Determination of local meteoric water lines along a precipitation gradient, Namibia

Abstract

Precipitation is the main input parameter in the hydrological balance and plays important role in groundwater recharge. Isotopic fingerprints are a tool to trace this component. In this study, isotopic composition of precipitation was determined along a precipitation gradient at three sites namely: Tsumeb (600 mm/a precipitation; Waterberg (450 mm/a) and Kuzikus/Ebenhaezer (240 mm/a precipitation). Precipitation samples from Tsumeb and Waterberg were collected during the rainy season from 2017 to 2018, while Kuzikus/Ebenhaezer samples were collected between 2014 and 2015. A total number of 83 precipitation samples were collected. Precipitation samples were analysed using a Los Gatos water stable isotope spectro-analyser at the University of Namibia. Precipitation isotopic values  for δ¹⁸O(‰) range from -9.08  to 5.19 for Tsumeb, -15.96 to 5.09 for Waterberg and -12.54 to 4.75 for Kuzikus/Ebenhaezer, while δ²H(‰) isotopic values for Tsumeb, Waterberg and Kuzikus/Ebenhaezer range from -73.30 to 46.70; -117.50 to 40.60 and -82.50 to 47.80, respectively. Scattering of rain samples along the global meteoric water line in the areas could be attributed to a seasonal effect. Local meteoric water line equations for Tsumeb, Waterberg and Kuzikus/Ebenhaezer were obtained using a linear regression method and are δ²H = 7.78 δ¹⁸O + 6.74, R² = 0.95; δ²H = 7.37 δ¹⁸O + 5.77, R² = 0.97; δ²H = 7.16 δ¹⁸O + 9.88, R² = 0.96 correspondingly. All the slopes obtained from three study sites are lower than that of a global meteoric water line equation. A lower slope could be an indication that the local precipitation has experienced some subcloud evaporation, leading to enrichment of heavy isotopes. The effect is more pronounced at Kuzikus/Ebenhaezer where the slope is 7.16. Our findings could serve as baseline for these three study sites with regards to further isotopic investigations in the study areas especially in tracing the origin of groundwater.