tracing the origin of water using o- and h-isotopes...introduction because the major constituents of...
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INTRODUCTION
Because the major constituents of water are O and H, their isotopes are the most conservative of tracers, and allow
various hydrological processes to be recognised, including evaporation and mixing of water from different sources and,
in favourable circumstances, estimation of the rate of recharge of aquifers. Here, we present one example of our
research, where the isotope composition of Cape Town springs help to constrain the nature of the aquifer(s) and the rate
of recharge. There are mny other potential applications.
Tracing the origin of water using O- and H-isotopes Chris Harris, Roger Diamond, Keo Halenyane, Sli Zondi, Aquinar Malatjie, Fayrooza Rawoot, Sherissa Roopnarain,
Greg Netshilindi. Stable Isotope Laboratory, Department of Geological Sciences, University of Cape Town
ACKNOWLEDGEMENTS Our research is funded partly by the NRF and WRC, and partly by funds generated from contract analytical work
EXAMPLE IMAGE
• The O- and H-isotope composition of water is normally presented in delta notation where (e.g.) δ18O is the degree of enrichment of the heavy isotope relative to standard seawater (SMOW).
• The starting point of any isotope hydrology project should be the determination of the composition of ambient rainfall. For 20 years, we have collected and analysed monthly rainfall on the roof of the Geological Sciences Building.
• Four Cape Town springs have distinct O- and H-isotope composition that are best explained as being due to variation in the average height of recharge. The springs do not come from a single aquifer; rather they are derived from discrete aquifers within alluvial fans.
• Comparison of temporal changes in the isotope composition of the springs and rainfall (see below) indicate that recharge is rapid (50% recharge in < 2 years.
Cape Town was favoured as a site
of settlement due to the
presence of springs
Springs are a source of
clean water
O- and H- isotope are powerful traces of
water origin
We understand the nature of the aquifers
supplying spring water much better
These techniques can be applied to many problems
where water from different
origins is involved
The change in spring water composition
follows that of rainfall and allows
us to estimate recharge rate
Main Spring Put to good use
UCT rain water 1996-2008
Table Mountain springs
‘Deuterium excess’ (d)
Global meteoric water line (GMWL)
has the equation dD = 8*d18O + 10
d refers to an individual sample, and
d = dD – 8*d18O
A sample that plots on the GMWL
will have a d of 10
If d > 10, sample plots above the
GMWL; if d < 10, sample plots below
GMWL
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