sr value of 0.70917), and anthropogenic inputs (87Sr/86Sr value of 0.7110 and high Ca/Sr, due to coal combustion or automobile exhausts). The human activity inputs are likely the major sources of atmospheric contaminants in the Ordos rainwater. From back-trajectory analysis, principal component analysis and a comparison between the major ion compositions of other selected sites in China, we conclude that the Ordos rainwater ion composition is more significantly influenced by desert and soil dust and anthropogenic sources (primarily industrial and traffic emissions from the surrounding cities) than by marine sources.

A new geochemical perspective on hydrochemical evolution of the Tibetan geothermal system

The uplift of the Tibetan Plateau has caused the development of many high-enthalpy geothermal fields that are rich in exploitable rare and dispersed resource elements. However, the mechanism of the unusual enrichment of these resource elements is still unclear. From a geochemical viewpoint, including major chemical compositions, some rare and dispersed resource elements and trace elements in geothermal water and some river samples from the northern Lhasa block (saline lake area), the southern Lhasa block (Gangdise volcanic belt) and Tethyan Himalaya in Tibet, this study provides new insights into the mechanism of the hydrochemical evolution of the Tibetan geothermal system. The Cl-type geothermal waters in the Gangdise volcanic belt and Tethyan sedimentary area show similar chemical characteristics that are apparently different from that of surface cold waters. The concentrations of Sb, Tl, As, K, Cs, Li, Rb, Ga, B, Cl, Th, Sc, Mn, V, and Ti in Cl-type geothermal waters are at least one order of magnitude higher than those in surface cold waters, but the concentrations of Ca, Mg, Ni, Cr, Zn, Fe, Co, Cu, Pb, and U in Cl-type geothermal waters are slightly higher or even lower than those in surface cold waters. Some rivers and streams in Tibet also show high concentrations of toxic elements. These rivers and streams are mainly polluted by geothermal spring discharge and are unsuitable for drinking. Some ions and elements (such as Ca, Mg, Ba, Sr, SO42− and Mn) in HCO3− —type geothermal waters from sedimentary rocks are affected by the availability of soluble minerals such as calcite, dolomite and gypsum. However, the other dissolved elements in HCO3−–type geothermal waters show the characteristic of mixing Cl-type geothermal waters with surface cold waters. The origin of deep fluids in Cl− and HCO3−—type springs is related, and this origin probably involves the contribution of crustal partial melting rather than single rock leaching. Thus, deep circulating groundwater mixes with residual magmatic fluids and evolves into the unusual enrichment of geothermal mineral resources.

Isotope fractionation of sandy-soil water during evaporation – an experimental study

ABSTRACT Soil samples containing water with known stable isotopic compositions were prepared. The soil water was recovered by using vacuum/heat distillation. The experiments were held under different conditions to control rates of water evaporation and water recovery. Recoveries, δ18O and δ2H values of the soil water were determined. Analyses of the data using a Rayleigh distillation model indicate that under the experimental conditions only loosely bound water is extractable in cases where the recovery is smaller than 100u2005%. Due to isotopic exchange between vapour and remaining water in the micro channels or capillaries of the soil matrix, isotopic fractionation may take place under near-equilibrium conditions. This causes the observed relationship between δ2H and δ18O of the extracted water samples to have a slope close to 8. The results of this study may indicate that, in arid zones when soil that initially contains water dries out, the slope of the relationship between δ2H and δ18O values should be close to 8. Thus, a smaller slope, as observed by some groundwater and soil water samples in arid zones, may be caused by evaporation of water before the water has entered the unsaturated zone.