wAbstract
@The Minqin Basin is a type area for examining stress on groundwater
resources in the Gobi Desert, and has been investigated here using
a combination of isotopic, noble gas and chemical indicators.
The basin is composed of clastic sediments of widely differing
grain size and during the past half century over 10000 boreholes
have been drilled with a groundwater decline of around 1 m a-1.
Modern diffuse recharge is unlikely to exceed 3 mm a-1,
as determined using unsaturated zone profiles and Cl-1
mass balance. A small component of modern (50 a) groundwater
is identified in parts of the basin from 3H-3He
data, probably from irrigation returns. A clear distinction is
found between modern waters with median Β18O values
of 6.5}0.5ρ and most groundwaters in the basin with more depleted
isotopic signatures. Radiocarbon values as pmc range from 0.6
to 85 modern, but it is difficult to assign absolute ages to
these, although a value of 20 modern C probably represents the
late Pleistocene to Holocene transition. The Β13C compositions
remain near-constant throughout the basin (median value of -8.1ρΒ13Cj
and indicate that carbonate reactions are unimportant and also
that little reaction takes place. There is a smooth decrease in
14C activity accompanied by a parallel increase in
4He accumulations from S-N across the basin, which
define the occurrence of a regional flow system. Noble gas temperatures
indicate recharge temperatures of about 5.6 for late Pleistocene
samples, which is some 2-3 cooler than the modern mean annual
air temperature and the recharge temperature obtained from several
Holocene samples. groundwaters in the Minqin Basin have salinities
generally below 1 g/L and are aerobic, containing low Fe but elevated
concentrations of U, Cr and Se 'mean values of 27.5, 5.8 and 5.3Κg
L-1, respectively). Nitrate is present at baseline
concentrations of around 2 mg L-1 but there is little
evidence of impact of high NO3 from irrigation
returns. Strontium isotope and major iron ratios suggest that
silicate reactions predominate in the aquifer. The results have
important implications for groundwater management in the Minqin
and other water-stressed basins in NW China - a region so far
destined for rapid development. The large proportion of the water
being used at present is in effect being mined and significant
changes are urgently needed in water use strategy.x
1. Introduction
2. Regional geology and hydrogeology
3. Methods
4. Results and discussion
@4.1. Recharge to the aquifer
@4.2. Origin and age of the groundwater
@4.3. Geochemical indicators of origin and groundwater evolution
@@4.3.1. Chloride and major ions
@@4.3.2. Strontium isotopes and groundwater provenance
@@4.3.3. Redox controls and trace element evolution
5. Conclusions
Acknowledgments
References