Geochemical studies of the geothermal systems in Kenya: II. The Majimoto geothermal field

Journal of African Earth Sciences, Vol. 14, No. 3, pp. 387-391, 1992. Printed in Great Britain

0899-5362/92 $5.00+0.00 © 1992 Pergamon Press Ltd

Geochemical studies of the geothermal systems in Kenya: II. The Majimoto geothermal field MWAKIO. P. TOLE

School of Environmental Studies, Moi University,P. O. Box 3900, Eldoret, Kenya (First received 30 August 1990; revised version received 15 March, 1992) Abstract - The Majimoto geothermal field discharges at the boundary between metamorphic schists and gneisses of Precambrian age, and Pleistocene volcanic ashes. The waters are near neutral, low salinity waters. Calculated reservoir temperatures are about 90°C. Oxygen and hydrogen isotopes indicate that the

thermal waters are meteoric in origin. The field is suitable for development for uses in spa therapy, crop drying, milk pasteurisation, leather processing and house warming. INTRODUCTION

In c o n t r a s t to the high e n t h a l p y geothermal r e s o u r c e s , the low e n t h a l p y g e o t h e r m a l r e s o u r c e s of Kenya have so far b a r e l y a t t r a c t e d m o r e t h a n s u p e r s t i t i o u s a n d academic curiosity (Walsh, 1969; Tole, 1985; 1986; 1988; 1990a; 1990b). In several c o u n t r i e s , h o w e v e r , low e n t h a l p y g e o t h e r m a l s y s t e m s c o n s t i t u t e v a l u e d economic r e s o u r c e s that are p u t to s u c h u s e s a s s p a c e heating, s p a therapy, a q u a c u l t u r e , industrial p r o c e s s heating, a n d green h o u s e farming, cooling a n d refrigeration, a m o n g others (see, for example, Lindal, 1973; G u d m u n d s s o n a n d Lund, 1987). The u s e s to which a particular geothermal s y s t e m is p u t to will d e p e n d on the location of the r e s o u r c e s v i s a vis w h a t economic activities take place in the area, the chemical c o m p o s i t i o n of the geothermal fluids, a n d t h e t e m p e r a t u r e a n d size of the r e s o u r c e at depth. The p u r p o s e of this series of p a p e r s is to report on t h e r e s e a r c h t h a t h a s b e e n c a r r i e d o u t to d o c u m e n t t h e locations, chemical compositions, reservoir t e m p e r a t u r e s , a n d to s u g g e s t possible economic u s e s of s o m e of Kenya's low e n t h a l p y g e o t h e r m a l r e s o u r c e s . This p a p e r f o c u s e s on the Majimoto g e o t h e r m a l s y s t e m , e n c o m p a s s i n g the Majimoto area proper, a n d the N a r o s u r a w a r m springs area. FIELD AND LABORATORY

METHODS

The hot spring sites were visited, a n d the geological f e a t u r e s at the site noted. The t e m p e r a t u r e s of the d i s c h a r g e s were m e a s u r e d to d e t e r m i n e the highest t e m p e r a t u r e at each site. The total flow r a t e s for the sites were determined. The pH w a s also m e a s u r e d in t h e field.

Three w a t e r s a m p l e s were collected in duplicate, using a plastic w a t e r jug, after it h a d b e e n rinsed three t i m e s with tile w a t e r to be coUected. The s a m p l e s were d e c a n t e d into 5 0 0 ml Azlon plastic b o t t l e s to e x c l u d e visible s u s p e n d e d m a t t e r . S a m p l e s w e r e not filtered. The s a m p l e s were treated a n d a n a l y s e d for Na, K, Ca, Mg, Si, Cl, F, SO 4, CO 2, H2S, a n d conductivity, according to t h e m e t h o d s given in Tole {1986). Afew s a m p l e s w e r e collected in McCartney bottles for d e t e r m i n a t i o n of oxygen a n d h y d r o g e n isotopic compositions. T h e s e d e t e r m i n a t i o n s were carried o u t at the British Geological S u r v e y laboratories at Wallingford, England. RESULTS Location

and general

description

of the sites

As d e s c r i b e d in this paper, the Majimoto geot h e r m a l field c o m p r i s e s the Majimoto hot spring's area a n d t h e N a r o s u r a w a r m spring's a r e a (Fig. I). The two sites are s e p a r a t e d b y a d i s t a n c e of 28 km. The Majimoto hot spring's site is located s o m e 34 k m S o u t h w e s t of Narok town. The c o o r d i n a t e s of the location are 35 ° 42.48' E, I ° 20.05' S. Two discharge points, one of w h i c h is 100 m d o w n s t r e a m from the other, form the h e a d w a t e r s of t h e perennial Majimoto s t r e a m , w h i c h d r a i n s into t h e E w a s o Ngiro river. The h o t s p r i n g s discharge at the c o n t a c t b e t w e e n Tertiary volcanic tufts a n d u n d e r lying g n e i s s e s a n d s c h i s t s of t h e P r e c a m b r i a n M o z a m b i q u e belt s y s t e m , r e s i s t a n t m e m b e r s of which rise to form the c h a i n of Loita hills to t h e Southwest. Surface discharge t e m p e r a t u r e s range from 5257 ° C. The total s u r f a c e flow rate w a s e s t i m a t e d to be 20 t o n n e s / h r (20 m3/hr). The Majimoto area is arid to semi-arid, t h e m a i n

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Geochemical studies of the geothermal systems in Kenya economic activity being livestock r a n c h i n g . Crop cultivation is practiced on a very small scale. The MaJimoto t r a d i n g c e n t e r a n d a m i s s i o n c e n t e r are located w i t h i n 2 kin of the hot springs. Also p a s s i n g close to the site is t h e r o a d to t h e Masai Mara g a m e reserve. C u r r e n t l y , the only u s e of t h e s e hot spring w a t e r s is in t h e w a t e r i n g of livestock. The N a r o s u r a w a r m s p r i n g s o c c u r 54 krn s o u t h of Narok town. The c o o r d i n a t e s of the locality are given by 35 ° 49' E, 1° 32.7' S. The site is one of extensive w a r m seepage, e x t e n d i n g over a 5 m radial distance, forming t h e h e a d w a t e r s of t h e N a r o s u r a s t r e a m , w h i c h d r a i n s into t h e Mabokoni s t r e a m , w h i c h e v e n t u a l l y d r a i n s into the Ewaso Ngiro river. The w a r m s p r i n g s discharge from m e t a m o r p h i c quartzites of t h e Mozambique belt s y s t e m , w h i c h rise to the west a n d s o u t h west to form t h e c h a i n of Loita hills. Surface d i s c h a r g e t e m p e r a t u r e r a n g e s from 2931 ° C, c o m p a r e d to a m b i e n t air t e m p e r a t u r e s of 15 ° C at t h e time of sampling. The total s u r f a c e flow rate w a s e s t i m a t e d to be more t h a n 180 t o n n e s / hour. The N a r o s u r a a r e a is arid to semi arid, a n d livestock r a n c h i n g is the m a i n economic activity. Irrigated crop cultivation a n d growing of dry l a n d crops (sun flower) is practiced. The Narosura trading c e n t e r is located 5 k m a w a y from the w a r m springs.

Chemical and i s o t o p e analyses The r e s u l t s of t h e c h e m i c a l a n d isotope a n a l y s e s are given in Table 1. The Majimoto hot springs have slightly alkaline pH, low salinity (TDS less t h a n 250 ppm), high Mg waters. Application of the

389

q u a r t z g e o t h e r m o m e t e r gives reservoir temperatures in t h e r a n g e of 85-92 ° C, while t h e Na-K-Ca g e o t h e r m o m e t e r gives a reservoir t e m p e r a t u r e in the range of 101-104 ° C. Due to t h e high Mg correction required in application of t h e Na-K-Ca g e o t h e r m o m e t e r a n d the fact t h a t t h e r e is no cold spring w i t h i n a 15 k m r a d i u s of t h e h o t spring's site, the q u a r t z g e o t h e r m o m e t e r t e m p e r a t u r e of 92 ° C is a d o p t e d as t h e reservoir t e m p e r a t u r e for this site. Oxygen a n d h y d r o g e n isotope c o m p o s i t i o n s fall close to t h e local K e n y a Rift Meteoric W a t e r Line (KRMWL) of Allen a n d Darling (1987), (Fig. 2), indicating a r a i n w a t e r s o u r c e for t h e hot springs. The N a r o s u r a w a r m s p r i n g s similarly have n e a r n e u t r a l pH, low salinity (TDS less t h a n 250 ppm), high Mg waters. The q u a r t z g e o t h e r m o m e t e r gives t e m p e r a t u r e s of 58 a n d 67 ° C, while t h e Na-K-Ca g e o t h e r m o m e t e r gives t e m p e r a t u r e s of 79 a n d 80 ° C. The q u a r t z g e o t h e r m o m e t e r t e m p e r a t u r e of 67 ° C is a m i n i m u m t e m p e r a t u r e for this site, since there were cold s p r i n g s d i s c h a r g i n g in the immediate vicinity of t h e w a r m springs: t h e possibility of mixing of the g e o t h e r m a l w a t e r s with shallowcirculating cold w a t e r s c a n therefore n o t be discounted. Oxygen a n d h y d r o g e n isotope c o m p o s i t i o n s are also close to t h e KRMWL (Fig. 2), indicating a rainw a t e r source.

DISCUSSION The similarity in c h e m i c a l a n d isotopic compositions of the w a t e r s from the two sites are indicative

Table la. R e s u l t s of c h e m i c a l a n a l y s e s of w a t e r s from t h e Majimoto G e o t h e r m a l Field Sample ID

Disch. T~C

Fiel pH

Lab Conduct Na K pH ppm ppm

Ca pmm

Mg SiO2 CI F pmm pmm prom pmm

H2S pmm

CO2 pmm

SO4 pmm

52.0 52.0 52.0 52.0 57.0

7.0 7.0 7.0 7.0 7.0

8.10 7.24 8.15 7.82 7.88

310 310 350 340 340

34 34 37 39 37

17 17 15 15 14

ND ND 20 19 19

ND ND 12 12 11

38 37 36 34 41

20 20 0.6 0.6 0.6

ND ND <1.0 <1.O
ND ND <0.07 <0.07 <0.07

88 110 106 110 106

ND ND 18 19 19

31.0 31.1 22.0

7.0 7.0 5.0

7.56 7.64 7.78

220 210 230

16 16 15

7 8 8

12 12 13

28 28 28

17 22 24

0.6 0.4 0.4

0.I 0.1 0.i

0.I 0.i 0.1

48 53 53

I0 18 27

Majimoto: M~. 1 M~. 2 031/001 031/002 031/003 Narosura: 034/001 034/002 034/003

Table lb. Results of Isotope analyses of waters from the Majimolo Geothermal Field Sample ID 031/001 031/003 034/002

Conductwi~ ~/cm 350 340 210

8D

8~°0

-32 -31 -31

-5.5 -6.0 -5.7

TDS Salinity 245 ppm 248 ppm 157 ppm

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Fig. 2. Plots of isotopic compositions of water from the Majimoto geothermal system SMOW (Standard Mean Ocean Water), WMW (World Meteoric Water Line), KRMW (Kenya Rift Meteoric Water Line). Table 2. Calculated reservoir t e m p e r a t u r e s for waters from the Majimoto Geothermal System SAMPLE ID

Quartz TC

Chalcedony TC

Na-K-Ca TC

MaJimot0: Maj. 1 Maj. 2 031/001 031/002 031 / 003

90 89 88 85 92

61 60 59 56 64

ND ND 102 104 I 01

58 67 71

29 39 43

79 80 81

Narosura: 034/001 034/002 034/003

of a similar origin for the geothermal fluids. The fluids circulate within metamorphic rocks, and apparently do not react very extensively with them. The calculated reservoir temperature is 92 ° C for the Majimoto geothermal system, and 71 ° C for the Narosura field. Nyblade and Pollack (1990) give values of temperature gradients in the Mozambique belt in Kenya as ranging between 13 ° C / k i n and 47.5 ° C/kin.

Using these figures for the Majimoto field, the water can be heated to the observed t e m p e r a t u r e s by circulating to depths anywhere between 2 and 7 km, while at Narosura, the observed temperatures wiU be attained b y circulation to a depth of between 1.5 and 5.5 km. Geothermal gradient values are, however, not very reliable in this area, due to interferences by shallow water circulation (Baticci, 1987), and the values quoted above are only indicative.

Geochemical studies of the geothermal systems in Kenya More d a t a are n e e d e d to d e t e r m i n e conclusively t h e c o m m o n origin or otherwise of t h e fluids at t h e two discharge sites. The reservoir t e m p e r a t u r e s in t h i s field, a n d the economic activities c u r r e n t l y practiced in the surr o u n d i n g a r e a s u g g e s t t h a t t h i s r e s o u r c e would b e s t be developed for (a) spa t h e r a p y u s e s , (b) m i l k pasteurisation, (c) leather processing, (d) crop drying, (e) h o u s e warming. SUMMARY The location a n d c h e m i s t r y of the Majimoto geot h e r m a l s y s t e m are given. The w a t e r s are dilute, n e a r n e u t r a l to, slightly alkaline pH fluids, with a fairly high Mg c o n c e n t r a t i o n . Chemical geothermom e t e r s indicate a reservoir t e m p e r a t u r e of 92 ° C for the w a t e r s d i s c h a r g i n g at MaJimoto, a n d 71 ° C for t h e w a t e r s d i s c h a r g i n g at Narosura. Oxygen a n d h y d r o g e n isotopes indicate t h a t the w a t e r s are e s s e n t i a l l y m e t e o r i c in origin, b u t have b e e n slightly modified by equilibration with the s u r r o u n d i n g g n e i s s e s a n d schists. The c o m m o n origin of the w a t e r s at t h e two sites is indicated, b u t f u r t h e r work is required to prove t h i s conclusively. The field c a n be developed for s p a t h e r a p y , milk p a s t e u r i s a t i o n , l e a t h e r processing, crop drying, a n d h o u s e warming.

Acknowledgements- This research was conducted with funds from the UNDP project KEN/82/002, and National Council for Science and Technology project NCST/5/ 003/F/5. The author is grateful to the personnel of the geochemical laboratory at the Olkaria geothermal power station for facilitating the chemical analyses. The BGS is thanked for carrying out the isotope analyses. REFERENCES

Allen, D. J. and Darling, W. G. 1987. Kenya Rift Valley Geothermal Project: Interpretation of fluid sample

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analyses from Magadl-Sllale area. BGS Report WD/ OS/87/3. 16 p. Batlcci, G. 1987. Thermal conditions of the CentralSouthern sector of the Kenyan Rift Valley from the data collected in the boreholes drilled for water. Report for the UNDP/DTCD project Ken/82/002, Nalrobi, 42 p. Gudmundsson, J. S. and Lund, J.W. 1987. Direct uses of earth heat. In: Applied Geothermlcs (edited by Economides, M. and Ungemach, P.) Chapter 10, John Wiley and Sons, Ltd. Lindal, B. 1973. Industrial and other applicaUons of geothermal energy. In: Geothermal Energy, (edited by Armstead, H. C. H.)0 135-148, UNESCO, Paris, France. Nyblade, A. A. and Pollack, H. N. 1990. Terrestrial Heat Flow in Eastern and Southern Africa. Journal of Geophysical Research 95, B11, 17371-17384. Taylor, H. P. 1979. Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits. In: Geochemistry of Hydrothermal Ore Deposits. (edited by Barnes, H. L. 1979.) 2nd ed. J. Wiley and Sons, p. 236-277. Tole, M. P. 1985. Geochemical studies of the geothermal area east of the Jombo hill intrusion, Coast Province, Kenya: I. Hot water reservoir temperatures. Kenya Journal of Science and Technology. Series A. 6/2 91-101. Tole, M. P. 1986. Geochemical Studies of Low Enthalpy Geothermal Areas of Kenya. Report on a Reconnaissance Study. UNDTCD Report for project KEN/82/ 002.33 p. Tole, M. P. 1988. Low enthalpy Geothermal Systems in Kenya. Geothermics 17 (5/6), 777-783. Tole, H. P. 1990a. Stable isotope studies of some low enthalpy geothermal systems in Kenya. Journal of African Earth Sciences I I , 33-37. Tole, M. P. 1990b. Geochemical Studies of Low Temperature Geothermal Areas of Kenya. Report NCST/5/OO3/F/5 to National Council for Science and Technology 28 p. Walsh, J. 1969. Mineral and thermal waters of Kenya. XXIII Internation~d Geological Congress. 19. 105110.