Estimation of groundwater recharge in savannah aquifers along a precipitation gradient using chloride mass balance method and environmental isotopes, Namibia

Uugulu, S.; Wanke, H.

Abstract

© 2020 Elsevier Ltd The quantification of groundwater resources is essential especially in water scarce countries like Namibia. The chloride mass balance (CMB) method and isotopic composition were used in determining groundwater recharge along a precipitation gradient at three sites, namely: Tsumeb (600 mm/a precipitation); Waterberg (450 mm/a precipitation) and Kuzikus/Ebenhaezer (240 mm/a precipitation). Groundwater and rainwater were collected from year 2016–2017. Rainwater was collected monthly while groundwater was collected before, during and after rainy seasons. Rainwater isotopic values for δ18O and δ2H range from −10.70 to 6.10‰ and from −72.7 to 42.1‰ respectively. Groundwater isotopic values for δ18O range from −9.84 to −5.35‰ for Tsumeb; from −10.85 to −8.60‰ for Waterberg and from −8.24 to −1.56‰ for Kuzikus/Ebenhaezer, while that for δ2H range from −65.6 to −46.7‰ for Tsumeb; −69.4 to −61.2‰ for Waterberg and −54.2 to −22.7‰ for Kuzikus/Ebenhaezer. Rainwater scatters along the GMWL. Rainwater collected in January, February and March are more depleted in heavy isotopes than those in November, December, April and May. Waterberg groundwater plots on the GMWL which indicates absence of evaporation. Tsumeb groundwater plots on/close to the GMWL with an exception of groundwater from the karst Lake Otjikoto which is showing evaporation. Groundwater from Kuzikus/Ebenhaezer shows an evaporation effect, probably evaporation occurs during infiltration since it is observed in all sampling seasons. All groundwater from three sites plot in the same area with rainwater depleted in stable isotopic values, which could indicates that recharge only take place during January, February and March. CMB method revealed that Waterberg has the highest recharge rate ranging between 39.1 mm/a and 51.1 mm/a (8.7% – 11.4% of annual precipitation), Tsumeb with rates ranging from 21.1 mm/a to 48.5 mm/a (3.5% – 8.1% of annual precipitation), and lastly Kuzikus/Ebenhaezer from 3.2 mm/a to 17.5 mm/a (1.4% – 7.3% of annual precipitation). High recharge rates in Waterberg could be related to fast infiltration and absence of evaporation as indicated by the isotopic ratios. Differences in recharge rates cannot only be attributed to the precipitation gradient but also to the evaporation rates and the presence of preferential flow paths. Recharge rates estimated for these three sites can be used in managing the savannah aquifers especially at Kuzikus/Ebenhaezer where evaporation effect is observed that one can consider rain harvesting.

• Journal Article Type: Article
• Acceptance Date: Jan 29, 2020
• Online Publication Date: Feb 2, 2020
• Publication Date: Apr 1, 2020
• Deposit Date: Apr 22, 2020
• Publicly Available Date: Jul 29, 2022
• Journal: Physics and Chemistry of the Earth, Parts A/B/C
• Print ISSN: 1474-7065
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 116
• Article Number: 102844
• DOI: https://doi.org/10.1016/j.pce.2020.102844
• Keywords: Geochemistry and Petrology; Geophysics; Chloride mass balance; Groundwater recharge; Isotopic values; Precipitation gradient
• Public URL: https://uwe-repository.worktribe.com/output/5916909
• Additional Information: This article is maintained by: Elsevier; Article Title: Estimation of groundwater recharge in savannah aquifers along a precipitation gradient using chloride mass balance method and environmental isotopes, Namibia; Journal Title: Physics and Chemistry of the Earth, Parts A/B/C; CrossRef DOI link to publisher maintained version: https://doi.org/10.1016/j.pce.2020.102844; Content Type: article; Copyright: © 2020 Elsevier Ltd. All rights reserved.