Stable isotope hydrology of the Table Mountain group

Rain was collected from 2010 to 2012 at 15 locations around the Cape Fold Belt, at the same time as samples from rivers, springs, seeps and boreholes, totalling 435 samples. Precipitation ranged from -75 ‰ to +40 ‰ for δD and -12 ‰ to +8 ‰ for δO, showing seasonal patterns, with lower δ values in winter and higher in summer. Certain anomalous δ values can be attributed to individual weather events, such as thunderstorms. Using weighted data, the meteoric water line is δD = 6.15 δO + 8.21, which is similar to previous equations. The best fit line for groundwater δ values is δD = 7.09 δO + 10.08, the steeper gradient and higher intercept reflecting the predominance of heavy rainfall events with lower δ values in recharge, known as selection. The range of -47 ‰ to 0 ‰ for δD and -8 ‰ to -1 ‰ for δO values for all groundwater data is about half that of the rain values, due to the averaging effect from mixing during groundwater flow. Rainfall isotope composition is negatively correlated with continentality, as defined by the product of distance to the Atlantic and the closest coast. Isotope composition of rainfall is also strongly negatively correlated with altitude, with gradients from -0.48 to -2.2 ∆δD 100m and -0.075 to -0.34 ∆δO 100m . Sites that are elevated within the landscape have a reduced altitude effect, such as tall peaks, whereas mountain valleys display enhanced altitude effects. Temporal and spatial variations in the strength of the amount effect reveal meteorological variability and emphasise the need for long term monitoring. Groundwater-fed surface waters show little systematic variation in isotope composition within catchments, indicating groundwater mixing. Differences between catchments are significant and reveal continental and altitude effects across the Cape Fold Belt. There is no significant change in hot spring isotope compositions over 40 years, confirming deep, well mixed groundwater flow. In the greater Cederberg region, groundwater abstracted from the Peninsula Formation in the Olifants River Mountains appears to be locally recharged and not in the Cederberg mountains. Abstraction in the Rietvlei Formation north and south of the Hex River anticline is being recharged at higher altitude, around 1200-1600masl, probably in the Goudini or Skurweberg Formation and moves up the stratigraphy prior to discharge. Discharge of increasingly negative δ values, a sign of deeper groundwater from higher recharge areas, could warn against overabstraction. Similar relations occur at Gamkaberg (Little Karoo) and Rooihoogte (Great Karoo), where groundwater, having been recharged at higher altitude, moves up the stratigraphy in the Skurweberg aquifer. At Gamkaberg, selective recharge of isotopically more negative large rainfall amount events, or interannual shifts in isotope composition of rainfall may account for the more negative δ values of groundwater than rain and warns against overinterpretation of short term precipitation monitoring. The δD and δO values of springs around Table Mountain correlate positively with distance from the mountain, an amount effect. The altitude effect gradient revealed that the typical spring is recharged an average of 1km upslope of the spring at an average elevation of 330masl, into the scree aquifer. Shifts in annual mean δ values of precipitation are mimicked in the springs, proving that a component of groundwater flow is fast and shallow, whilst a deeper component must also exist to account for the steady discharge of perennial springs. An average hydraulic conductivity of 15–20m/day was calculated for the scree aquifer.