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

Highlights

• Based on the onsite parameters, groundwater from all three sites is safe for human consumption.

• High recharge rate at Waterberg is due to fast infiltration via preferential paths..

• A low recharge rate at Kuzikus/Ebenhaezer is attributed to evaporation..

• Roof rainwater harvesting should be considered for the Stampriet Basin.

Abstract

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.

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.