Estimation of groundwater recharge in savannah aquifers along a precipitation gradient using chloride mass balance method and environmental isotopes, Namibia
Introduction
Namibia is a dry sub-Saharan country with limited surface water resources due to the fact that all rivers inside Namibia are ephemeral and all perennial rivers are shared with neighbouring countries. Groundwater is therefore the main source of water in the country both for domestic and agricultural purposes.
Estimating groundwater recharge in arid and semi-arid regions like Namibia can be difficult, because such regions are characterized by generally low recharge compared to the average annual rainfall or evapotranspiration, and thus making it difficult to quantify precisely (Scanlon et al., 2002). Recharge occurs to some extend in even the most arid regions and, as aridity increases, direct recharge is likely to become less important than localized and indirect recharge, in terms of total aquifer replenishment (Alsaaran, 2005) (De Vries and Simmers, 2002).
Accurate quantification of recharge rates is vital for proper management and protection of valuable groundwater resources. For proper management systems the recharge to the aquifer cannot be easily measured directly but usually estimated by indirect means (Lerner et al., 1990).
Chloride mass balance (CMB) method and environmental isotopes have been commonly used in water resource development and management (Subyani, 2004). CMB method is based on the law of conservation of mass, whereby chloride is considered as a conservative tracer. The input of chloride deposition by both dry and wet deposition is assumed to balance out the output of chloride concentration by infiltration and mineralisation.
CMB method has been successfully applied in several studies to estimate groundwater recharge rates in semi-arid areas. Sharma and Hughes (1985) estimated groundwater recharge using CMB method in the deep coastal sands of Western Australia, Gieske et al. (1990) in south eastern Botswana and Subyani (2004) in Saudi Arabia with 15, 2.5 and 11% of the average annual precipitation respectively.
Environmental isotopes are widely used as tracers to understand hydrogeological processes such as precipitation, groundwater recharge, groundwater-surface water and vegetation interaction. A comparison of the δ18O and δ2H isotopic compositions of precipitation and groundwater provides an excellent tool for evaluating the recharge mechanism (Yeh et al., 2014). This method can only be used to understand groundwater recharge processes rather than quantifying groundwater recharge, and therefore needs to be used hand in hand with other groundwater recharge estimation methods such as CMB.
Vogel and Van Urk (1975) compared δ18O content of the precipitation at Grootfontein with the δ18O content of the groundwater from the Etosha National Park, assuming a north western discharge of groundwater from the Grootfontein district. His conclusion was that recharge only takes place under exceptional circumstances, when precipitation tends to have lower heavy isotope content. Hoad (1993) considers that recharge to the confined Kalahari aquifer occurs by through flow from the unconfined Kalahari aquifer. The unconfined aquifer between Namutoni Gate and Otjikoto Lake is defined as the recharge area where direct diffuse recharge is thought to be the dominant recharge mechanism to the unconfined Kalahari aquifer. Groundwater recharge estimation using the saturated volume fluctuation approach revealed annual recharge ranging between 0.33% and 4% of the mean annual precipitation for both Kalahari and Otavi dolomite aquifers (Bäumle, 2003).
Mainardy (1999) estimated groundwater recharge rates based on the CMB method and on fracture aperture measurements. Recharge amount ranging between 3.2 and 4.8% of the mean annual precipitation were determined for bare, fractured sandstone in the western part of the Waterberg. Much lower recharge values of 0.2–1.8% of the mean annual rainfall in the area were derived for quartzite outcrops of the Nossib Group and for meta-sediments belonging to the Damara Sequence.
Külls (2000) estimated groundwater recharge in the north-eastern part of the Omatako Basin ranging between 0.1 and 2.5% using a water balance model. He also used CMB method that gave recharge values ranging between 2% and 3.3% of the mean annual rainfall.
Külls (2000) observed only little isotopic enrichment by evaporation in the western part of the Waterberg area. However, the isotopic composition of groundwater from the secondary aquifers in the Damara Sequence north of the Waterberg indicates some evaporative enrichment due to shallower depths to the water table.
Taapopi (2015) estimated groundwater recharge rates in the unsaturated zone at Ebenhaezer farm in the Stampriet Basin using CMB method. Her findings ranged from 0.18% to 0.71% of the mean annual precipitation. Stone and Edmunds (2012) estimated groundwater recharge rates in the Kalahari dune field, Stampriet Basin using CMB method in the unsaturated zone. Their findings indicated recharge values between 4% and 20% of the mean annual precipitation, with chloride profiles representing between 10 years and 30 years of rainfall infiltration. JICA (2002) determined groundwater recharge rates of the Auob aquifer system, Stampriet Basin and found out that the recharge is 1% of the long-term mean annual precipitation.
Although groundwater recharge studies have been carried out in Namibia, a seasonal sampling along a precipitation gradient has not been carried. This study thus aims at identifying groundwater recharge rates as well as processes using a CMB method and water stable isotopes δ2H and δ18O along a precipitation gradient in the savannah aquifers, therefore from Tsumeb area in the north, Waterberg in the central part and Kuzikus/Ebenhaezer further south-east of Namibia.
Section snippets
Location
The study was carried out along a precipitation gradient in the following areas: Tsumeb, Waterberg and Kuzikus/Ebenhaezer. The study areas indicate a precipitation gradient (Fig. 1). Tsumeb area lies within the south-eastern part of Cuvelai-Etosha Basin, having the highest annual precipitation rate of about 600 mm/a and an annual potential evaporation rate ranging between 2000 and 3000 mm/a. Waterberg area lies within the south-western part of Omatako Basin. The area receives an annual
Materials and methods
Seasonal field campaigns were carried out between 2016 and 2017 for Tsumeb, Waterberg and Kuzikus/Ebenhaezer areas. A total of 20 rainwater samples were collected monthly throughout the rainy seasons using a rain collector at all three study sites. Groundwater sampling was done before rainy season (around November), during rainy season (March), and after rainy season (June). As a result, a total of 28, 25 and 58 groundwater samples were collected from boreholes in Tsumeb Karst Aquifer, Tsumeb;
Groundwater physio-chemical parameters
Groundwater pH ranges between 6.0 and 7.2 for Tsumeb; 5.4 to 8.4 for Waterberg; and 6.2 to 8.0 for Kuzikus/Ebenhaezer, thus groundwater in these areas is slightly acidic to slightly alkaline in nature with an exception of Onyoka spring in the Waterberg area which is mildly acidic, covered by algae and not captured due to its low yield. With an exception of Onyoka spring in Waterberg, pH for all sites are within a range of 6–9 therefore, groundwater at all three study areas can be classified as
Groundwater physio-chemical parameters
Groundwater samples from Onyoka spring in the Waterberg area are mildly acidic due to the presence of algae. The presence of algae in water reduces its pH due to the fact that the pH of the water is lowered during respiration, where carbon dioxide is produced and hydroxide levels decreases (Assmy and Smetacek, 2012).
Waterberg groundwater having the lowest electrical conductivity, followed by Kuzikus/Ebenhaezer can be explained by the fact that groundwater at these two study sites are hosted in
Conclusion
The water quality assessment based on the onsite parameters show that groundwater at all three sites is mostly safe for human consumption. Chloride Mass Balance method revealed that Waterberg area has the highest recharge rate compared to the other two study sites despite Tsumeb having a higher mean annual precipitation amount, followed by Tsumeb area and Kuzikus/Ebenhaezer area having the lowest. High recharge rates in the Waterberg can be related to the absence of evaporation as indicated by
Declaration of competing interest
Authors declare that there are no conflicts of interest.
Acknowledgements
Authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding this reasearch through OPTIMASS project (01LL1302A).. We would like to also acknowledge and thank all the farmers who allowed us to sample their private boreholes. Special thanks goes to the maintenance team at Waterberg Namibia Wildlife Resort for field assistance in sampling their springs.
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