unsaturated zone of southeastern Botswana: a study of chlorides and environmental isotopes. A. GIESKE. University of Botswana, Geology Department, Private ...
Bemonalizatian in Hydrology (Proceedings of the Ljubljana Symposium, April 1990). IAHS Publ. no. 191, 1990.
Groundwater recharge through the unsaturated zone of southeastern Botswana: a study of chlorides and environmental isotopes A. GIESKE University of Botswana, Geology Department, Private Bag 0022, Gaborone, Botswana
E. SELAOLO & S. McMULLAN Geological Survey Department, Private Bag 14, Lobatse, Botswana
Abstract Research has been done in sandy loam soils of southeastern Botswana over the past two years, to gain an understanding of solute and isotope distributions as a function of depth below the surface in order to quantify moisture fluxes through the unsaturated zone. Preliminary analysis of 16 profiles in a small study area, has shown that in comparison with results obtained in similar semiarid environments, the chloride concentrations in southeastern Botswana are rather low. Chloride concentrations in general do not seem to increase in the root zone of the common Acacia tree species and most enrichment takes place near the surface. The analysis of samples taken after the rainy season of 1987-1988 with an above average rainfall of 850 mm, has indicated that the mobility of the solutes and isotopes in the unsaturated zone is very high and that one-dimensional steady-state infiltration models are not applicable. Recharge des eaux souterraines par la zone non-saturée du sud-est du Botswana: une étude des chlorures et des isotope du milieu Résumé Deux années des recherches ont été poursuivies dans les régions couvertes avec du sol argileux au sud-est de Botswana, pour obtenir une comprehension meilleure des distributions isotopiques et géochimiques en fonction des profondeurs au dessous de la surface à fin de déterminer les mouvements d'eau dans la zone non-saturée. L'analyse préliminaire de 16 sondages dans une petite région d'investigation, a montré que les teneurs en chlorures sont assez basses en comparaison avec des résultats obtenus dans des régions semi-arides similaires. Les teneurs en chlorures en général ne semblent pas d'augmenter dans la zone racinaire des arbres de l'espèce Acacia, et la concentration augmente du bas vers la surface. L'analyse des échantillons de sol extrahis après les pluies extraordinaires de 1987-1988, atteignantes 850 mm (pluviosité moyenne 550 mm), a indiqué que la mobilité des chlorures et des isotopes est très grande et que les modèles d'infiltration d'une seule dimension et en régime stationnaire, ne s'appliquent pas. 33
A. Gieske et al.
34
INTRODUCTION The main aquifer systems of eastern Botswana have developed in arenitic and dolomitic rocks and because of the geological and structural control on the geomorphology these groundwater basins normally correspond to surface catchments. The fractured and permeable zones are therefore covered with colluvial material and alluvial soils. It has been recognized (Jennings, 1974; De Vries & Gieske, 1990) that groundwater is recharged along various routes, i.e. direct infiltration at outcrop areas, hillslope runoff channelled into colluvial fans, riverbed infiltration and diffuse infiltration through the alluvial soils. For an appraisal of the relative importance of these contributing factors, an understanding of the moisture transport through the unsaturated zone is essential. Research in Botswana has been limited to an early tritium study by Jennings (1974) and to the tritium and chloride work by Foster et al. (1982) in the eastern Kalahari margin near Letlhakeng. The present paper describes an ongoing study of moisture transport through the alluvial soil cover of a small dolomitic aquifer near Lobatse in southeastern Botswana (Fig. 1). Twenty holes, varying in depth from 1 to 10 m, were drilled in an area of 200 by 200 m. Samples taken at regular depth intervals have been analysed for moisture, grain-size distributions, tritium, chloride and stable isotopes. Most of the holes were drilled in the period from September to November 1987, just before the rainy season. Since the total rainfall during that season reached a near record figure of 850 mm, 300 mm above the average rainfall, it was decided to drill five more holes in the area after the season, in order to study the impact of the rains on the study area. A fully automatic weather station and five neutron probe access tubes to a depth of 10 m have been installed on the site. Regular moisture monitoring combined with determination of soil moisture retention and diffusivity curves will enable us to make a long-term analysis of the soil physics in the area under investigation. This paper will concentrate mainly on the preliminary findings of the chloride and isotope analysis. STUDY AREA The study area is located approximately 10 km north of Lobatse (Fig. 1) and lies in the Pitsanyane basin which is described by De Vries & Gieske (1990). The site lies just off the main valley axis on the Manyelanong fault and is bounded by a railway track on the northwestern side, and by dolomite outcrops to the east. As can be seen from Fig. 2, the area is sloping gently in the order of 2%. Soil thickness is about 10 m in the centre, but decreases to less than 1 m on either side of the valley. Below the alluvial soils, there is approximately 20 m of mixed dolomitic and quartzitic boulders which form the transition to dolomite bedrock. The piezometric level is about 35 m below the surface. The area is densely vegetated with a continuous well developed grass layer and an open discontinuous layer of shrubs and trees. Tree species consist mainly of Acacia tortilis (dominant), Acacia karroo, and Ziziphus mucronata, while shrubs and bushes consist of Acacia mellifera, Dichrostachys
35
YZâ
——350
Groundwater recharge in southeastern Botswana
Location of the study area within the Pitsanyane and Nnywane basins
Mean annual rainfall in m m
~ < u J j j j > J - i - Limit of Kalahari sand cover; bedrock on ticked side
®
Network showing rainfall sample points for tota! chloride deposition
Fig. 1 Map of Botswana, indicating study area. cinerea, Asparagus spp. and Rhus spp. Most common grasses are Eragrostis rigidior, Aristida congesta, Cynodon dactylon and Digitara pentzii, indicating a normal grassland type which is grazed and has a fair amount of moisture. The tree species also indicate deep soils with relatively good water supply.
36
A. Gieske et al.
Canopy cover is about 30%. Not much is known about the rooting depth of the dominant Acacia species. Soil samples generally showed fine root material up to a depth of 3 m. A cattle kraal is situated on the northern side of the area (Fig. 2). The number of cattle at present, however, is fairly limited and disturbance of the vegetation in the study area is not considered abnormal. Soils in the area can be classified as ferric luvisol (FAO) of alluvial or parent derived origin ranging from sandy loam to sandy clay loam near the surface while below 3 m depth sandy clay is more common. A horizon of iron-manganese concretions is usually found at depths varying from 0.5 to 2 m. 100
50
o
-50-100
o in
