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Tredoux, G. et al., 2009. Artificial recharge of the Windhoek aquifer, Namibia: Water quality considerations. Boletín Geológico y Minero, 120 (2): 269-278ISSN: 0366-0176

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Artificial recharge of the Windhoek aquifer, Namibia:Water quality considerations

G. Tredoux(1), B. van der Merwe(2) and I. Peters(3)

(1) CSIR, Natural Resources and the Environment, P O Box 320, Stellenbosch, 7599 South Africae-mail: [email protected]

(2) Environmental Engineering Services, PO Box 6373, Windhoek, Namibia(3) City of Windhoek, P O Box 59, Windhoek, Namibia

ABSTRACT

For managed aquifer recharge (MAR) to succeed, an aquifer is needed which allows easy access to a relatively high storage capacity.Whereas the country rock hosting the Windhoek Aquifer, consisting largely of quartzite, has no primary porosity, secondary porosity devel-oped for several reasons. The brittle nature of the quartzite led to better secondary porosity development than in the adjacent schistoseterrain. The degree and intensity of fracturing decreases as the mica content of the quartzite increases. Therefore, a larger storage capac-ity is found per unit volume of the fractured quartzite compared to an equal volume of fractured schist. Equally, the success of MARdepends on a thorough understanding of the ambient water quality in the various parts of the aquifer together with potential water-rockinteractions. This study was aimed at developing water quality guidelines with specific management options for ensuring sustainable arti-ficial recharge of the aquifer. Ample groundwater quality data are available for the saturated zone but a major aspect that can only beassessed to a limited extent is the water-rock interactions that will take place when the unsaturated zone becomes saturated during injec-tion. To some extent unsaturated water-rock interaction can be inferred from the hydrochemistry of the saturated zone and its evolutionin the aquifer. Water quality criteria for the recharge water are included.

Key words: artificial recharge, hydrochemistry, Namibia, quartzite, Windhoek

La recarga artificialdel acuífero de Windhoek, Namibia: consideraciones sobre la calidaddel agua

RESUMEN

Para que la recarga artificial de acuíferos tenga éxito, es preciso disponer de acuíferos con una capacidad de almacenamiento relativa-mente importante. Las rocas que confroman el acuífero de Windhoek, constituidas en gran parte por cuarcitas, no poseen porosidad pri-maria, pero sí sencundaria, que se ha desarrollado por la acción de diferentes procesos. La frágil naturaleza de la cuarcita ha originadoen esta roca un mejor desarrollo de la porosidad secundaria que en los terrenos esquistosos adyacentes. El grado de intensidad en la frac-turación decrece a medida que aumenta el contenido en mica en la cuarcita. Por consiguiente, la cuarcita fracturada tiene una capacidadde almacenamiento por unidad de volumen más grande que el mismo volumen de esquisto fracturado. De igual modo, el éxito de la recar-ga artificial depende de la adquisición de un sólido conocimiento de la calidad del agua en diferentes zonas del acuífero, así como de laspotenciales interacciones entre la roca y el agua. El propósito del estudio que se presenta ha sido desarrollar directrices relativas a la cali-dad del agua con distintas opciones de gestión que aseguren la recarga artificial sostenible del acuífero. Se dispone de mucha informa-ción sobre la calidad del agua en la zona saturada, pero la interacción agua-roca que se produce cuando la zona no saturada se saturacomo consecuencia de la inyección del agua de recarga solamente ha podido ser evaluada en extensiones limitadas del acuífero. Hastacierto punto, la interacción agua-roca en la zona no saturada se puede inferir a partir de la hidroquímica de la zona saturada y su evolu-ción dentro del acuífero. También se han incluido criterios de calidad para el agua de recarga.

Palabras clave: cuarcita, hidroquímica, Namibia, recarga artificial, Windhoek

Introduction

Namibia is located along the arid south-west coast ofAfrica. The capital, Windhoek, is situated in a semi-arid region in the central highlands of the country andriver systems originating in the Auas and ErosMountain ranges, are draining away from the city inall directions. As a result, local water resources arevery limited and most of the city’s water supply is

obtained from surface impoundments located at adistance of tens to hundreds of km from the city.Large fluctuations in annual rainfall aggravate the si-tuation. The lack of water has lead to direct recyclingof reclaimed wastewater and the implementation ofwater demand management. The main local sourceof water is the Windhoek Aquifer found in the AuasFormation quartzite and other quartzitic formationslocated on the southern side of the city. The aquifer

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can provide approximately nine per cent of the pre-sent demand of about 20 Mm3/a on a sustainablebasis. Periods of drought have led to overexploitationand water levels in some parts of the aquifer arepresently tens of metres lower than initially when theboreholes were drilled. For this reason, artificialgroundwater recharge, or managed aquifer recharge(MAR), provides an attractive option for augmentingthe natural groundwater recharge and subsurfacestorage from surplus surface water. The physical fea-sibility of artificial recharge by injection of surfacewater into boreholes was proved by four pilot scaletests, one of which extended over seven months(Murray and Tredoux, 2002). The cost of other variouspotential water supply schemes were compared andusing artificial groundwater recharge for augmentingthe water supply proved to be the most cost efficient.Successful introduction of an MAR scheme

requires a thorough understanding of the ambientwater quality in the various parts of the aquifertogether with potential water-rock interactions. Thesources of water for injection are the Von Bach Damand the reclaimed water. For both these sources andfor the Windhoek Aquifer, considerable amounts ofwater quality data are available also providing trendsover time. However, the expected water-rock interac-tion and the hydrochemical evolution have to beinferred from the groundwater data. Another impor-tant aspect relates to the inundation of at least partsof the unsaturated zone. The water-rock interactionsthat will take place when the present unsaturatedzone becomes saturated during injection can only beassessed to a limited extent.The conclusion from a preliminary investigation

into the quality of the recharge water sources wasthat none of these was of sufficiently high quality tobe used as such for injection into the WindhoekAquifer (Tredoux, 2003). The presence of dissolvedorganic carbon (DOC) at levels often exceeding5 mg/L in the treated Von Bach Dam water rendersthis source unacceptable for injection. On the otherhand the salinity of the reclaimed water is such thatthe water would only be suitable for injection in cer-tain parts of the aquifer. Subsequently, the issuesregarding injection water quality were addressed in areport by Van der Merwe et al. (2005) on managementoptions for the sustainable artificial recharge of theWindhoek Aquifer. The improvement of the sourcewater quality was also considered in detail.The purpose of this paper is to evaluate the

impacts of full scale injection of aerated surface wateron ambient groundwater chemistry in the WindhoekAquifer. Hydrochemical results of the pilot scale injec-tion tests are interpreted against the background of

the natural groundwater quality and give an indica-tion of the expected impact of water-rock interactionand potential water quality deterioration. Injectionwater quality requirements are provided and injectionpractices may need to be adapted to ensure sustain-able management of the aquifer in terms of waterquality.

Hydrogeology

The Windhoek aquifer is located to the south of thecity extending northwards from the Auas Mountainsfor 20 to 25 km as far as the city centre. It forms partof a highly complex metamorphic environment. Thegeological formations within the area were folded inthe process of orogenesis, and subjected to a numberof episodes of faulting including thrusting and rifting.Quartzite and schist horizons with transverse faultsand fractures are prevalent throughout the aquifer.The quartzite, being brittle, is highly fracturedbecause of folding and faulting and has developedsecondary porosity and permeability. The schist onthe other hand is ductile and does not have welldeveloped secondary permeability (Murray &Tredoux, 2002; Carr Barbour & Associates, 1999).Hydrogeologically the aquifer can be divided intothree main units of decreasing permeability:quartzite, micaceous quartzite, and schist. The AuasFormation quartzite in the south is relatively pure butthe mica content in the quartzite horizons increasesnorthwards and quartzite of the Kleine KuppeMember (Kuiseb Formation) in the north is highlymicaceous. The degree and intensity of fracturingdecreases as the mica content of the quartziteincreases.The Windhoek Aquifer is bounded by imperme-

able formations on all sides. In the south it is boun-ded by gneiss, in the north by amphibolite of theMatchless Belt, and in the east and west by low per-meability formations consisting predominantly ofschist. The succession hosting the aquifer dips fromthe Auas Mountains northwards at an angle between15 and 30°. The water level configuration in 2001 isshown in Figure 1. Elevated water levels occur in thenorth at the hot springs in the centre of the city (nearboreholes 4/2 and 4/3), in the central area in shallowboreholes in the schist (e.g. near borehole 9/5), and inthe west around the Kaiser Wilhelm Mountain (seeFigure 1). This latter area also includes theKupferberg waste disposal site where saline wateroccurs at elevated topographic levels. Groundwaterleakage from the aquifer is only possible along theAretaregas River valley in the north-western part of

Tredoux, G. et al., 2009. Artificial recharge of the Windhoek aquifer, Namibia: Water quality... Boletín Geológico y Minero, 120 (2): 269-278

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the aquifer (see Figure 1). The hydraulic relationshipbetween the elevated levels in shallow boreholes inthe schist and water levels in the main water bearingformations needs further investigation. Radiometricage determinations using 14C indicate that rechargeoccurs in, and possibly also just south of, the AuasMountains. The overall water level gradient indicatesthat water flows from the Auas Mountain range to thenorth and northwest. This is in general agreementwith the topographic gradient and with naturalrecharge mainly taking place in the highly fracturedAuas Formation quartzite, with limited recharge in themicaceous quartzite, and practically no recharge inthe areas underlain by schist. This was also borne outby the groundwater modelling (Zhang et al., 2002).The micaceous schist is not conducive to ground-

water flow. The same applies to the thin layers ofamphibolite, with their orientated fabric and commonassociation with thrusts. Secondary porosity deve-

loped for several reasons including sympathetic frac-turing of the country rock flanking the tension faults(Carr Barbour & Associates, 1999). Weathering, due tothe breakdown of disseminated iron sulphide (pyrite)in the quartzite, is another potential source of se-condary porosity. The oxidation of the sulphide gene-rally extends down to the lowest historic water levels,but may extend to greater depths within the faultzones.

Hydrochemistry

The hydrochemical evolution of groundwater in theaquifer takes place along the flow direction. Initially,in the Auas Formation quartzite, the water representsrecently recharged water containing mainly calcium-magnesium-bicarbonate. Even in the quartzite matrix,sufficient calcium carbonate is present to allow the

Figure 1. Water levels and piezometric heads in the Windhoek Aquifer in 2001. Contour labels are oriented to indicate higher level abovethe numberFigura 1. Niveles de agua y cota piezométrica en el acuífero de Windhoek en 2001


development of groundwater of such hydrochemicalcomposition. Along the flow path sodium chloridedissolves from the aquifer matrix to increase the so-dium content of the groundwater. In the case of theWindhoek Aquifer, shale and clay are largely absentand ion exchange plays a minor role. Only in thedeep-circulating groundwater this process canprogress to near completion as the residence time ofthe water is long enough.The salinity distribution in the aquifer, as por-

trayed by the chloride distribution (Figure 2), con-firms that the groundwater has a very low salinity inthe Auas Formation quartzite in the south where chlo-ride is below 10 mg/L. As part of the hydrochemicalevolution in the aquifer, the salinity increases gra-dually in a northerly, north-westerly and westerlydirection and reaches the highest levels in the micaschist underlying the southern part of the residentialarea, and in the west along the Aretaregas River. The

deep circulating water emanating in the city centre ashot water have a higher chloride concentration,reflecting the long residence times at depth. The high-est salinities in the area occur at the Kupferbergwaste disposal site (in the south-western part of theaquifer) where the chloride reached 315 mg/L.However, this might be part of a perched aquifer inthe micaceous schist and at this stage, it is unknownhow much, of this water will eventually reach themain aquifer, and at what rate. On this relativelycoarse scale, this is the main pollution point sourceidentified based on the hydrochemical data.Figure 3 shows the distribution of sulphate in the

aquifer and it is evident that sulphate occurs in highconcentrations practically throughout the aquifer. Thereason is that iron sulphide in the form of pyrite is dis-seminated throughout the host rocks of the WindhoekAquifer. Dissolved oxygen entering the aquifer withthe rainwater during natural recharge oxidizes the


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Figure 2. Chloride concentration in groundwater in the Windhoek Aquifer. Contour labels are oriented to indicate higher level above thenumberFigura 2. Concentración de cloruro en el agua del acuífero de Windhoek



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sulphide to sulphate. In this way, sulphate is genera-ted in the aquifer until the dissolved oxygen isexhausted. As a result, the groundwater is largelydevoid of dissolved oxygen and high in sulphate.Under natural conditions, the groundwater easilyattains sulphate concentrations of 200 to 300 mg/L. Inareas where the soil has been disturbed, as in the re-sidential area, the sulphate concentration increasesas more of the pyrite is exposed to oxidation. This isthe case in the central part of the area where shallowmonitoring boreholes show high levels of sulphate.Also at the Kupferberg waste disposal site the naturalsoil and rock structure is disturbed and high concen-trations of sulphate are being generated. This isreflected in the groundwater down-gradient of thesite where the sulphate exceeds 1000 mg/L. However,at this stage it is evident that these high sulphate le-vels are not reflected in the main aquifer underlyingthe perched aquifer in these areas.

In view of the high sulphate levels in the aquifer,the relationship between sulphate and electrical con-ductivity was investigated (Figure 4). The strong co-rrelation between sulphate and electrical conductivitysuggests that the increase in the salinity is largely dueto the dissolution of sulphate ions. The down-gra-dient observation borehole at Kupferberg, with high-est sulphate is not shown in the figure. The graphalso indicates that there is more than one populationof points. Whereas most of the water plots near astraight diagonal a certain number of points plotbelow the line. These include the hot water emana-ting in the city, the associated wellfields, as well asboreholes in the west and northwest.During the oxidation of iron sulphide, iron also

enters into solution in the form of Fe+2 ions. In theunsaturated zone, sufficient oxygen may be availableto oxidize the iron to the Fe+3 form, which is insolubleat neutral or alkaline pH. However, in the saturated

Figure 3. Sulphate concentration in groundwater in the Windhoek Aquifer. Contour labels are oriented to indicate higher level above thenumberFigura 3. Concentración de sulfato en el agua del acuífero de Windhoek


zone Fe+2 ion remains in solution once the dissolvedoxygen is consumed. Inspection of analytical datashowed that iron is widely distributed throughout theaquifer. This needs further investigation but is consis-tent with the high levels of iron sulphide present inthe mineralised fault zones in this area. High iron con-centrations occur at the Kupferberg waste disposalsite and a maximum of 22 mg/L was recorded in theslightly acidic water. Both Fe+2 and Fe+3 ions are solu-ble in such groundwater.

Discussion

Most aspects of the hydrochemistry in the WindhoekAquifer were discussed above using the data provi-ded. However, little attention has been devoted to theoccurrence of organic compounds. The analyticalresults indicate that such compounds are generallyabsent from the aquifer except in polluted areas suchas the vicinity of the Kupferberg waste disposal site,and in shallow boreholes in the schist.

Potential hydrochemical interactions and changesin water quality during and after injection are relatedto a number of factors. These include chemical diffe-rences between the injected water and the ambientgroundwater, e.g. oxygen content and chemical com-position, which will lead to re-establishment of che-mical equilibria. Other important interactions will takeplace between the injected water and the aquifermatrix, both in the saturated and unsaturated zones.The oxidation-reduction potential of the injectedwater and the groundwater as well as the carbondioxide partial pressure will determine the extent ofsuch reactions. From the available data, it wouldseem that changes in the oxidation-reduction poten-tial in the subsurface might be the main factor affec-ting the hydrochemical environment of the WindhoekAquifer.Borehole injection is presently considered the

preferential technique for applying managed aquiferrecharge at Windhoek. Where pure, inert quartzite isinvolved, this may have little effect on the hydro-chemistry but at Windhoek the quartzite contains


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Figure 4. Relationship between sulphate and electrical conductivity in the Windhoek AquiferFigura 4. Relación entre sulfatos y conductividad eléctrica en el acuífero de Windhoek



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pyrite and graphitic schist. The aquifer is highlyreducing with no dissolved oxygen in the ambientgroundwater. Injection of surface water saturatedwith oxygen will change the redox conditions andcause interaction with the rock matrix. At Calvinia(Northern Cape, South Africa), where surface waterwas injected into a breccia pipe with ideal hydraulicproperties for subsurface water storage, pilot studiesshowed that it had adverse hydrochemical effects(Cavé, 1999, 2000; Murray & Tredoux, 2002).However, it is important to note that the abstrac-

tion of the natural groundwater from any aquifer isalso accompanied by the ingress of oxygen into thereduced environment. This shifts the redox potentialof the system, creating disequilibrium and promotingoxidation reactions in the subsurface. Subsequentrises in the water levels, natural or induced, will dis-solve soluble chemical species and cause changes inwater chemistry. At Windhoek, the widespread dis-semination of iron sulphide in the aquifer affects thequality of the water stored in the aquifer. The intensi-ty of the effects will depend on the recharge tech-nique, method, and regime. Injection through a deepborehole at depth will be the preferred technique ascascading of the water through the unsaturated zone

will exacerbate any adverse reaction, such as the oxi-dation of pyrite.Although the chemical speciation of iron is

unknown, considerable information is available onthe distribution of total iron in the aquifer.Manganese is also present in the aquifer but littledetail is available on its distribution. However, it isalso mobilized by changes in the redox potential inthe aquifer and the study of the Kupferberg wastedisposal site (Tredoux & Barbour, 2004) has con-firmed that it is present in sufficient quantities towarrant attention.Protection of the aquifer against pollution has to

have a high priority. Once artificial recharge is fullyoperational, the water level will rise and the need forprotection will increase.In view of the wide distribution in the aquifer and

the high iron levels, borehole clogging is a distinctpossibility. For this reason, future monitoring shouldbe designed to allow research into the concentrationsof the various iron species in order to get a betterunderstanding of the situation in the aquifer withregard to the clogging potential.

Quality guidelines for recharge water

As the recharge water will be injected directly into theaquifer without seeping through the unsaturatedzone it was concluded that the injection water qualityneeds to conform at least to drinking water require-ments. The final water quality from the Von Bach DamWater Treatment Plant is presented in Table 1. Thesalinity (EC) and inorganic constituents are relativelylow but the 95th percentile as well as the median of thedissolved organic carbon concentration is very high.On the other hand, the DOC values of the NewGoreangab Water Reclamation Plant are relativelylow, but the salinity and chloride concentrations arehigh (Table 2). Even a 4:1 or 5:1 blend of Von Bachwith Goreangab water would produce water that isunsuitable for injection. Thus further treatment isessential, particularly better removal of DOC at VonBach, and partial desalination as a future option atGoreangab.Assuming that the water from the treatment plantscould be improved, the quality criteria were set basedon the following guiding principles (Van der Merwe etal., 2005):The recharge scheme should meet a number of

demands, which are listed below:1. No negative environmental impact of significance.2. Sustainable use of water from the Windhoekaquifer for drinking water purposes preferably

Table 1. Final water quality Von Bach Dam Water Treatment PlantTabla 1. Calidad del agua final de la planta de tratamiento de VonBach Dam


without treatment or at most with limited treat-ment such as stabilisation and disinfection:a) The recharge water should meet modern drink-ing water standards (e.g. Rand Water and RSA)

b) No additional health risk for the residents ofWindhoek as compared to present sources (2004)

3. No technical problems should arise due to injec-tion water quality, such as clogging, corrosion anddemand for extensive treatment of water beforedistribution.

4. Accept a deterioration of certain quality parame-ters of the water within the aquifer provided thatthe water quality after abstraction complies withacceptable water quality guidelines (RSA & RandWater)Van der Merwe et al. (2005) also recommended that:

1. The Rand Water (95% percentile values) and SouthAfrica (Class 0) water quality guidelines be adop-ted as the general guideline for the injection water.

2. The provisional guidelines as summarised in theTable 3 be accepted on an interim basis

3. The guidelines be revisited after 2 years of inten-sive data collection and evaluation after furtherpilot injection studies.Determining the longer term water quality impact

will only be possible through mass transport model-

ling and this has to be considered as a next step forpredicting the sustainability of the operation. Themass transport model will be based on the existinghydraulic simulation model. The outcome willstrengthen the scientific base for refining the waterquality requirements.

Conclusions

The Auas Formation quartzite constitutes the mostimportant part of the Windhoek Aquifer. This is wherenatural recharge is taking place, the best qualitygroundwater is found, and high permeability com-bines with high storativity. Only low salinity waterfree of organic compounds should be injected in thispart of the aquifer.Low salinity water occurs over most of the aquifer.

Groundwater in the Auas Mountains generally hasvery low chloride concentrations ranging from 4 to10 mg/L and increasing to a maximum of approxi-mately 60 mg/L in the deep circulating hot water issu-ing in the city and the associated wellfield area. In con-trast, the chloride concentration exceeds 300 mg/L inthe polluted part of the aquifer at the Kupferbergwaste disposal site. It is also high in shallow boreholeslocated in the schist. From the observations it wouldseem that these polluted parts may have limitedhydraulic connection with the main aquifer.Major changes in salinity and sulphate were associ-

ated with pilot scale injection tests, but also due to vari-ations in the abstraction regime due to fluctuations inthe water level in the aquifer, the concomitant enhancedoxidation of sulphide, and subsequent dissolution ofsulphate. High intensity natural recharge events mayalso cause salinity fluctuations in intake areas. Such aphenomenon was observed at the Kupferberg wastesite following the 1999/2000 rainy season.The detailed water quality assessment showed

that the presence of sulphate throughout the aquiferis an important characteristic of the hydrochemistry.Sulphate reaches very high concentrations of severalhundred mg/L in certain areas. Its source is the ironsulphide (pyrite) which occurs widespread through-out the aquifer. Disturbing the soil and rock enhancesweathering and oxidation of the sulphide. In dis-turbed areas, such as the Kupferberg waste disposalsite, sulphate reaches several thousand mg/L.Iron is ubiquitous in the groundwater in the

Windhoek Aquifer. Similarly, manganese is present inthe aquifer. The solubility of iron depends on a num-ber of factors and data on the chemical speciation ofiron are needed for a full understanding of the condi-tions in the aquifer.


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Table 2. Final water quality New Goreangab Water ReclamationPlantTabla 2. Calidad final del agua de la planta de depuración de NewGoreangab



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The presence of pyrite in the aquifer is of someconcern and injection should be controlled to mini-mize the oxidation of pyrite. Deep injection of thewater, i.e. directly into the saturated zone, would benecessary to prevent cascading of the water throughthe unsaturated zone. It is suggested that injectionpoints (levels) should be determined according to thecharacteristics of each individual borehole but shouldbe at least 10 metres below the rest water levels. Therapid rise and fall in the water tables during injectionand abstraction over long periods would also releasesulphate in the water as a result of the oxidation ofpyrite as a result of the variation in the groundwatertable. Where possible, the change in water tableshould be minimised.Mass transport modelling will be essential for

determining the longer term impact of the injectedwater on aquifer water quality. Managing injectionwater quality according to the set criteria should go along way to maintaining the high quality of the waterstored in the aquifer. It may be possible to improvethe water quality in certain higher salinity areas, e.g.in the western part of the aquifer, by artificialrecharge.

Acknowledgements

Thanks are due to the City of Windhoek for fundingthe research and providing water quality and otherdata. Tribute is also paid to the men and women thatmeticulously gathered the production volumes, waterlevels, and data on other parameters over manydecades without ever knowing what this data wouldmean one day.

References

Carr Barbour & Associates, 1999. “Windhoek WellfieldExtension Project. Part 1: Final Report”. City ofWindhoek.

Cavé, L C, 1999. “Geochemistry of artificial groundwater re-charge into the Kopoasfontein breccia pipe nearCalvinia, Karoo”. MSc dissertation. University of CapeTown.

Cavé, L C, 2000. “Geochemical modelling and laboratoryex-periments to test the feasibility of artificial recharge”.In: Sililo et al. (eds), Groundwater: Past achievementsand future challenges. Proc. IAH Congress, Cape Town:465-470. Rotterdam: Balkema.

Murray, E C, & Tredoux, G, 2002. “Pilot artificial rechargeschemes: testing sustainable water resource develop-ment in fractured aquifers”. Report to the WaterResearch Commission, WRC Report No 967/1/02,Pretoria. ISBN 1 86845 883 0

Tredoux, G, 2003. “Windhoek artificial groundwaterrecharge: Preliminary investigation into quality require-ments of injected water”. Report submitted to City ofWindhoek. CSIR Report No ENV-S-C2003-141.

Tredoux, G, & Barbour, E A, 2004. “Interpretation of hydro-chemical data for waste disposal sites aroundWindhoek”. Volume I: Kupferberg. Report submitted tothe City of Windhoek, CBA & CSIR (ENV-S-C 2004-118),December 2004.

Van der Merwe, B, Tredoux, G, Johansson, P O, & Jacks, G,(Windhoek Aquifer Joint Venture Consultants), 2005.Water quality requirements for artificial recharge of theWindhoek Aquifer. City of Windhoek, Tender No INF260/2003, Final Report.

Vogel, J C, & Van Urk, H, 1975. Isotopic composition ofgroundwater in semi-arid regions of Southern Africa. J.Hydrol., 25, 23 – 36.

Zhang, J, Tredoux, G, & Murray, E C, 2002, “Windhoekaquifer groundwater model”. Report submitted to theCity of Windhoek, CSIR Report No ENV-S-C 2002-067.

Recibido: junio 2009Revisado: septiembre 2009Aceptado: septiembre 2009Publicado: octubre 2009

Table 3. Provisional water quality guidelines for recharging the Windhoek AquiferTabla 3. Estándares provisionales de calidad para la recarga del acuífero de Windhoek


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