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