Elsevier

Applied Geochemistry

Volume 23, Issue 6, June 2008, Pages 1563-1580
Applied Geochemistry

The hydrochemistry of a semi-arid pan basin case study: Sua Pan, Makgadikgadi, Botswana

https://doi.org/10.1016/j.apgeochem.2007.12.033Get rights and content

Abstract

This study presents results on the fluid and salt chemistry for the Makgadikgadi, a substantial continental basin in the semi-arid Kalahari. The aims of the study are to improve understanding of the hydrology of such a system and to identify the sources of the solutes and the controls on their cycling within pans. Sampling took place against the backdrop of unusually severe flooding as well as significant anthropogenic extraction of subsurface brines. This paper examines in particular the relationship between the chemistry of soil leachates, fresh stream water, salty lake water, surface salts and subsurface brines at Sua Pan, Botswana with the aim of improving the understanding of the system’s hydrology. Occasionally during the short wet season (December–March) surface water enters the saline environment and precipitates mostly calcite and halite, as well as dolomite and traces of other salts associated with the desiccation of the lake. The hypersaline subsurface brine (up to TDS 190,000 mg/L) is homogenous with minor variations due to pumping by BotAsh mine (Botswana Ash (Pty) Ltd.), which extracts 2400 m3 of brine/h from a depth of 38 m. Notable is the decrease in TDS as the pumping rate increases which may be indicative of subsurface recharge by less saline water. Isotope chemistry for Sr (87Sr/86Sr average 0.722087) and S (δ34S average 34.35) suggests subsurface brines have been subject to a lithological contribution of undetermined origin. Recharge of the subsurface brine from surface water including the Nata River appears to be negligible.

Introduction

Sua Pan (surface area 3400 km2) is located in the Kalahari (Fig. 1A) (approx location 20.55° S, 26.28° E) it represents the eastern portion of the Makgadikgadi Pan complex (Fig. 1B) and is the second largest of countless smaller pans in Botswana. The northern part of Sua Pan (Fig. 2) which is at the heart of this study is subject to a major brine extraction, is an important wetland of significance to the breeding of flamingos (McCulloch et al., 2007), subject to seasonal flooding and a significant source of regional dust (Bryant et al., 2007) with impact upon the regional soil chemistry (Wood et al., 2008, White and Eckardt, 2006) in the western sector of the Makgadikgadi.

It was fortunate that river and lake water was sampled during a particularly active period of flooding, being augmented by samples of subsurface waters and soil leaches in the pan catchments. This allowed examination of the chemical relationship between the different types of water, which provides a rare insight into the workings of such a large system. The prevailing processes operating within the basin were established, as has been achieved elsewhere (Eugster, 1980, Bryant et al., 1994a, Bryant et al., 1994b, Rosen, 1994) and it is aimed to classify the hydrology on the basis of water and salt chemistry.

The Makgadigkadi pans are in part oriented along a Tertiary graben of the Pan African rift and occupy the lowest point (890 m a.s.l.) in the Kalahari, which in the past received water from the proto Zambezi resulting in a significant inland lake (Cooke, 1980) of around 66,000 km2. Sua is currently fed primarily by the ephemeral Nata River (12,000 km2) and other smaller intermittently flowing stream catchments (Semowane 1500 km2, Mosetse 1500 km2, Mosopo Rivers 3000 km2 depicted in Fig. 2) all of which drain gently sloping terrain to the east, consisting of thick Kalahari sediments and arenosols which are punctured by Archaean granite outcrops and underlain by Carboniferous-Jurassic sandstone and basalts (Thomas and Shaw, 1991). The northern end of Sua Pan undergoes the most pronounced seasonal flooding and drying and is subjected to significant subsurface brine extraction. Ntwetwe pan the western half of the Makgadikgadi (4700 km2) is linked to an overflow from Okavango Delta via the Boteti River, which has remained dry for more than a decade now (Fig. 2).

On average 450 mm of rain falls annually between the months of December and March at Sua Town on the eastern edge of the pan. It has been shown that for the last 25 a, flooding was linked to El Niño Southern Oscillation cycles and landfall of Indian Ocean cyclones (Bryant et al., 2007). The pan surface dries between April and November when evaporation greatly exceeds rainfall. Nevertheless, wet muddy patches can remain on the pan surface in particular near the Nata River Delta. The smooth pan surface consists of clay, silt and sand and some calcrete cementation at depth. Sua pan is the dustiest surface in Botswana and the Makgadikgadi ranks high amongst the world’s major dust sources (Washington et al., 2003) and disperses dust in the entire subcontinent (Resane et al., 2004). Dustiness at Sua is a direct result of Nata River discharge, pan flooding and drying cycles (Bryant et al., 2007) while pan surface evaporation processes control dust composition and chemistry.

Currently a BotAsh (Botswana Ash (Pty) Ltd.) mine manages 98 wells, an extensive network of pipes, evaporation ponds and processing facilities (Fig. 3) covering an area of almost 400 km2 on the northern end of Sua Pan. The mine has been operational since 1991 and extracts water at a rate of 2400 m3/h from an average depth of 38 m. The brine deposit is estimated to exceed 1 billion m3 in volume and the mine currently produces NaCl, Na2CO3, Na2SO4 and NaHCO3 salts. Despite the commercial nature of this operation, remarkably little is known about the chemistry and hydrology of the entire system.

Section snippets

Methods

Soil leach samples were obtained from the catchment of the Makgadikgadi basin in July 1999 (Table 1), floodwater was collected at road bridges between November 1999 and February 2000, lake samples were collected between December 1999 and July 2000 (Table 2) and subsurface brine water was extracted in November 1999 (Table 3). Sampling of surface water occurred against the backdrop of the unusually wet period of 1999–2000. Rains commenced early in November 1999 shortly after the collection of

Wet chemistry

All analyses of water samples are shown in a Piper Plot (Fig. 4). Soil leaches in general are rich in Ca2+, K+, HCO3- with high values of Mg2+ and some Na+ and Cl in particular towards the pan margin. Inflow water samples are relatively fresh and range from 87 mg/L to 1711 mg/L TDS (average −0.43 log Cl mmol/L). Lake samples range from 258 mg/L to 17,800 mg/L in December 1999 (−0.19 to 2.3 log Cl mmol/L) up to 709–31,200 mg/L in April 2000 (0.77–2.65 log Cl mmol/L). The southern pan with the highest

Water sources

There appear to be two types of floodwater:- those that are similar to leaches in the proximity of pans (salty) and those that are similar to leaches of typical Kalahari soils (fresh). Floodwater and soil leaches have similar chemical composition with regard to Mg2+, HCO3- and Ca2+, but floodwaters are not as rich in K+ as soil leaches. These fresher waters are largely of the Ca–HCO3 type. The salty floodwater is comparable with soil leaches from the saline pan margin (Ca–Na–HCO3). Early floods

Conclusions

This study captured surface water inputs and revealed the subsequent lake water evolution and associated evaporation products for Sua pan and the Makgadikgadi, which is amongst the largest arid zone, saline, inland pan systems in the world. After high initial Na–Cl concentrations for Nata River floodwater, seasonal inflow into Sua can be characterized as being Ca–HCO3 in type, while concentrated lake waters produces Na–Cl brines. The soils in the catchment add much of the Ca2+, HCO3-, Mg2+ and K

Acknowledgements

We would like to thank BotAsh Pty. Ltd. and in particular appreciate the support of Godfrey Nkala for his assistance in accessing the Sua pan wellfield brine. This study was carried out while the corresponding author was based at the University of Botswana. We would like to thank UB and the Department of Environmental Science for support and financial assistance (Research Project R 509) and Marty McFarlane for comments on the manuscript.

References (36)

  • M.L. Coleman et al.

    Direct reduction of sulfates to sulfur for isotopic analysis

    Anal. Chem.

    (1978)
  • H.J. Cooke

    Landform evolution in the context of climate change and neotectonics in the Middle Kalahari of north-central Botswana

    Trans. Inst. Brit. Geogr. New Ser.

    (1980)
  • J.A. Day

    The major ion chemistry of some southern African saline systems

    Hydrobiologia

    (1993)
  • J.I. Drever et al.

    Cyclic wetting and drying of the soil zone as an influence on the chemistry of ground water in arid terrains

    Am. J. Sci.

    (1978)
  • S. Ehrlich et al.

    Direct high-precision measurements of the 87Sr/86Sr isotope ratio in natural water, carbonates and related materials by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS)

    J. Anal. Atom. Spectrosc.

    (2001)
  • H.P. Eugster

    Geochemistry of evaporitic lacustrine deposits

    Ann. Rev. Earth Planet. Sci.

    (1980)
  • Fishman, M.J., Friedman, L.C., 1989. Methods for determination of inorganic substances in water and fluvial sediments....
  • I. Gavrielli et al.

    Mechanisms of sulfate removal from subsurface calcium chloride brines: Heletz-Kokhav oilfields, Israel

    Geochim. Cosmochim. Acta

    (1995)
  • Cited by (0)

    View full text