Determination of isotopic composition of rainwater to generate local meteoric water line in Thohoyandou, Limpopo Province, South Africa

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.

Hydrochemical Processes and Isotopic Study of Geothermal Springs within Soutpansberg, Limpopo Province, South Africa

“…Samples were flushed using a mixture of either H 2 or CO 2 in He, which was also used as the equilibration gas. The water samples were equilibrated along with platinum (Pt) catalyst in preparation for deuterium measurements [34]. The equilibration time of the water samples with hydrogen gas was 40 mins, whereas carbon dioxide gas was equilibrated with water samples in about 20 h. Laboratory standards calibrated against international reference materials were analysed with each batch of the samples.…”

Precipitation Over Southern Africa: Moisture Sources and Isotopic Composition

“…The isotopic data of our water samples are shown in Figure 6 plotted against the global meteoric water line (GMWL: δ 2 H = (8.17 ± 0.06) δ 18 O + (10.35 ± 0.65) with r 2 = 0.99; Craig [1961a], refined by Rozanski et al., 1993) and African meteoric water line (AMWL: δ 2 H = 7.4 δ 18 O + 10.1; Durowoju et al., 2019) to illustrate the deviation from the lines. The δ 18 O and δ 2 H values vary from −15.56‰ to 7.09‰ and from −115.98‰ to −40.18‰, respectively.…”

“…The isotopic data of our water samples are shown in Figure 6 plotted against the global meteoric water line (GMWL: δ 2 H = (8.17 ± 0.06) δ 18 O + (10.35 ± 0.65) with r 2 = 0.99; Craig [1961a], refined by Rozanski et al, 1993) and African meteoric water line (AMWL: δ 2 H = 7.4 δ 18 O + 10.1; Durowoju et al, 2019) The resulting regression line from all collected samples is δ 2 H = 7.2 (±0.1) δ 18 O + 5.8‰ (±0.6). Regarding precipitation samples only, the regression line is δ 2 H = 7.3 (±0.1) δ 18 O + 7.5‰ (±1) with a higher intercept, but a similar slope.…”

Investigating stable isotope effects and moisture trajectories for rainfall events in Johannesburg, South Africa

“…The results from this work shall be useful to researchers interested in the use of stable isotopes to assess the provenance of water and recharge mechanisms, particularly for the Johannesburg area. This also improves the understanding of spatial isotopic composition of South African precipitation, adding onto work done on precipitation in other parts of South Africa (Diamond and Harris, 1997;Harris et al, 2010;IAEA/WMO, 2018;Durowoju et al, 2019).…”

“…Diamond and Harris (1997) and Harris et al (2010) studied the stable isotope effects in precipitation in the west coast of South Africa. Durowoju et al (2019) specified the isotopic effects in rainfall of Thohoyandou in Limpopo Province and generated the Thohoyandou Local Meteoric Water Line (TLMWL) as δ 2 H = 7.56δ 18 O + 10.64‰ (n = 12). Leketa et al (2018) determined the Johannesburg Local Meteoric Water Line (JLMWL) using 3 years daily rainfall data (δ 2 H=6.7δ 18 O + 10‰, n = 260 and R 2 = 0.92) (Fig.…”