Challenges in African Hydrology and Water Resources (Proceedings of th<" Harare Symposium, July 1984). IAHS Publ. no. 144.

Environmental isotope hydrology in southern Africa

B. Th, VERHAGEN Nuclear Physics Research Unit, University of the Nitwatersrand, Milner Park, Johannesburg, South Africa

ABSTRACT Hydrogeology has come to rely increasingly on environmental isotope techniques. In southern Africa the growth in their application has been particularly marked due to the existence of expertise within the region, fostering approaches suitable to particular regional problems. Some of the achievements to date are highlighted as well as some of the pitfalls encountered. Brief resumes are presented of four widely differing studies: an investigation of a village water supply which produced an early warning of pollution; an environmental isotope study of a major dewatering project at an open-cast mine; the rapid elimination of a suspected source of pollution and finally the contribution of environmental isotope studies to the understanding of the hydrology of the Kalahari.

Etudes hydrologiques par les isotopes du milieu en Afrique australe RESUME L'hydrogéologie tend à se baser de plus en plus sur les techniques des isotopes du milieu. En Afrique australe, le développement de leurs applications a été particulièrement remarquable par suite de l'existence d'expertises dans ia région ce qui a permis de mettre au point des approches bien adaptées à problèmes notamment régionaux. Quelques-unes des réalisations effectuées à ce jour sont présentées dans cette communication ainsi que certains des pièges rencontrés. On présente des résumés de quatre études très différentes: une étude pour l'alimentation en eau d'un village laquelle a conduit à un système d'alerte à la pollution; l'étude par les isotopes du milieu du dénoyage important d'une mine à ciel ouvert; l'élimination rapide d'une source, supposée de pollution; et enfin une contribution à des études par les isotopes du milieu en vue de la compréhension de l'hydrologie du Kalahari.

INTRODUCTION

Hydrogeological studies have come to rely increasingly on the application of environmental isotope techniques. Such techniques present a unique approach to the study of groundwaters (Moser & Rauert , 1984). Isotope effects and radioactive decay alter the isotopic ratios of the elements making up the water molecule and of

161

162 B.Th.Verhagen

the dissolved constituents. These changes can be related to the origins and history of the water since its infiltration into the ground. The advantage of these environmental tracers (be they naturally produced or anthropogenic), along with the hydrochemistry to which they are closely related, is that they provide in situ information at the moment of sampling. A working hypothesis or "model" is required to interpret this information; on the other hand the data themselves can suggest a model and confirm or eliminate existing hypotheses.

Amongst the array of possibly useful isotopic species which can be employed in hydrology, there is a small number which, on account of ease of sampling and relative ease of measurement and interpretation have become the "workhorses" of isotope hydrology.

2 18

These are the isotopes H and 0 the concentrations of which (about 0.015% and 0.2% respectively with respect to their common isotopes) undergo small changes expressed as promille (%o) deviation from a standard, on evaporation and condensation of water. In addition, there is the cosmogenic (and, during the past three decades, nuclear fallout-produced) radioactive H or tritium, half-life 12.4 years at natural concentrations in rainwater around 10~17 or 10 TU. Ap art from these labels of the water molecule itself, there are the isotopes of the dissolved inorganic carbon 13C at an isotopic concentration of 1.1% affected by biological and carbonate exchange processes and radioactive C of similar source as 3H, at atmospheric concentration about 10"12 (or 100% modern carbon (pmc)) and with half-life of 5730 years.

This paper will deal with the application of mainly these isotopes to hydrogeological problems in southern Africa. Much of the area has a semiarid to dry-tropical climate, with highly seasonal (summer) rainfall. Potential evaporation greatly exceeds precipitation. Significant recharge events to groundwater are usually sporadic in time and space. Under thick sand/soil cover the piezometric response can be so severely damped as to be barely discernible.

A very general problem is the poor or often complete absence both of records on existing usually privately drilled boreholes and of longer-term hydrological observations which frustrates the initial investigation of an area with a view to groundwater development. Environmental isotopes have proved to be well suited to overcome some of these problems. Local expertise in this field has been developing in southern Africa over the past two decades, resulting in numerous studies in a variety of hydrological situations. Apart from in South Africa, some of this expertise has been developed for and applied in other countries such as Botswana (Verhagen et al., 1970, 1974ab; Verhagen 1984; Mazor et al., 1974, 1977, 1980), Namibia (Vogel & van Urk, 1975) and Zimbabwe (Wurzel, 1983).

SOME PROBLEMS IN INTERPRETATION

Environmental tritium was present in rainwater up to the middle 1950's at a concentration around 5 TU. Nuclear fallout increased these levels up to several thousand TU in the northern hemisphere, with a southern Africa maximum of about 60 TU in 1964 (IAEA,1981).

Environmental isotope hydrology in southern Africa 163

Since then, levels have been declining worldwide. Tritium tracing of recently recharged waters in the northern hemisphere became fairly simple. Meaningful tracing in the southern hemisphere, where levels have dropped to about twice the "natural" value at present, requires much greater measurement sensitivity and has in some instances been abandoned. Low-level counting preceded by isotope enrichment provides a practical means of reaching sensitivi­ties of around 0.2 TU. Hydrological systems with relatively short (̂ 50 years) turnover times can therefore still be studied with environmental tritium. Ambiguities can occur as a result of the rising of and subsequent falling of input levels produced by the fallout "peak". Post-bomb waters can be identified by tritium concentrations in excess of 1.5 TU. Lower values can be interpreted as "dating" the water from the natural level over another two to three half-lives.

In order to "date" groundwater with lkC it is necessary to know: (a) the initial concentration with which the dissolved

bicarbonate reaches the saturated zone; (b) the subsequent fate of the dissolved bicarbonate, i.e. the

possible exchange with and dilution by aquifer material resulting in non-radioactive reduction in C concentration;

(c) the degree of mixing of "older" and "younger" waters and their relative bicarbonate concentrations. Radiocarbon or lkC is introduced into groundwater by the reaction:

C0„ + H 0 + CO - - % 2(HC0„)~ 2 2 3 3

where, on a simplistic model, half the carbon in the dissolved bicarbonate is biogenic, the carbonate being derived from fossil limestone. Taking the pre-bomb C concentration in the atmosphere as 100 pmc, this model would lead to recent water containing not much above 50 pmc. In semiarid environments, soil carbonate may be partly pedogenic with xl*C > 0 pmc; there may be practically no soil carbonate available, so that all the carbon will be biogenic, the balancing cations resulting from the decomposition of feldspars (Mazor et al., 1980).

Several schemes have been proposed by which the correct initial concentration may be calculated (cf. Mook, 1976) involving the carbonate chemistry of the water. As the parameters involved in these schemes are numerous and the chemical pathways, especially in semiarid to arid environments diverse, an "age" interpretation based on the radiocarbon concentration, can be somewhat artificial. Further complications can arise in alkaline, sodium bicarbonate waters often involving remobilization of ancient evaporites.

As much of the groundwater in southern African is encountered in the secondary porosity of crystalline rocks, water is often struck in boreholes well below the piezometric level. Such groundwater appears locally as sub-artesian but can regionally be regarded as under water table conditions. Boreholes are often drilled to considerable depth to enhance yield. Water pumped from such a hole is therefore a mixture derived from a number of different levels and residence times. A measured lkC model age of several thousands of years can therefore be misleading (cf. Vogel & Van Urk, 1975) as the surface layer of the saturated zone might be actively recharged.

164 B.Th.Verhagen

Such cases require a more detailed treatment (Bredenkamp & Vogel, 1970) but the heterogeneity of most aquifers defeats simplistic models. Data obtained during drilling can be used in a qualitative, rather than pseudo-quantitative fashion (Verhagen, 1984), with 14C data given as concentrations rather than "ages" and interpreted in context.

Radiocarbon concentrations in atmospheric CO2 rose from the natural production level of 100 pmc to 160 pmc in the southern hemisphere and have since been declining. The resulting bomb C pulse in groundwater along with the H bomb peak allows for further time resolution in younger groundwaters. Figure 1 suggests a pre-bomb recent lkC value of 70-90 pmc in the range first proposed by Munnich (1957), but the scatter in the data again cautions against an over­simplified approach.

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GROUNDWATER STUDIES AND POLLUTION AT SEROWE

The village of Serowe dates from the nineteenth century when it was sited on account of the existence of springs at the base of a low scarp. With continued development, the increased demand for potable water was met by the drilling of private and government boreholes within the confines of the village (Pig.2), which by 1970 had become the largest traditional village in Botswana. The hydrogeology had been studied by Jennings (1974) and an environmental isotope study was undertaken in 1969, which ran through several years (Verhagen et al., 1974b). Recently, further isotopic data were collected.

Serowe is underlain by the Stormberg series of the Karoo System. To the south and southwest of the major fault the succession consists of amygdaloidal basalt overlying the main sandstone aquifer. To the northeast of the fault and the dolerite dyke which intruded the arcuate section of the fault, the upthrown block exposes the less

Environmental isotope hydrology in southern Africa 165

FIG.2 Map of Serowe village, showing borehole locations, tritium and some carbon-14 concentrations.

permeable lower section of the sandstone. Rain recharge to the groundwaters was assumed to occur mainly on

the exposed sandstone, along the fault and the dyke which is expressed by hills, where a piezometric mound exists. From there water would then move to the boreholes in the south and southeast. Chemical variations and rest level fluctuations had been observed over a number of years' observation at certain of these boreholes. Mean tritium values observed over several years and some radiocarbon concentrations are shown in Fig.2. These show clearly that, although recently recharged water was present in the vicinity of the dyke, very recent water was also to be found at several other sites scattered around the village. The chemical composition of the groundwater was correlated with the tritium contents, which in its turn is probably related to the degree of decomposition of as well as joints and fissures in the basalt cover at individual sites.

Averaging all the tritium concentrations in the Serowe groundwaters showed that recharge was more general than previously assumed and explained the relatively small drawdown observed in spite of sharply increasing abstraction. With more refined level contours a more realistic assessment of storage within the sandstone aquifer was possible (Verhagen et al., 1974b).

Of perhaps greater importance was the conclusion, which was based on relatively few tritium analyses of water from boreholes scattered around the village, that the water supply in certain parts was under immediate threat of bacteriological and chemical pollution. This threat was fully realized by the latter 1970's when the village boreholes were closed down and alternative supplies developed outside

166 B.Th.Verha gen

inhabited areas. Several boreholes, with vanishing 3H and low lkC concentrations,

showed that sections of the sandstone aquifer within the village might have remained safe for some time into the future. A subsequent instance of pollution has been studied and rectified in another eastern Botswana village (Lewis et al. , 1978).

When further development, or the short to medium term safety of existing village supplies is to be assessed, a few 3H, and possibly lhC measurements on existing groundwater supply points can indicate the extent to which rain recharge is occurring within the confines of the existing and possible future extension of the village and the attendant pollution threat.

A STUDY OF A MAJOR MINE DEWATERING PROJECT

The open-cast workings of Sishen iron ore mine reached groundwater levels in 1967. Exceptionally heavy rains, up to 3 times the average of 330 mm year- occurred during 1973/1974 and two subsequent seasons, causing groundwater levels to rise by 8-20 m and flooding of the mine workings. Dewatering, originally at 0.3 x 10 m3year"~ was increased to 19 x 106 m3year_1 in 1980/1981. With a decline in mining activity and drought conditions over the last few years, abstraction has recently been substantially reduced.

Geologically and geographically the mine is divided into a northern and southern section by a major dyke (Fig.3(b)), known to be highly decomposed at the surface. During the early stages of dewatering, the piezometric level over the whole mining area was practically horizontal and identical on both sides of the dyke, suggesting a single highly transmissive aquifer. However, early measurements of 2H and 0 showed that there was a small but consistent difference in the isotopic content between the water pumped from the south and north mines, indicating hydrological separation and different recharge origins (Fig.3(a)).

Rest levels in the north mining area stood higher than in the south mine during the period immediately following the flood. By late 1977, following increasing dewatering, the differential was reversed, with a step of 20 m across the dividing dyke, demonstrating its impermeability at depth. This fact was effectively foreseen by the stable isotopic composition of the water before three years of intensive pumping proved this to be the case.

In spite of the considerable rise in piezometric levels following the massive recharge events, the tritium concentration of the pumped water remained at 0 and the radiocarbon concentration of around 50 pmc suggested water of several thousands of years mean residence time. With increasing pump rate, the lkC values* of the north mine waters remained largely constant whilst in the south mine the values increased to about 65 pmc, the H remaining at 0 TU. This gave rise to a model of a two-component system for the south mine, one at about 50 pmc, the more recent at 80 pmc, mixing in proportions determined by the pump rate (Verhagen et al., 1979a).

Figure 4 shows a nearly 3 year record of the pumped water from the south (SW 275/SW 258) and north mine (SW 240-241/SW 330) well fields. In addition, sampling points to the north of the mine

Environmental isotope hydrology in southern Africa 167

FIG. 3 (a) 8D-61B0 diagram for Sishen groundwaters, showing rain line and north and south mine compositional groups. (b) Map of Sishen area, showing borehole locations and dykes.

(SW 280, SW 118) are shown, as well as the Lohatla well which monitored shallow dolomite water in the presumed catchment and Lylyveld/SW 374, in a likely highly transmissive zone for water flowing towards Sishen mine from the recharge area. Apart from fluctuations of "*C concentrations during 1979 which were related to major changes in pumping rate, values became quite constant at constant pumping rates up to the end of the series of observations. Even though by that time about 108 m3 had been pumped from the mine, no evidence was yet seen of the arrival of recently recharged water, i.e. by a rise in lkC and measurable 3H levels. All the water had therefore been derived from relatively deep storage in fractured sections of the ore-bearing formation and extensive karstification in the underlying and adjacent dolomite, the latter constituting the probable recharge area.

Although relatively little was known of the hydrogeology of the surroundings and rocks below the ore-bearing strata, the environmental isotope data provided important insights into some parameters governing the movement and storage of the large quantities of groundwater involved. When more information on the hydrogeology is forthcoming the available isotope data can be reinterpreted more quantitatively.

A CASE OF APPARENT POLLUTION

An example is given of an apparent groundwater pollution problem which was rapidly solved by only a few isotopic analyses.

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Environmental isotope hydrology in southern Africa 169

Seepage water entering a diamond mine at a number of different levels was pumped out and used in the beneficiating plant and for domestic supply. A check of water quality revealed unexpectedly high pH values of between 9 and 10 in the pumped water. Concern was expressed that surface pollution was finding its way underground. A descaling agent, sodium hexametaphosphate had been used in the plant to remove carbonate accumulation in the water pipes and had been dumped along with the mine's tailings. The agent could produce high pH values in water infiltrating from the surface dump.

In order to test this supposition, seven samples were taken of seepage water at mining levels around 240-290 m depth and around 490-520 m depth. Three parameters were measured: 3H, 180 and pH. These are plotted against sample depth in Fig.5.

The pH for the two depth ranges is similar, but the 180 values at the greater depth are considerably more negative and more uniform. The seepages at these two depths are therefore derived from two independent groundwater systems. This conclusion was confirmed by a few 3H measurements which showed that the shallower water was being rapidly recharged from rain, whilst the deeper water has a residence time in excess of 50 years. The high pH of the deeper water is therefore not derived from any

surface pollution and appears to be indigenous to the groundwater. It is now thought that the high pH values are rather being generated by contact with freshly fractured kimberlite, the result of blasting. The same mechanism probably operates also for the shallower groundwater.

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RAIN RECHARGE IN THE KALAHARI

The Kalahari is a vast sand-covered area, stretching from the northern Cape province in South Africa, through Botswana, eastern Namibia and western Zimbabwe further northwards. The mantle of sand,

170 B.Th.Verhagen

which may contain local argillaceous and calcareous sections as well as gravel and sandstone lenses is collectively known as the Kalahari Beds. They overlie a shallow basin of the almost horizontally layered Karoo succession, with different members forming the local subcrops. Groundwater rest levels are usually deep and stand in the Karoo aquifers. Occasionally water levels stand in the Kalahari when they are perched or when the Kalahari cover is thick.

According to Martin (1961) and some subsequent workers (cf. Boocock & Van Straten, 1962; Foster et al., 1982) no infiltrating rainwater will reach the groundwater table under present climatological conditions as a result of quantitative évapotranspiration losses from the Kalahari Beds. Evidence put forward for this contention are the usually deep rest levels, no observable rest level fluctuations and depth to water table dependent on the thickness of the Kalahari Beds cover. It was postulated that groundwater is either derived by underflow from areas of thin or no Kalahari cover ("recharge areas") or represents remnants of earlier "pluvial" periods.

Early isotope observations (Verhagen et al., 1974a) showed that recent recharge was present and groundwaters were generally active (Fig.l) in areas with varying Kalahari Beds cover in northern Botswana. Further studies (Mazor et al., 1974, 1977, 1980) have shown that Kalahari groundwaters are by and large receiving rain recharge. Tritium concentrations are usually vanishing as a result of the long delay times in the unsaturated zone (Verhagen et al., 1979b). Although C concentrations often lie around 50 pmc and lower in water pumped from boreholes penetrating Karoo (Ecca) sandstones, as has been pointed out in section 2, this can signify an active, stratified groundwater body. Evidence is accumulating that borehole depth below rest level as well as pump rate can determine the C concentration or mean "age" of the water produced.

Groundwater in sandstone overlain by basalt has very low to vanishing 1'*C, largely as a result of the lack of displacement in the deeper sections, with the fluctuations confined to the basalt and the Kalahari Beds. Such a local recharge model is also entirely compatible with the variability of water chemistry as is observed in the Kalahari (cf. Bath, 1980), which is difficult to explain on the assumption of regional underflow.

A striking example of the local nature of groundwater is given by the ôD-ôi80 relationships for groundwater in Gordonia, the southernmost section of the Kalahari (Verhagen, 1984). The area is traversed in the north by two ephemeral river beds, which flowed during 1974/1977 for the first time in 50 years. In the east the pre-Kalahari rocks outcrop as low ridges and hills. In this area also, the groundwaters below the Kalahari Beds were assumed to be derived by underflow from these features which would allow recharge from the surface, gradually salinizing down-gradient westwards and southwestwards. In particular, fresh water found in a trough of deep Kalahari sediments stretching some 50 km from the Kuruman River bed was taken to be derived from the occasional flow in the river (Fig.6, inset).

The ÔD-61B0 diagram (Fig.6) clearly shows that the isotopic signal for the fresh trough waters differs from groundwaters below or close to the river bed which are localized and represent infiltrated river water. The fresh trough waters resemble

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isotopically most other groundwaters found well away from the river beds, which form a tight cluster on the diagram and lie somewhat to the right of the rain line. This relates to the uniform physiography of the greater part of the region, which consists of parallel dune ridges, and its effect on rainfall infiltration. This will be direct in the dunes, but often slightly evaporated due to surface collection in the dune valleys or "streets". The waters are therefore of local origin and not derived from underflow. These conclusions have been corroborated by lkC and l3C measurements as well as by the chemistry of the water.

The highly saline waters in the west of the area lie on an evaporation line. These waters are encountered under fairly shallow water table conditions. High sodium bicarbonate concentrations make the 14C data difficult to interpret and 3H concentrations are low. However, it is clear that the highly evaporated saline waters have an association with small overlying pans or playas. The high salinities and alkalinities may in part be remnants of an earlier major evaporation basin. Again, the association with surface conditions and the implications of local recharge are clearly demonstrated by the isotopic content.

CONCLUSIONS

It has been shown by these few examples that in southern Africa, where environmental isotope facilities exist, steady and growing use is made of the available methods in the study of a variety of hydrogeological problems. It should be stressed that the success and development in approach has been due to a large extent to the long-term interaction between the local hydrogeologists and isotope workers. This interaction is producing a local expertise in isotope hydrology with a "feel" for its application to and interpretation of data in the context of regional problems. Although much good work is being done by international organizations in the form of aid and consultancies, these are in the long run no substitute for and should stimulate the development of such local expertise.

Although some cooperation is taking place, it is perhaps too much to hope for existing facilities to be shared on a truly regional basis at present. The power of environmental isotope techniques has been demonstrated; they are highly cost-effective and at a technological level which lies within the reach of several countries in the region. Based on either existing or newly established facilities, the extended application of isotope hydrology in southern Africa is one of the important challenges in water supply development for the subcontinent.

REFERENCES

Bath, A.H. (1980) Hydrochemistry of the Karoo aquifers in southern Botswana. Unpubl. Report GS10/11, Botswana Geological Survey.

Boocock, C. & Van Straten, O.J. (1962) Notes on the geology and hydrology of the central Kalahari region, Bechuanaland Protectorate. Trans. Geol. Soc. S. Afr. 65, 113.

Environmental isotope hydrology in southern Africa 173

Bredenkamp, D.B. & Vogel, J.C. (1970) Study of a dolomitic aquifer with carbon-14 and tritium. In: Isotope Hydrology 1970, 349. IAEA, Vienna.

Foster, S.S.D., Bath, A.H., Farr, J.L. & Lewis, W.J. (1982) The likelihood of active groundwater recharge in the Botswana Kalahari. J. Hydrol. 55, 113.

IAEA (1981) Statistical treatment of environmental isotope data in precipitation. Tech. Report Ser. no.206, IAEA,. Vienna.

Jennings, C.M.H. (1974) The hydrology of Botswana. Unpubl. PhD Thesis, Univ. of Natal, South Africa.

Lewis, W.J., Farr, J.L. & Foster, S.S.D. (1978) A detailed evaluation of the pollution hazard to village water supply boreholes in eastern Botswana. Unpubl. Report GS10/4, Botswana Geological Survey.

Martin, J. (1961) Hydrology and water balance of some regions covered by Kalahari sands in South-West Africa. In: Inter-African Conf. on Hydrology (Nairobi), 450. CCTA Publ. no.66.

Mazor, E., Verhagen, B.Th., Sellschop, J.P.F., Robins, N.S. & Hutton, L.G. (1974) Kalahari groundwaters: their hydrogen, carbon and oxygen isotopes. In: Isotope Techniques in Groundwater Hydrology, 1974, vol.1, 203. IAEA, Vienna.

Mazor, E. , Verhagen, B.Th., Sellschop, J.P.F., Jones, M.T., Robins, N.E., Hutton, L. & Jennings, C.M.H. (1977) Northern Kalahari groundwaters: hydrologie, isotopic and chemical studies at Orapa, Botswana. J. Hydrol.34, 203.

Mazor, E. , Bielsky, M. , Verhagen, B.Th., Sellschop, J.P.F., Hutton, L. & Jones, M.T. (1980) Chemical composition of groundwaters in the vast Kalahari flatland. J. Hydrol. 48, 147.

Mook, W.G. (1976) The dissolution-exchange model for dating groundwater with C. In: Interpretation of Environmental Isotope and Chemical Data in Groundwater Hydrology, 213. IAEA, Vienna.

Moser, H. & Rauert, W. (1984) Determination of groundwater movement by means of environmental isotopes - state of the art. In: Relation of Groundwater Quantity and Quality (Proc. Hamburg Symp., August 1983). IAHS Publ. no.146 (in press).

Munnich, K.0. (1957) Messungen des "*C-Gehaltes von hartem Grundwasser. Naturforschung 44, 32.

Verhagen, B.Th., Sellschop, J.P.F. & Jennings, C.M.H. (1970) Contribution of environmental tritium measurements to some geohydrological problems in southern Africa. In: Isotope Hydrology 1970, 289. IAEA, Vienna.

Verhagen, B.Th., Mazor, E. & Sellschop, J.P.F. (1974a) Radiocarbon and tritium evidence for direct rain recharge to groundwater in the northern Kalahari. Nature. 249, no.5458, 643.

Verhagen, B.Th., Sellschop, J.P.F. & Jennings, C.M.H. (1974b) A study of the effectiveness and application of environmental tritium as a groundwater tracer in a semi-arid region of Botswana. Unpubl. Final Report 691/R1/RB, IAEA, Vienna.

Verhagen, B.Th., Smith, P.E. Dziembowski, Z. & Oosthuizen, J.H. (1979a) An environmental isotope study of a major dewatering operation at Sishen mine, northern Cape province. In: Isotope Hydrology 1978 vol.1, 65. IAEA, Vienna.

Verhagen, B.Th., Smith, P.E. & Dziembowski, Z.M. (1979b) Tritium profiles in Kalahari sands as a measure of rainwater recharge.

174 B.Th.Verhagen

In: Isotope Hydrology 1978, vol.2, 733. IAEA, Vienna. Verhagen, B.Th. (1984) Environmental isotope study of a groundwater

supply project in the Kalahari of Gordonia. In: Proc. Int. Symp. on Isotope Hydrology in Water Resources Development. IAEA, Vienna.

Vogel, J.C. & Van Urk, H. (1975) Isotope composition of groundwater in semi-arid regions of southern Africa. J. Hydrol. 25, 23.

Wurzel, P. (1983) Updated radio isotope studies in Zimbabwean ground waters. Ground Water 21 (5), 597.