Annual shifts in O- and H-isotope composition as measures of recharge: the case of the Table Mountain springs, Cape Town, South Africa

Tracing the flow of groundwater with little hydrochemical variability over short distances is challenging. Stable isotopes offer a solution where there is considerable variation in relief. This study used stable isotopes to delineate recharge areas and pinpoint the aquifer responsible for the many springs that issue from the lower slopes of Table Mountain in South Africa. Table Mountain rises 1,086 m above sea level (masl) and, like the surrounding south-western Cape region, experiences a Mediterranean climate. The stable isotope composition of rainfall and springs issuing from the lower slopes was measured over the 2010–2012 period, allowing comparisons over space and time. An amount effect, an altitude effect (δD = −0.48‰/100 m, δ¹⁸O = −0.075‰/100 m) and significant interannual variations (year to year ~10‰ δD and ~1‰ δ¹⁸O) in isotope composition of rainfall were observed. This enabled estimation of the average altitude of recharge to be ~300 masl, which indicates that the spring sources are aquifers comprising the scree aprons ringing the mountain, with an average groundwater flow path of about a kilometre. Matching the shifts in average annual isotope composition and deuterium excess of the springs with that of nearby rainfall suggests that some groundwater flow is fast: recharge to discharge within months. However, steady discharge rates over seasons and years indicate that flow is generally slow. Hence, a hydrogeological model is proposed with fast, recent water flowing in shallow parts of the aquifer at the same time as slow, older water in deeper parts of the aquifer.