Tracing stable isotope values from meteoric water to groundwater in the Cape Flats, South Africa: indicative tool for resource management

Urbanization and water resources management remains a central issue in the twenty-first century, partly because water is rapidly becoming a scarce resource and partly due to increasing population. Therefore, the global focus relating to water resources has shifted from a water development perspective to one of water management. In the Western Cape of South Africa water resources management even becomes crucial. This is a semi-arid region that receives precipitation amounts from 500-800 mm annually. Analysis of the long-term rainfall pattern has shown a drier condition in the last few decades. Stable isotopes have been measured for rainwater, surface water and groundwater samples around Cape Town in order to understand the inter-relationships between water resources of this area. This is being used in a qualitative sense to demonstrate present day recharge to the groundwater. The long-term weighted mean of precipitation for Cape Town is -12.8 per mille and -3.28 per mille respectively for δ2H and δ18O. Surface water (rivers and reservoirs) falls on the evaporation line. The isotopic data indicate seasonal variations in the isotope composition of recharge to groundwater. The Cape Flats aquifer waters show similar stable isotope values but are distinguishable from those of the springs and the Table Mountain Group aquifer. The isotopic ratios tend to decrease with increasing distance from the coast. The indications and mechanism of recharge are integrated into the wide range of data, which are translated into a common language of the diverse actors in the water management process. (author)