ISOTOPE HYDROLOGY IN THE EXPLORATION OF THE KATWE, BURANGA AND KIBIRO GEOTHERMAL SYSTEMS, UGANDA

Godfrey BahatiGodfrey BahatiDepartment of Geological Survey and Mines, Department of Geological Survey and Mines,

P.O Box 9, Entebbe, UgandaP.O Box 9, Entebbe, Uganda

Second African Rift Geothermal ConferenceSecond African Rift Geothermal ConferenceImperial Resort Beach Hotel Entebbe, Uganda,Imperial Resort Beach Hotel Entebbe, Uganda,

November 24November 24--28, 200828, 2008

Outline Outline

• Objectives of the study• Study areas• Methodology• Geology of the western Rift Valley• Sampling points• Stable isotopes• Tritium and mixing processes• Sources of solutes • Water–rock interaction • Conclusions

Objectives of the study

• To elucidate the origin of the geothermal fluids,

• identify the recharge mechanisms,• estimate subsurface temperature using

isotope geothermometry,• trace the source of solutes,• improve the conceptual geothermal

models of the study areas.

Study areasFigure: a) East African Rift System (EARS), b) Locations of the

geothermal areas of Uganda.

Methodology

• 118 water samples from hot and cold springs, dug wells, rivers and lakes,

• 13 samples from surface outcrops of different types of rocks,

• Samples analyzed for chemical and isotopic compositions.

• Isotopes analyzed included hydrogen (δD and tritium), Oxygen (δ18O in water and sulphate), sulphur (34S in sulphate), and strontium (87/86Sr in water and rock).

Geology of the western Rift ValleyGeology of the western Rift Valley

• Katwe: Volcanics• Buranga:

Sedimentary environment

• Kibiro: sedimentary west of the escarpment and crystalline environment east of the escarpment (granites/gneisses)

Katwe: sampling points

Buranga: sampling points

Kibiro: sampling points

Stable isotopes in water

-30

-20

-10

0

10

20

30

40

50

60

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

δ18O

δ2 H

hot springs

groundwater

lakes

rivers

LMWL

GMWL

Katwe: stable isotopes in water

-30

-20

-10

0

10

20

30

40

50

60

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

δ18O

δ2 H

Kitagata HSRiversCrater LakesCold springsGroundwaterKibenge-HSFresh LakesKatwe HS Mixing line

LMWL

Buranga: stable isotopes in water

-30

-25

-20

-15

-10

-5

0

5

10

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

δ18O

δ2 H

Hot Springs

Rivers

Groundwater

Kibenge-HS

LMWL

Buranga: deuterium concentration in water

165000 170000 175000 180000 185000 190000 195000 200000 205000

Eastings (m)

60000

65000

70000

75000

80000

85000

90000

95000

100000

105000

Nor

thin

gs (m

)

1234

56

7

8

9 10

11

12

13

14

15

1617

1819

2021

2223

242526

2728

-20-19-18-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1012

Buranga

Bundibugyo

Rwenzori Mountain Ranges

Kibuku

Kichwamba

Bukuku

Kabarole

D

Kibiro: stable isotopes in water

-30

-20

-10

0

10

20

30

40

50

-5 -4 -3 -2 -1 0 1 2 3 4 5 6δ18O

δ2 H

Hot Springs

Rivers

Lakes

Groundwater

LMWL

Kibiro: deuterium concentration in water

Deuterium in water Geology of Kibiro

Tritium and mixing processes

-1

0

1

2

3

4

-6 -5 -4 -3 -2 -1 0 1 2

Oxygen-18 (per mil)

Triti

um C

once

ntra

tion

(TU

)

Kibiro hotKibiro coldKatwe hotKatwe coldBuranga hotBuranga cold

Isotope and chemical geothermometry

0

2

4

6

8

10

12

14

16

18

20

-10 -8 -6 -4 -2 0 2 4 6 8 10

δ18O in H2O

δ18O

in S

O4

KibiroBurangaKatwe

150°C

50°

200°C250°C 300°C

100°C

Solute geothermometry:

• Katwe: 140-200˚C

• Buranga: 120-150 ˚C

• Kibiro: (a) 150˚Cand (b) 200-220 ˚C

sulphate-water (S18O4-H218O) isotope geothermometer

Sources of solutes

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30δ18O(SO4)

δ34S(

SO4)

KibiroKatweBuranga

Devonian to Lower Triassic

Magmatic Continental evaporites

Cenozoic

Modern marine SO4

Atmospheric SO4

Water-rock interactions

Basalt

DioriteDolerite

Gneiss

GraniteGraniteSiliceous clayTravertine

TravertineXenolith

0.6800

0.6900

0.7000

0.7100

0.7200

0.7300

0.7400

0.7500

0.7600

0.7700

0.7800

0.7900

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00Strontium (mg/L)

87/8

6 Sr

0.6800

0.6900

0.7000

0.7100

0.7200

0.7300

0.7400

0.7500

0.7600

0.7700

0.7800

0.7900

Kibiro Buranga Katwe Kibenge Rocks

ConclusionsConclusions

•• Recharge to the geothermal areas is from high Recharge to the geothermal areas is from high ground: ground: RwenzoriRwenzori mountains for mountains for KatweKatwe and and BurangaBuranga; ; MukihaniMukihani--WaisembeWaisembe Ridge for Ridge for KibiroKibiro..

•• Movement of the fluids from recharge areas to the Movement of the fluids from recharge areas to the reservoirs and hot springs is fault controlled.reservoirs and hot springs is fault controlled.

•• Subsurface temperatures predicted by isotope Subsurface temperatures predicted by isotope geothermometry are highest for geothermometry are highest for BurangaBuranga (200(200°°C), C), and 140 and 140 –– 160160°°C for C for KatweKatwe and and KibiroKibiro. .

•• Reservoir rock types are most likely basalt in Reservoir rock types are most likely basalt in KatweKatwe, and granites/gneisses in , and granites/gneisses in BurangaBuranga and and KibiroKibiro..

•• The major source of solutes is rock dissolution The major source of solutes is rock dissolution with with magmaticmagmatic input.input.