- © 2010 Geological Society of South Africa
The O- and H-isotope composition of rainfall collected over a variety of time periods at the University of Cape Town (UCT) between 1996 and 2008 has been determined. A continuous record of monthly rainfall from 1996 to 2008 has a range in δD and δ18O values from −57 to +18‰ and −8.1 to +3.5‰, respectively. These data show limited but discernable temperature and amount effects. Daily rainfall between June 2000 to September 2001 ranges in δD and δ18O from −57 to + 27‰, and, −9.0 to +6.4‰ respectively, showing only a marginally greater temperature and amount effect than the monthly data. Rainfall collected at < hourly intervals during two storms in July and August 2000 showed changes in δD and δ18O value of 26 and 3.3‰, respectively in as little as 30 minutes during the passage of a cold front. Unusual events such as snow and hail in the area have much lower δD and δ18O values (−60 and −10.9‰). The deuterium excess values were significantly higher in the storm samples (average 20.0) and the hail and snow samples (average 30.1) than in the monthly rain samples (average 12.4).
The UCT monthly samples define a meteoric water line whose equation is δ= 6.41*δ18O + 8.66 (r = 0.88). The daily and hail/snow samples show a better correlation (r = 0.93) due to the greater spread of values, with an equations of the line of best fit of δD = 6.64δ18O + 11.89. Rain water collected during two storms in July and August show an excellent correlation between δD and δ18O (r = 0.97) with an equation of best fit of δD = 7.89 δ18O + 19.35. There has been no systematic change in annual amount, mean temperature or weighted mean isotope composition from 1996 to 2008, but 1996 (16.1°C) was, on average, 1.3°C colder than 1997 to 2008 (ave. 17.3°C). Annual rainfall at UCT has varied from ~1000 mm (1997) to 1700 mm (2001), and the weighted mean annual δD and δ18O values calculated from the monthly samples varies from −16 to −7‰ and −3.8 to −2.6‰, respectively. The years 2005 and 2006 showed a significantly lower deuterium excess (~6) compared to typical values of about 16, which suggests that these years experienced less frontal rain. This difference in isotope composition of rainfall is detectable in the isotope composition of water collected from springs on the lower slopes of Table Mountain and suggests that the Table Mountain aquifer(s) are recharged by approximately 50% within three years.