Report GS10/7 JULY 1979
AN INVESTIG,'~I;lOl'l INTO SUPPLENEN'J'.IW
GROUNDW',TEH SOUHCES ~;OH
AUGMi;;'TNrION OF THe; GA1lOHUNE/LOBi,'1':C:::
WA'l'EH SUPPLY
1. Geophy::lcist, Geological Survey De par (.o'en t ~- Ap! ,8 :idi X A
2. Electronics Bngineer, Geolog;ca1 Su.rvey Department - Appenri.ix B
G810 PROJECT REPORT SERIES
GSIO/l (October 1976) Botswana: the role of hydrogeology in water resource planning 8 S D Foster and J L Farr
GSIO/2 (September 1977) Compilation of groundwater maps - - - -experience in the use of data on the GS boreho1e archive Jane H Whitelaw, J L Farr and 8 8 D Foster
G810/4 (August 1978) A detailed evaluation of the pollution hazard to village water-supply boreholes in eastern Botswana W J Lewis, J L Farr and 8 8 D Foster
G810/5 (September 1978) An appraisal of the groundwater resources of the Ecca Series of Central Kweneng in relation to Jwaneng Diamond Mine development J L Farr and 8 8 D Foster
G810/6 (December 1978) The like1yhood of recent groundwater recharge in Central Kweneng 8 8 D Foster
G810/7 (July 1979) An investigation into supplementary groundwater sources for a'lgmentation of the Gaborone/Lobatse water supply J L Farr and J H Baron
G810/S (January 1979) Hydl'ogeological investigations into the Karoo formations of the Dibete area, Kgatleng District J L Farr, Jane H Baron, E Milner
GSIO/9 (in preparation) The Groundwater search for the extension of livestock grazing - technical and economic appraisal of a sample problem Jane H Baron, R J Peart, J L Farr
CCi'lTENTS
1. BACKGROUND
2. GEOLOGY AND INVESTIGATION DRILLING
2.1 Geology 2.2 Drilling Summary
3. HYDROLOGY AND HYDROMETEOROLOGY
4. HYDROGEOLOGY
4.1 Groundwater Gradients 4.2 Hydrogeological Pumping Tests 4.3 Analysis of Existing Borehole Records 4.4 Hydrochemistry
5. GROUNDWATER RECHARGE
6. GRCUNDWATER RESOURCES
7 . SUMJI'lARY
O. RECOMMENJ)ATIONS
9. REFERENCES
JU'PENDIX A - Surface Geophysical Investig8.tions -Pitsanyane Basin/Nuane Subbasin
APPENDIX B - Bcrehole Geophysics
APPENDIX C - Borehole Data for' Transvaal Dolomite boreholes - Lobatse Area.
1. BACKGROUND
It was proposed (Foster and Farr 19(6) that GSIO Project should undertake a short study order to assess the potential of formations within the Transvaal and Waterberg Supergrou as supplementary groundwater sources to be u to augment the Gaborone - Lobatse water supp In 1978 Water Utilities Corporation, the operators of both the Notwane and Nuane Dams expressed interest in any groundwater source that could be developed along the length of pipeline link, particularly near any existin break-pressure points, with groundwater bein added directly to the system or (lower quali waters) blended with water from surface so~r Su.ch developments would be used to increase normal rate of offtake from both da'ns (enabl operation at a hi.gher risk of failure) and t supplemer,t the system during periods of peal< demand.
During late 1978/early 1979 the storage in t Nuane Da,m reached a critically low level ane during the same period the supplementary grc water supplies from the Lobatse Estates wellfield>? began to fail. These factors, tc with the suddenly increased water demand in Lobatse due to expanded production at the Botswana Meat Commission, put considerable, on the Lobatse/Gaborone pipeline link and necessi tated continuous purr.pi.ne; from Gaboror Lobatse. Such a situation gave an added iml and urgency to the current study.
Addi tiono.l detail s on the background to the present work may be found in GSIO Technical No. 3 (Sept. 19(8).
2. GEOLOGY AND INVESTIGATORY DRILLING
2.1 Geology
ThE geology of the area between Lcbatse and Ramotswa is well known (Crokctt, 19(1) and published as Quarter Degree Sheet 2525B (Ma) part of the original draft of 2525B reduced 1:100 000 scale). Examination of previous literaturc;ndicates that the dolomites of Transvaal Supergroup and the sands tones/ cor,,;lomerates of the Wate!'berg Supergroup o. the best grouLdwater poter:tial and hence investigativns were concentrated on these
lithologies. However, it "as decided from the outset that the "rea of Transvaal dolomites around Ramcts"a "as too far removed from the Lobatse/Gaborone pipeline to warrant immediate study and efforts "ere concentrated on the area between Otse and Lobatse.
2.1.1 Dolomi tic rocks of the Transvaal SUl'argroup occur to the north of Lobatse in the area around Pitsanyane and Bathoen. The lo"er parts of the dolomites ~ome 1850 metres thick) consist of dark blue-grey dolomite wit.h chert beds; the chert content becoming considerable in t.he top 160 metres. The dolomi tes may be mangan.i.ferous and a manganese 'wad' (mud) accumulates in weathered zones and fissures o Such material was encountered in investigation boreholes D13/Dl4 and D? Semi-karstic weathering plays an important role in groundwater movement and storage in such formations and considerable sized fissures were encountered in boreholes D13 and DB. Sites Dl? and mB did not per:etrate the dol.omites but terminated in the overlying black/ dark-grey shales of the Lephal.a FOrn1e.tion (previously known as the Timeball Hill Stage of the Pretoria Series).
2.1. 2 The Waterberg SUl'ergrollp is divided into four informal lithologic units ranging from chert conglomerate (Wl) through shale/siltstone (W2), rub by slump conglomerate (W3) to red pebbly sandstones (W4), after Crockett (1968). Unit W3, penetra.ted by boreholes Dl/D3/D4/mO (and not Transvaal dolomites as indicated on Map A) and uci t. Wl (toreholes D2/D9) were assumed to offer the highest groundHater potential of this Supergroup. Unit W2 "a2 drilled at sites DlS/D16 and proved to h&ve poor potential.
2.1. 3 The whole area ts highly disturbed tectonically and both pre- and post-Transvaal Suy:ergroup rocks are affected by major gravity sLdes accompanied by 8 complex pattern of reverse and tranccurrent faults. Such structural features and major transposition of strata have clearly had a great influence on the gecmcrphological development of the area and the location and extent of increased subsurface "eathering.
2.2 Drilling Swnn,ary
Drilling was undertaken in hlo lJeriocis (September mid October and November/December 19'(8) and a total of 19 holes (1260 metres) were drilled on nine sites. Seventeen boreholes totalling 1078 metres were drilled by Seismograph Services Ltd. (contractor for GSlO) and two boreholes totalling 182 m were drilled by the Drilling UnIt, Geological S\U'Vey Department. Coring ems attempted at one site but proved very difficult and since core '.las not essential to the study the method was discontinued and all holes were sunk by air hammer techniques. All holes vere completed at 127 mm (5 inch) di81lleter Hith the exception of D9, DIO and D20. Borehole geological logs Hi th static water levels are summarised in Fig. 1.
HYDROLOGY AND HYDROJVlE'l'EOIiOLOGY .~~>--"~,-"--, .. -.
Jennings (1974) states that 'eight geologi.oally discrete basins are to be found :Ln the Lotatse area' (p. 206) and that they 'ma.y be regarded as virtually closed systems' (p. 224), Reviewing the mass of data accumulated hy Jennings there seems to be no valid reason to disput', his assertion since geologlcal/etructural. evidence 9
hydrochemical data and isotofic determinations all tend to indicate the existence of such sep::rate groundwater basinso TIll e correspondence of these basins to the major ,·urface catchment areas is, as a result of the geolop,·ical and structural cor:trol over the geomc,I'l)hology of the region, in general fairly close; and hydrol.ogical data may be related to groundwccter eondi tions in a straightforward manner" As et result~ and in line with the terme of refennce of the study (Section 1) investigations \-.,rere concentrated on the potential aquifer formations occuring in the relatively undeveloped basins to the north of Lobatse Le. the Pitsanyane B"sin (including Jennings' Going Subbasin), tLe Nuane Subbasin (th"t part of the Nume Basir, belO\; the Nuane Dam) and the Moroekwe Basin.
301 SUI'faCt catchment boundar les Wf're drEnm from the J: 50 000 topographic maps ("heet 2'j25Bl/B2) and geological inf0rrnation to ell2.ble the delineation of grour;dwater basins Ha3 taken from Crockett (1968). Catchment/basin boundar.ie2 are shoHn on Map B. Geomorphological deVelopment
GSIO O/~ 2
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KEY:
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11 15 16 17 18 19
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FIGURE j: Summary of bore hole logs, GSJO drilling prog-amme,Lobatse area.
m o
10
20
·30
40
50
60
70
80
90
100
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of both the (lower) Nuane and Moroekwe catchme",:. ha~ clearly been influenced by the E/W faul tin!' wLich cuts the pre-Transvaal SupergT:'up format; ':)ns forming the range of hills runDing 'J£th/south from Lobatse. The Pitc:anyane catchment has, however, resulted from preferential ,:eathering and erosion along a major NNW/SSE thrust plane affecting the Transvaal Supergroup formations very close to their marginal contact with the Ventersdorp (Lobatse Volcanics) and earlier rocks. Minor subcatchments wi thi.n the Pi tsanyane catchment are again E/W fault controlled gullies through hills composed of pre-Transvaal Supergroup rock-units (tbe Lobatse Volcar'i.cs).
3.2 There is no evidence of any surface water flow out of the Pi tsanyane catchment aHhough both the lower Nuane and the Mcroekwe catcPJ1lents have stream channels incised into alluviwn which flow briefly after 0xtended periods of heavy rainfall 0 No stre8JI.1flow records are available for either catchment.
3.3 Rainfallreccrds are "sohered at a number of sites around Lot.atsc (JfJar ;3). From ex:,,-nination of these records i:~ WGu1.d appe~;l,:r· tl" -:3 .. 1" iT.can annual rainfall over the tlllc;8 catchments is very close to 600 mm/annum with a S88S0Uc11
variabiIity of 30 - 40'/0. Most precipi t"ciC:1 occurs during the period JaYluary - March vJhile the months June - August are frequently totally dry.
Evaporation data has been collected at Sur:nyside Ranch (Ministry of Agricul turo - Animal Production Research Unit) from a Class A Evaporation Pan for the period 1975 - 78. A comparison betwec;n rainfall and evaporation is shown in Pig. 2.
4. HYDROGEOLOGY
4.1 Ground;;ater Gradients
Data from Jennings (1974) in the Lobatse Eastern anC. Westerr: Basins together Yli th sparse data from the Pitsanyane Ba.sin (Going's Subbasin) indicates a groundwater divide coinciding with the surface catchment divide in the vicinity of the Lobatse Plantation. Groundwater gradient is to the north/northwes t in the Pi tsanyariC .Basin
320
310
300
290
280
270
260
250
240
230
220
210
200
190
180
170
160
160
140
130
* 120
E 110
~ lOO
90
80
70
GO
50
40
30
20
1976
-
10l
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10
20
30
40
50
60
70
80
90
100
Potential Infiltration
FIGURE 2
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1977
Rainfall ----
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1978
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x 1979
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Comparison between rainfall and evaporation - Sunnyside Farm, Lobatse
but there is insufficient information to establish the magnitude of the gradient. No data is available for either the Nuane or Mar- h'" Basins but gradients are assumed to be to tL, east or northeast away from potential recha.I'ge areas.
4.2 Hydrogeological Pumping Tests
A total of s,ix sistes were subjected to controlled pumping tests using an engine-driven Mono pump system. Test duration varied from 2 to 24 days and discharge rates ranged from 0.3 to L'e/sec. A summary of the test site conditions and calculated aquifer parameters is shown in Table 1.
One of the sites located on Tra~svaal Supergroup dolomi tes (D6/7) and the one site (D2/9) on Waterberg Supergroup conglomerate show moderate values :of transmissivi ty (betHeen 50 and 80 m2 /d) and confined aquifer conditions. THO (1er sites, both on dolomites, indicate fairly hig! \Talues of transmissi vity (> 100 rrP / day), with 'JO,c site (D13/14) having a water table dorage c,:efi'icient of approximately 0.1. The hiGhest yiel.ling borehole (site D3/4/10) driD,A at 1',:rO(';c'd2 F3.rm into Waterberg conglomerate cl ",hcated (luting drilling the presence of la:tf_~,1.' /.)lwr.e::,:, nf groundHater. After some considerable delay the site Has subjected to a 24 day pumping test. using a BH 200 Mono pump giving an average disch~rge of 12.1 + 0.6 ;;/8ec). Wate.r was discharged into the nearby (dry) Nuane river. Time-drawd.owll and residual recovery plots are shown in Figs 3a, 3b. Some difficulty Was experienced in maintaining a steady discharge rate due to engine speed fluctuation during the later stages of the test, resul ting in a slight recovery in water levels in 0,11 three boreholes.
Examination of pwnpi.ng test results, surface geophysical information (Appendix A) and borehole geophysical logs (Appendix B) from site D3/4/10 indir;ate a semi-confined fract.ure system, Hith the confinement caused by the soil and weathered zone. The piezometric surface stands at approximately 12.5 metres b. g.l. and the effecti VB
base of the aquiferous zone occurs at approximately 32 metres b.g.l. (Appendix A and B). Important fractures occur at 14, 20, 23 and 30 metres b.g.L with the major inflow thought to take place from the lOHerrrJost fracture horizon (30 mG tres) immediately above the lithology change (Appendix A and B). Pumping test results inoicc,ted a
Site
i
Geological Conditions
1 n.w ......
mbgl
._",~:~Z:r-:;:;;~.:c',
2adius and. No. of
Cbserva tion Eh,
Dura tio!} of
Test (day,)
Calculated ?::::a.r.smissivi ty (:nz / day)
Jacob
Ilrawdown. Data Recovery Data
Theis 'hlton (obs)
,"veraGe
Discharge f------------------r------------f-----------------Rate
(1/ ,ec)
Coe:fic:ent of
Storage
?'..L':",P Obs \ Early \ Late \ Purr.p 'Dh. \ Obs Bh.
I :J t "b -" -1 - "4' 1 2 . ~('-2 (*) iD3 "/10 ,~a e_ e_ 6 ) .) \JJ I • X "-v_2 i . 'Conglomerate 28.0 (~3) 2.9 x 10 (*) i--
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Transvaal Dolomite
Tra..'lsvaa:C Dolomi..te
'1)1 "i/16 'Iia te::::'oerg i ' i Shale
!-
b· (18 Timeball Sill Shales
30.92 I NIL 4.7
a 7. /" v 27.5 (16) 2.6
19.92 61.0 (Dl3) :4.0
9.54 42.7 (Dl6) I 2.9
28.99 32.7 (018) 4.7
3.2 ;!: 0.2 1',9.8
).7.::°.314.3 I 41.7 67.6 I 30.2 I
1.3..:t 0.1 approx SCO
0.4 1:. 0.05 2.3 3.2 1.9
0.6..:t 0.05 36.5 42.7 12.2
23.6
14.9 72.9
0.9
14.1
7.7 ,., lC-~ (*) 16.2 x J.C'-~ (+)
6.;' Y lC-':: (+)
app.;:-ox 0.1
5.3 X.lO:§ (*) 8.3 x 10 _ (+) 7.2 x 10-) (+)
6.4 1.4 2.tl
x 10-; ( .. ) x 10=3 (+) x 10 (+)
I ,
I Spec
Capac 1 day
fie I ty at I lis/m) I
1 1.32
1.85
c.165
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(+) - ;';al ton al1a:ysis
Remarks
Observe:. ticn borehole :::locke(~ by date 0: test
Only 5 cm d::::2Wdo;.;n ::1 ohs. H.< afte>:: 14 cays pu.7.?ing
No pump borer:ole data - access Hmi tee
;';0 pu..:p bo:rehole data - access limi ted
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regular decline in water levels (no equilibrium was attained during the high-rate 24 day test) in all 3 boreholes and later stage t.ransrnissivi ty of around 50-60 m2 /day. Tentative c""js::tion of drawdown results, assuming no ";"ajor aquifer discontinuities, indicates drawdown to the main inflow zone (jilld the bc.se of the aqu.; rer) may occur after 1- to 2 years continuous pumping at high rates an~ would result in the c.2watering of the aqui fer. However, using Logans approximation formula and a minimum late stage transmissivity of 50 m2 /d and allowing a maximum drawdown of 15 metres (Le. 28 m b.g.l.) a constant discharge of approximately 7 1/28C could probably be maintained witho~t this occuring.
I,ater level recovery measurements :c8.de at site D3/4/10 indic8.te 8. fairly slow recovery response (in Bh. D4 approximately 4 metres recovery in 10 days), a factor which may have 'oonniderable influence on any production abstraction regimeo
4.3 AnalYsis of Existing Borehole Records
Records of all boreholes penetrating the dolomites and the Waterberg SUjCcrgroup formations in the Lobatse/Otse areas ,-'ere examined and th:,oretical specific capacities calculated using origi.nal drillers test data* (Tables 2, 3). Calculated spec.; fic capaci ties are presented gr"phically in Fig. 4, and indicates that in the lower range distribution of specific capacities is basically :oimilar for both the dolomite and Watcrberg Supergroup wells but that the :!laximUIl' range and mea~ values are very different as a result of a number of very high specific capacities in the dolomites. These are almost certainly due to zones of highly developed karsti.c Weathering within the dolomites.
Figure 5 shows the probability of obtaining a particular specific capacity in the waterberg and Transvaal dolomite for·natior,s. If a specific capaci ty of 5 x 10-3 is regarded as a technically dry hole then there is approximately 75% chance in the Waterberg and 6CP;6 chance in ti:0 'rransvaal SUj.,ergroup dolomi tes of a dry hole occur-
Theoretical specific capacity = test yield
RWL - depth of pump suction
LOBATSE AREA - TRANSVAAL DOLOMITE BOREROLES
STATISTICAL DATA
Order Cumulative Borehole Yield Specific Capacity Percentage Number (m3 /hr) (m3 /hr/m)
o - 24 35.8 Dry Boreholes 25 37.3 z 164 0.09 1.25 x 10::? 26 38.8 Z 135 1.14 9.53x10 3
27 40.3 Z 147 1.36 1.08 x 10"""2 28 41.8 902 0.59 1.11 X 10"""2 29 43.3 919 . 0.45 1.31 x 10"""2 30 44.8 Z 144 2.05 1.62 X 10"""2 31 46.3 1457 2.73 1.82 x 10"""2 32 47.8 952 2.21 2.09 x 10"""2 33 49.2 860 0.73 2.18 x 10"""2 34 50.7 334 1.82 2.61 x 10"""2 35 52.2 327 0.91 2.66 x 10"""2 36 53.7 Z 131 2.95 3.21 x 10"""2 37 55.2 Z 274 1.04 3.54 x 10"""2 38 56.7 917 2.73 3.94 x 10"""2 39 58.2 Z 157 4.80 4.46 x 10"""2 40 59.7 Z 201 1.36 4.47 x 10"""2 41 61.2 Z 448 . 1 .37 5.50 x 10"""2 42 62.7 638 3.41 I 5.87 x 10"""2 43 64.2 Z 449 2.96 6.30 x 10"""2 44 65.7 P 210 1 .1 ~ 6.83 x 10"""2 45 67.2 Z 626 2.71; 6.88 x 10"""2 46 68.7 Z 190 0.91 7.29 x 10"""2 47 70.2 1263 '4.09 8.39 x 10"""2 48 71.6 2278 2.05 9.59 x 10"""2 49 73.1 P 709 2.27 ""l! 9.94 x 10_1 50 74.6 2269 3.07 1.01 x 10_1 51 76.1 Z 199 3.19 1.07 x 10_1 52 77 .6 1276 0.86 1.18x10_1 53 79.1 Z 154 1.64 1.35 x 10_1 54 80.6 Z 148 3.31 1.45 x 10_1 55 82.1 . Z 407 2.27 1.49 x 10_1 56 83.6 1245 3.86 1.81 x 10_1 57 85.1 P 772 6.59 1.94 x 10_1 58 86.6 7, 88A 5.45 2.63 x 10_1 59 88.1 Z 387 4.09 2.98 x 10_1 60 89.5 Z 412 3.27 5.37 x 10_1 61 91.0 P 777 9.55 5.69 x 10_1 62 92.5 Z 89 9.10 5.97 x 10_1 63 94.0 Z 295 6.37 6.74 x 10_1 64 95.5 Z 394 5.45 8.52 x 10_1 65 97.0 Z 191 2.94 9.64 x 10 66 98.5 467 11.52 1.22 67 100.0 Z 153 26.45 3.15
Table 2
_.-. -_._--_._--_ ... _-... - .,. __ .. -
30
00 ,Cc
le
20
c
10
o
""Izt"terberz borehcles, Lob:;-tS9 rea
nu.mber 15 282..'1 = 0.0647 std. clov. = 0.0375 Skev.'Tl8SS 0.2308 kurtosis = 1.8200 x Din = 0.0040 x m?J = 0.1340 range = C.1300
0.1 0.2
Dclo;::i te borehGles, Lobatse Area nu:;:;ber = 41 mean = 0.1666 std. dev. :;:: 0.2407 ske"';rness 1.9203 kurtosis = 5.6615 x min = O. COl 3 x rr;2.J.: 0.9640 ranc:e = 0.9628
0.1 C~2 0.3 C.I!. CoS C.6 C.7 C.8 C, 0 . / 1 .. 0
I::-:cre:nents of tot.e.l ranEe of 2-; ec .. [.1 C C2.3:;.:{ci ty
Fig. 4 S::}ECI ?'IC ~:\I" '~~TL' ::. "·r; -':,'.< '-,:" ,'...'1 :-; ;--:rr: "P1 '~;~ICS
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5.0 f!/sec
10.0 .e/ sec
ing. There would appear to be 4~/o probability of obtaining the same specific capac .. ty (5.5 X 10-:2) i.n both format i OLC' hut" t, che l~/o probability level the chances "I' a ;igher specific capacity are much I':,eater in the dolomites than the Waterberg. If specific capaci ty is converted to theoretical. yield by assuming an available drawdown of 50 metres in a new borehole then the chances of obtaining particular yields in each formation are as indicated in the Table 1.
Table 4: Probability of obtaining particular yields :rom the Watcrberg and Transvaal fOl~ations
,--Waterberg Supel'group Transvaal SuperfTOtlp
Sandstones D:)lomi tes
75/0 6~/o
5<J}G 4(}/o
25% 3G'/o
15% 2("0.' _ )/0
5% 2~/o
0.5% l~/o
near zero 5%
(asswning 50 m available drawdown)
4.4 Hydrochemistry
Figure 6 is a triUnear plot showing the chemistry of the groundwaters derived from dolomites of the Lobatse area. All samples, apart from boro;'ole P209. c'Jnform to the expected chemical composition of dolomite derived groundwaters i.e. Ng-Ca-HCO -CO. Total dissolved solids "re generally low \"verage 277 p.p.f;).) cnd only tnree boreholcs are scightly greater than the accepted W.H.O. Linit of 500 p.p.m. Borehole P?09 is located on the Botswana Meat. Commission
DOLOMITES OF THE LOBAlSE AREA~ PIPER TRIUNEAR DIAGRAr,,1I ,4
---~~L-~ .'-':"
I:: ?209, 2;.. Z387, 3:2!35. '::~2t27
5:: 2136~ 6::665, 7::629,8=23C2j
9~'?7, :0::::3:6, ,1=:'?97, 12=2278,
'3~2,0, 14::767, 15·::6(,:€::;.::.;J;;,
:7=9S2, tf'::467, i3::' E
FIGURE 6,
11
~ ,\/~ (j . .1+ 1 \/._". >( i' \ 2' ~'O 7~':\~ ,C~~7\\ % ~ k_;-\~t~'_-" __ ,'_';1:- -'<--~:\
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' /-_. -? , , r ; / \ / / \( ,~- - , L' , , 'v . \' e " '. ' " \ ' '--- / .
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,
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SCALE
10mm = 10/
preuises and the NB-Cl enriched water is almost certainly due to pollution by ccbbatoir effluent.
5 GROUNDWATER RECHARGE
Three differing techniques of recharge evaluation were examined and their application to the Pi tsanyane/Nuane areas assessed.
5.1 The method of comparing the amount of groundwater abstraction with known rainfall during a period of constant groundwater sta.ge (neglecting evapotranspiration i.e. recharge = abstraction) has been used in the Lobatse Basin to the south (Jennings 1961) and an average figure of 5% M.A.R. (mean annual rainfall) as recharge determi.ned. However, in tlli' present areas there are no long term hydrograph records and the method is thus inapp::'.icable.
5.2 'rhe technique of examining the change in soil moisture content during periods of known meteorological conditions (rainfall a:cd evaporati.on) and trJ<:ing into account the typeof vegetation cover (Crindley 196,') has fY'C',cntly bee!! LJ.sed in groundv.!ater stUti,j (~8 to ev;,lu:i~e 'excess infiltration' (recharge) through the unBatur,,..I.ted zone.. In the Pi t3aI}Y~ ... P~_> drea p
however, and des})i te the excelle,>nt me lerorological data obtained from Sunnyside ILseC:Tch BI,nch, there is insufficient data on veget"tion cover and quantitative soil moisture contents to allow such a technique to be applied. However, examination of qualitative Eoil moL~ture profiles (r8f:i.ist811ce probes) i.n an area of tyr,ical, ;mcleared bush on Sunnyside Ranch would indicate thd the soil profile is almost always totally dry at a depth of one 'iletre and only becomes moist when the rainfall 'exceed" evanoTE,tion by "oflle considerable amount (March '77 l'ebruary '78) (Fig. 7). It would thus appear that general "real recharge from rr,infall through the unsaturated zone is probably infrequ8nt and does llOt occur every wet "eason. Quantification of recharge duri~g such events is currently not possible but ar;lounts are likely to be relatively small since the large soil moil,ture defici ts which undoubtably accumulate during the, remainder of the year have to be made up before any recharge can occur ..
~:t " If
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Depth of probes
~ 50cm.
-0- - ....... 100cm.
FIGURE 7
\
\ 1\ ., .v\,~\
"'I 1I
MOISTURE IN THE SOIL PROFILE
Sunnyside Farm January 1977 -August 1978
• /1 / 1
'I / I
"
• , I
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~ / , ,
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• •
JAN FEB MAR. APR . MAY. JNE JL Y AUG. SEP . OCT . NOV DEC JAN FEB. MAR. APR , MAY. JNE JLY , AUG 1977 x 1978
5.3 Utilizing an analytical technique for meteorological data developed by Pike (Pike 1964), ',.{1 estimation of annual runoff within the Pi tsanyane catchment was undertaken. Mean annual rainfall (R) ~~d open water evaporation (Eo) data are available for Sunnyside Research Ranch and actual evapotranspiration (Et) vas calculated using Pike's modification of TuI'c's formular relating annual evagotranspiration to rainfall and air temperature.
i. e. Et = ",RE=o __ _
If there is no subsurface outflow from the catchment and grvundwater storage changes are very small, then
Et = R - Q (2)
vhere Q is surface runoff. From equation (2) values of annual surface runoff may be calculated (Table 5) as a percentage of annual rainfall. If then, as is the case in the Pitsanyane catchment, there is no surface outflow from the catcruuent, this computed surface r~~off must become Y'Pcharge to ground',!;:; ter (evapotranspiration having already been accGunted for).
Table 5: Pitsanyane Catchment (Sunnyside data)
Water Years 11 Eo Et Q QjR ()b) (R-Et)
1976-77 828.5 1849·7 756.1 72.4 8·7
1977-78 765.3 1375.4 668.7 96.6 12.6
Results of this type of analysis using Sunnyside data for w"ter years 1976-77 Cenu "-977-78 .indicate approximately 10% annual rainCall may be recharged to the groundvater body. However, mean =ual rainfall (M.A.R.) at Sunnyside is considerably higher than adjacent stations (i'lap B) ,cnd the calculation was repeated using the overall
average M.A.R. for the three other raInfall stations around the catchment and the longterm ,werage open water evaporation figure (pan evaporation x 0.7 - Gibbs, 1969) from Gaborone Meteorological Station. (This latter also corresponds closely to the average value of Eo measured at Sunnyside).
Thus if: R = 600 mm/ annum Eo = 58 inches/annum = 1473 mm/ annum
then Et = 555.7 mm/annum and Q = 44.3.!1J1Il/annum
i .. e .. recharge to groundwater = 7.4% M.A.R.
5.4 A modified form of Jennings' recharge estimate was used by consulting engineers ~obatse Water Supply Investigations, 1969) in attempting an assesment of Lobatse groundwqter resources. 5% M.A.R. as recharge from maj0J:'L, dip slope areas was assumed, together with ~ from minor catchmGnts discharging occasionalfy to the main valley and 1% directly through main valley alluviwn. No adequate justification is given for these estimates and the resulting resources figure can probably be regsrded as an underestimate.
5.5 The evaluation technique discussed in section 5.3 would seem to indicClte that the generally accepted figu.re of 5% M. A. R. (section 5.1) contributing to groundwater recharge is an underestimate for this area. However, the results produced from the Sunnyside meteorological data are considered rather too high as a result of the somewhat anomalous nature of the Sunnyside precipitation figures. The best estimate of recharge is thought to fall between these two extremes and to be approximately 7.5% N.A.R., a figure also produced using average rainfall and evaporation data (Section 5.2).
Recharge volume per annum would then be 45 flUll
(7.591, M.A.R.) over' the arga of the catchment (36.89 km2
) Le. L.6 x 10 m3 /annum.
6 GROUNDWATEil RESOURCES
Using the calculated recharge figure of 7.5% M.A.R. (4) mm/annum) the annual rec;"arge in the basins under consideration is sh0wn below;
Pi tsanyane Basin
NU1Cne Sub-basin
1.6 x 106
m3 /annum
0.3 x 106
m3 /annum
Such inputs to the groundwater body constitute the maximum amount of water which theoretically could be extracted without depleting the groundwater reserve. However, these amounts must be regarded as the absolute maxima since they do not reflect the extreme variability of annual rainfall and they also neglect any losses due to underflow out of the basins. Therefore, assuming some unlierflow and allowing for errors in the estimation of recharge, an 'acceptable Qbstraction volume' of 5~h of the above figures is proposed for the Pitsanyane Basin and Nuane Subbasin. For the Moroekwe Basin too little data is available for a similar result to be evaluated.
6.2 In order more accurately to determine recharge, and hence groundwatcr resources, a long term groundwater monitoring network is necessary. Changes in groundwater levels may then be compared with rainfall event", more accurate aquifer parameters evaluated and a possible catchment model constructed. Such a model when caU brated against long term data would constitute an extremely valuable management tool for the groundwater resources of the two basins.
7 SUMMARY
Rcsul ts from boreholes already existing and from those drilled during the current investigations indicate that the Waterberg Supergroup conglomerate and Transvaal Supergrot; ,) dolomi tes of the Nuane Subbasin and Pitsanyane Basin have an excellent groundwater supply potential, probably of the order of 1600 to 1900 m3 /day (350,000 to 500,000 g.p.d.).
Aquifer tests indicate medium range transmissivity and fairly high v".lues of storage with water movement along dirocrete fracture zones, probably E:nlarged by solution in the dolorr.i tees. Geophysical evidence show::-,; tL~ geomorpholoKy, and hence subsurface karstic weathering, of the Pitsanyane Basin results from a major structural feature and that the Waterberg formation of the Nuane Subbasin possesses a fractured upper aquifer zone ,.hich may be Hidesllrcad.
8 RECOY~DATIONS
8.1 It is recommended that initially two production boreholes be sunk in the Fitsanyane Basin and that an additional two be sunk in the Nuane Sub-basin.
Boreholes may be expected to be within 2 kilometres of the Nuane Treatment Works and geophysical techniques as an aid to siting (for structural features and depth of fracture horizon) are recommended. However, due regard must be given to their spacing so as to avoid pumping interference.
8.2 Depth of production holes is not expected to exceed 100 metres and m~y be as little as 60 metres. Holes should be constructed such that they have a minimum completion diameter of 254 mm (la inches) and an access tube to allow water level monitoring should be inserted. It is recommended that consideration be given to installing a pressure transducer system to facilitate continuous water level monitoring in the prod'iction boreholes.
8.3 Both drilling and completion should be closely supervised by the Geological Survey Department as should any subsequent test pumping. An optimum abstraction regime may be recommended by Geological Survey but only after a trial production period of 4 - 6 months during which time an accurate record of pwnping water levels (if possible continuous), abstnj,ction r2,'~es, pumping hours and water quality should be kept. Ini hal abstr,"ction rates can be established on completion of pre-production pumping tests.
8.4 In addition to production boreholes, a number of observation holes should also be drilled and equipped with water level records in order that the groundwater regime of each b"sin can be monitored. It is therefore considered that an additional 6 to 8 observation holes of minimum completion diameter 100 mm (4 inches) will be required to expanded the network installed during the course of the investigation. An agreement should then be reached between Water Utili ties Corporation and Geological Survey Department on the regular collection and analysis on the data thus obtained.
0.5 Should Water utilities Corporation decide to develop the groundwater resources of these basins, Government should take the necessary steps to declare the basins water reserves in order to facilitate effective mar,agement of these resources.
9 REFERENCES
Crockett, R. N., "The Geology of the Country Around Gaborone and Lo ba tse, Botswana. (Interim Account)" Geol. Surv. of Botswana Unpub. report RNC/54/71. July 1971.
Farr, J. F. and Baron, J. H., "A Background to the GSIO Gaborone - Lobatse Groundwater Study" Geol. Surv. of Botswana Unpub. GSIO Technical Note No. 3. September 1978.
Foster, S.S.D. and Farr, J. L. Botswana: the role of hydrogeology in water resource planning. GSIO Report GSIO/l. October 1976.
Grindley, J., 1967 The Estimation of Soil Moisture Deficits. The Meteorological Magazine Vol. 96 No. 1137.
Jennings C.M.H., 1961 Report on a survey of the groundwater potential of the western Lobatse groundwater basin Geol. Surv. Botswana unpubl. rept.
Jennings, C.M.H., 1974 Hydrogeology of Botswana Ph.D. Thesis, Univ. Natal.
Livestock and Range Research in Botswana 1978 Animal Production Research Unit l1inistry of Agriculture, Gaborone.
Lobatse Water Supply Investigations - Stage 1 report 1969, Sir Alexander Gibb ana PFrtners, London.
Pike, J. G. 1964 The estimation of arnual run-off from meteorological data in a tropical climate. J. Hydrology (2).
!!{ /)
APPENDIX A - Surface Geophysical Investigations -Pitsanyane Basin/Nuane Subbasin
APPENDIX A Surface geophysical investigations -Pitsanyane Basin/Nuane Subbasin
Introduction
Geophysical surveys compr~s~ng electrical resistivity-, gravity- and magnetic traversing were undertaken on the fa.pns indicated below by the Geological Survey Department on behalf of the GS10 project. The objectives were twofold:-
(a) on the Farm Woodlands 8-JO: to site a high yielding borehole in weathered dolomites of the Transvaal Supergroup and
(b) on the Farm IYloroekwe 4-JO: to detect any concealed structure that might account for the high yields of boreholes D3 and D4.
Field work occupied five days (plus four days of line cutting) and involved:-
5,2 kms line cutting and pegging
4,2 kms Magnetometric traversing (25 m interval)
2,75 kms Electrical resistivity traversing (25 m interval)
0,45 kms Gravimetric traversing (25 m interval)
Field Method
Equipment used included:-
Resistivity:- 2,5 kW DC transmitter, digital voltage and current metres and auxiliary equipment.
Magnetometry:- McPhar GP70 proton precession magnetometer.
Gravimetry:- LaCoste-Romberg model G gravir.leter. Standard corrections were made to all geophysical measurements.
(a) Woodlands 8-JO: Three lines, 150 m separate and each approximately 1 km long, were cut on a bearing of 1100m from the main Lobatse - Gaborone road; the southernmost line Has sited approximately
700 m north of Pi tsanyane Halt. These three lines were surveyed by magnetometry and electrical resistivity (gradient array. current electrodes 1 km apart and potential electrodes 25 m apart) and part of the northernmost line was subsequently surveyed by gravimetry.
(b) Moroekwe 4-JO: Two lines were cut, on bearings of 254°m (1 km long) and 29l o m (500 m long) to intersect in the vicinity of the high yi~lding boreholes. Both lines were surveyed by magnetometry and electrical resistivity (gradient array, current electrode separation of 1 km on 245°m line and 500 m on 291 0 m line. Potential electrode separation of 25 m on both lines) •
Results and Conclusions
(a) Woodlands 8-JO: A large apparent resistivity anomaly occurs on all three lines at approximately peg 500 (i.e. 500 m east of the main road)- values to the east of this feature are erratic and high (generally in excess of 250 ohm-metres) while to the west the profile is smooth at a level of only some 50 ohm-metres.
A small anomaly appears on the magneto-metric profile at the same point- values to the west being some 20 gamnlas higher than to the east.
The gravimetric profile of the northernmost line displays a~ anomaly of approximately one mgal. across the same zone (centred at 462).
It is concluded that these anomalies indicate a shallow faulted contact at 460 (on the northernmost line). It is assumed that karstic weathering in the dolomite will have occured preferentially along the plane of this fault and a borehole was sited at 462 on the northernmost l~
(b) Moroekwe 4-JO: The high yielding boreholes are seen to be sited approximately 150 m to the SW of a large apparent resistivity anomaly and coincident magnetometric feature. It is concluded that these anomalies indicate faulting with a trend of approximately 3000 m. It will be possible to trace the supposed fault in more detail if further geophysical work is undertaken.
It may be of value to drill a borehole closer to the centre of the fault zone as indicated by geophysics.
The result of a vertical electrical sounding (Schlumberger array) made over the Moroekwe boreholes is shown in figure Al. The corrected curve was interpreted in the standard manner using a chart of two-layer curves and auxiliary diagrams to yield the five-layer geoelectrical model shown at the base of this figure. The computed curve for this particular model is also shown in Fig. Al, and this is seen to match the field curve closely.
The top layer, of specific resistivity 140 ohmmetres and thickness 0.9 metres, is dry soil, while the underlying layer of resistivity 56 ohmmetres is damper material. The increase in resistivity from 56 ohm-metres to 92 ohm-metres at a depth of approximately 6.3 metres possibly indicates an increase in fracturing. The rest water level at a depth of 12.5 metres is clearly detected on the curve. Depth to fairly massive bedrock (displaying a high resistivity) is seen to be approximately 34 metres and this depth correlates well with that seen on the resi.stivity borehole logs.
FIG.Al. "V.E.s. "AT"MOROEKWE"BOREHOLES.
SCALL. " " . " "" RH." ,.",,"" , DRAWN BY:
"" DATA INDEX REF"", DATE
'!!I ,
r
o
!
o
100 200 300 400 500 600 700 BOO 900 1000m,
' 400 '-",-,--,_,.""~_"--""--"", ~,~,' /.\
500
1 Po 300 " ...• - •...•• _ .••.• ' •.... _ .•.. , •
'sun - ' ••
'~~~I /---._/,-,/~ o "./ ./ .
500
400
~ 300
Am. 200
100
o 500
./ ~~_ .. _._o~-.-~--_Q-
/ __ 0_ .. - 0
O.s.
33
32
31
30
mgls_
400
8 300 o
Jtm·200
..... _ ....... -... It ~"""""""",,/ 1""'-"-- ."-' '
/':: )'\-""-""""---"
.-.-" ..
'-'-'-'-' /. /'/ ' 100
o 100 200
-._._.-._.- .........
300 400 500 500 700 800 900
30200 ~
30roox
]
30200'1'
30100X
30200¥
30100 r
1000 m,
FIG_A2Apparent resistivity. ma(,'1etometric and gravimetric profiles on the Form Woodlands 8JO • .-.-;----_. " ...• __ a-"·· .. -"'-.-"-"
--'>
... ,:-.:Q
+~ .--~:;,.
~,
OM.
N{fnost line
Centre line
S'most line
---'> OM.
---",-~~
·'t} L
f
~
120
110
100
90 Pa
80 SLm.
70
60
50
40
.\ !i;;'
i' !;:;
."r,,/\ / ._.". ;/_. \,/\/\ . \
...•...........•.....•..... ~ ...... -................ ~... "","" :.. \ . ',' '+-.+ .*--.-._-.-. . --.. --"--~--. \ '"" .. "'" i
prod.Bhs. \._.
'\.. ".
f
30250 ¥
30200r
[30150 Y
30100Y
.. ' ' '-> 500 400 300 200 100 0 100 200 300 400 500 m.
Pa
80
70
60
Jlm. 50
40
30
300
.-', /1 .,."._//\, \
T .•..•..•..•..•. \ ..• .~-~--.--.---- .. -- .. -
prod.Bhs.
200 100 o 100 200
1302501'
302001
30150,
300m. ~
OM.
FIG.A3 Apparent resistivity and magnetometric profiles on the Farm Moroekwe 4JO .
.-'-'--.-' . '-.- ... -"-
OM.
-":
APPENDIX B - Borehole Geophysics
APPENDIX B - Borehole Geophysic,'
i.) Temperature and Conductivl ty Logs 0[' Boreholes GSIO/4, 9, 14, 15 and 17
Instrumentation:
The equipment used comrrised:-
(a) Wuidart Engineering hand operated logging winch equipped with 6-conductor cable.
(b) Wuidart combined temperature/conductivity probe.
(c) Wayne-Kerr 1'1101 portable bridge.
(d) Bryants 28000 two channel chart recorder.
Log Repeatability and Validity
The passage of th logging pro be and cable is bound to disturb the equilibrium of tl:0 b0rehole temperature and conduct.i vi ty profile. '1'he first l:Jg to be record~d in each case was run downhole, and this log should be taken to as tll, best rc,cord of the equilibrium state.
TLe conducotivi ty cell h8-s an immedia ts response to changes in cO:1ductivity, so the direction of loggi.ng (up-hele or down-hole) should, arart from the reasons uu tl.ineci above, make no d~ fference to the validity of the log.
"l'he temperature element of the probe, being of th,·, thermistor type, does have a fin:'. te response j;imo, and logs will suffer vertical distortion. Th" effect is to snift the log response downward V.r)H"'n logging down and upwards when logging up. These shifts arc proportional to logging "peed, so assuming equ8-1 logging speeds up and down, the time position of temperature anOmaliE'2 is halfway between the positions indicated by the uphole and d'Jwnhole logs.
ii) Geophysical Logs of Borehole GS10/4
Instrumentation:
The equipment used was a Gerhart-Owen mODel X logl;er, fitted Ylith multi-conduotor logging cable. The probes used Cc'1d some of th8ir details are surr@~rised below:-
•
Probe Details
Caliper 3-arm motorised
3 curie Am-Be source, He detector Neutron with a source-detector s~aCing of
approx 400 mm
125 milli-curie C-137 soc.rce, geiger Bulk Density detector and 400 mm spacing,
decentralised
Hecistivity 16" and 64" normal electrode spacing
Gamma 1" x in sodium-iodide cryrto.,l detecior and photomultiplier
Ternpe rature Thermistor type terr:per&t,)re detector
First-order differentia~ of' ternper-Diff. temp. ature against time derived frcm
up-hole cjrcuitry
Intci'[;re tati on:
The caliper log inu"cates considerable fracturing throughout the hole to Q depth of 34.) metres, with p,rticularly large cavities from 8-12 ,"etres and at 14, 20, 23 811d 30 metres. The "u"k density log, being so depenJout on boreh0le rugosity, supports this. Also apparent from the hulk ciensity logs are the w"ter rect level ~t 12.5 metres and casing extending tc 8 metres. The low and fluctuating v'llues of bulk density behind the casing are due to borehole deformation caused by casing installation.
The neutron log indicates near uniform porosity from water rest level to 31 metres, a de(;r~ase from 31 to 34.5 metres ;,nd 10<1 porosity thereafter. Similarly the resistivity logs indicate '1
resistivity of about 70 ohm-metr'?s from 28 to 31
metres (due to tool geometry, 0-28 metres cannot be logged), approximately 100 ohm-metrr, ( from 31 to 32.5 metres, then a graded increase to highly resistive bedrock (greater than 400 Orli'll-metres) at 34.5 metres.
The difference in results between the 16 inch and 64 inch normal resistivity configurations is best understood in terms of the greater influence on the short normal electrode configuration of higher contact resistance experienced in regions of relatively low borehole fluid and rock conductivity.
The remaining logs, gamma, temper&ture and differential temperature yield little information on borehole lithology and hydroGsology. The temperature logs not unsurprisingly, because of the severe disturbance to thermal profiles due to local pump test activity.
· ,
1 I I I I I
I I I I I I
I I I I I ,I I-
G
APPENDIX C _ Borehole Data for Transvaal Dolomite boreholes - Lobatse Area.
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