Abstract
Hydrogen (D) and oxygen (18O) isotopic compositions of precipitation are useful tools to delineate the nature of precipitation, groundwater recharge and climatological investigations. This study investigated the isotopic composition of 12 rainfall occurrences at Thohoyandou, with the objective of generating the local meteoric water line (LMWL) and determining the factors controlling the isotopic composition of the rain. The delta (δ) values for D and 18O of the samples were determined using a Thermo Delta V mass spectrometer connected to a Gasbench. Thohoyandou rainwater showed a wide range of stable isotope values; δD values of the rainwater varied from −76.3‰ to +22.7‰ (SMOW) with a weighted mean of −9.8‰ and δ18O values ranged from −10.78‰ to +3.07‰ (SMOW) with a weighted mean of −2.7‰. δ-values of rainwater were more enriched during winter and more depleted during summer, due to the amount of rainfall and seasonal effect. The LMWL in Thohoyandou is defined by δD = 7.56δ18O + 10.64, which shows a similar slope to the global meteoric water line (GMWL) but with a slightly higher intercept, of 10.64‰ instead of 10‰. This implies that the process of rain formation in Thohoyandou occurred under equilibrium conditions which are not significantly affected by evaporation. The slightly higher d-intercept value above the GMWL reflects an additional supply of recycled moisture across the regions. This implies that there is no continental effect but inland moisture from various water bodies and vegetation.
Highlights
Thohoyandou is one of the fastest growing towns within the Soutpansberg Group in Thulamela Local Municipality of Vhembe District in Limpopo Province, South Africa (Vhembe District Municipality, 2007)
According to the International Atomic Energy Agency (IAEA) (IAEA-IMO 2015), warm regions are characterised by more enriched isotopic values (+ve) of D and 18O while cooler regions are characterised by more depleted isotopic values (-ve)
The amount effect, which is a profound characteristic of tropical low-latitude precipitation, has been observed in Thohoyandou, South Africa and in studies in other locations, such as Cameroon (Njitchoua et al, 1999; Kuitcha et al, 2012; Wirmvem et al, 2014), Nigeria (Mbonu and Travi, 1994), Niger (Taupin et al, 2000; Risi et al, 2008), Ghana (Adomako et al, 2015), Ethiopia (Kebede and Travi, 2012; Kebede, 2013), Costa Rica (Sa’nchez et al, 2013) and India (Rai et al, 2013)
Summary
Thohoyandou is one of the fastest growing towns within the Soutpansberg Group in Thulamela Local Municipality of Vhembe District in Limpopo Province, South Africa (Vhembe District Municipality, 2007). Variation in stable isotopes of hydrogen (D) and oxygen (18O) in precipitation forms the primary background data for groundwater recharge investigations (Ingraham, 1998; Gupta and Deshpade, 2003; Gat, 2010; Kortelainen, 2009). These include the sources and timing of recharge, retention time and circulation of groundwater (Kortelainen, 2009). Such a study will require long-term data for stable isotopes in rainfall (IAEA/WMO, 2015). There are limited and noncoherent datasets for δ2H and δ18O of rainwater in the Province, Received 12 March 2018, accepted in revised form 6 March 2019
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