Sand, salt and water in the Stampriet Basin, Namibia: Calculating unsaturated zone (Kalahari dunefield) recharge using the chloride mass balance approach

Aec Stone,Wm Edmunds

doi:10.4314/wsa.v38i3.2

Abstract

Estimating groundwater recharge rates in the Stampriet Basin is important for assessing the sustainability of the groundwater resource both within south-east Namibia, and across the borders of this transboundary resource into Botswana and South Africa. The 65 000 km2 basin contains a multi-layered aquifer system, of up to 9 superimposed aquifers, with a majorupper unconfined and 2 major artesian levels. The majority of water abstraction (66%) is from the unconfined Kalahari Aquifer. This research investigates the direct recharge pathway through the dune-sand cover of the southern Kalahari dunefield which overlies ~80% of the basin. This is undertaken using established chloride mass balance methods within 4unsaturated zone profiles (9 m to 11.5 m depth) from 2 regions of the basin. The results indicate that recent recharge rates are between 7 mm·y-1 to 46 mm·y-1, with profiles representing between 10 years and 30 years of rainfall infiltration. This is the first time that substantial direct (diffuse) recharge has been quantified for the Stampriet Basin, and this part of the southern Kalahari. These 4 profiles indicate potentially similar levels of spatial variability as seen in many other semi-arid, sand-rich unsaturated zones across the world. Further studies using the unsaturated zone should be employed in this region, to better inform our  understanding of recharge mechanisms across the basin. The chloride mass balance technique provides information about the unsaturated zone, and important insights about recharge in various sand-covered regions of the basin for which very little is known, or is possible to determine using conventional water balance approaches. Direct recharge rates estimated in this way constitute a vital part of constraining the water balance of the basin, and particularly of the Kalahari Aquifer from which the majority of groundwater abstraction occurs.

Highlights

The Stampriet Basin is a multi-layered aquifer covering ~65 000 km2 in Namibia (JICA, 2002; Tredoux et al, 2002) and extending an unknown distance into Botswana and South Africa (Fig. 1)
This paper provides the first application of the chloride mass balance (CMB) technique (Edmunds et al, 1988) in the unsaturated zone (USZ) above the SAB to address whether direct recharge through the Kalahari linear sand-dune cover is occurring as potential recharge to groundwater
Results for the unsaturated (or vadose) zone (USZ) recharge in the Stampriet Basin are in line with the recharge estimates using chloride in other semi-arid regions

Summary

Introduction

The Stampriet Basin (often referred to as the Stampriet Artesian Basin, SAB, whether or not the artesian layers are being considered) is a multi-layered aquifer covering ~65 000 km2 in Namibia (JICA, 2002; Tredoux et al, 2002) and extending an unknown distance into Botswana and South Africa (Fig. 1).

Results

Conclusion