Hydrogeology and water chemistry in the AYODELE OWOADE, L....

Groundwater Management- Quantity and Quality (Proceedings of the Benidorm Symposium, October 1989). IAHS Publ. no. 188,1989.

Hydrogeology and water chemistry in the weathered crystalline rocks of southwestern Nigeria

AYODELE OWOADE, L. G. HUTTON, W. S. MOFFAT & M. D. BAKO Department of Civil Engineering, University of Technology, Loughborough LEU 3TU, UK

Abstract Geophysical and hydrochemical techniques have been used to assess the groundwater prospects in the crystalline weathered regolith in southwestern Nigeria. The resistivity measurements gave an average depth of weathering of 34 m. From a consideration of the hydrodynamics of the weathering process, kaolinite was found to be the stable clay weathering product indicating that weathering reaction products are being flushed since otherwise feldspar-water equilibrium would have been attained considering the Pre-Cambrian age of the rocks. The presence of kaolinite was taken to imply that the groundwater is being recharged but it was not possible to use the available data to quantify the recharge. Analysis of recovery observations on one of the large diameter wells sampled gave an average permeability of 50 mm h"1 (1.2 m day"1). These results indicate that the weathered regolith in the study area could support modest water demands. Some re-thinking may be necessary regarding generalizations in the literature vis-à-vis the groundwater potential in hard rock terrains.

Hydrogéologie et chimie de l'eau dans les roches cristallines altérées du sud-ouest du Nigeria

Résumé Des techniques géophysiques et hydrochimiques ont été utilisées pour effectuer les prospections des eaux souterraines dans les régolithes cristallines altérées du sud-ouest du Nigeria. Les mesures de résistivité ont donné une profondeur moyenne d'altération de 34 m. A partir de considérations sur l'hydrodynamique du processus d'altération on a trouvé que la kaolinite était un produit d'altération argileux stable qui indique que les produits de réaction de l'altération sont lessivés; autrement l'équilibre eau-feldspath aurait été atteint compte tenu de l'âge pré cambrien de la roche. On a conclu de la présence de la kaolinite que ceci impliquait que la nappe est en train de se recharger sans qu'il ait été possible d'utiliser les données disponibles pour quantifier cette recharge. Les analyses des observations de remontée de l'eau dans un des puits à grand diamètre qui ont été choisis pour l'étude ont donné une

201

Ayodele Owoade et al. 202

perméabilité moyenne de 50 mm h"1 (1.2 m jour"1). Ces résultats indiquent que les régolithes altérées dans la zone étudiée pourraient supporter des demandes en eau modestes. Nous avons le sentiment qu'une certaine remise en question est nécessaire en ce qui concerne les généralisations défavorables dans la bibliographie vis-à-vis des potentialités de l'eau souterraine dans les sous-sols de roches dures.

INTRODUCTION

Southwestern Nigeria is underlain primarily by the basement complex rocks of pre-Cambrian age comprising gneisses, migmatites and schists. When fresh, such rocks have practically no porosity or permeability due to the interlocking crystal structure. The groundwater potential in crystalline rock terrains depends, therefore, on post-emplacement processes such as tectonism and weathering which could lead to the development of secondary porosity and permeability. This paper examines the weathered regolith developed over crystalline rock terrains in southwestern Nigeria. The primary objective is to assess the groundwater potential in the medium using geophysical, hydrochemical and hydrogeological techniques to evaluate the depth of weathering, the recharge potential and the flow characteristics respectively. A large number of people is dependent on the groundwater resource in the weathered regolith for basic water requirements and abstraction is largely through large diameter wells using a bucket tied to the end of a rope for withdrawing water. The rope-and-bucket arrangement is now being replaced in several households with centrifugal pumps resulting in larger quantities of groundwater withdrawal. The experience in other countries where ground subsidence has been attributed to groundwater overdraft e.g. Venice, Mexico City, San Joaquin valley, and several other cases documented by Poland & Davis (1969) has led to questions being asked as to the groundwater overdraft risks posed by this upsurge in groundwater abstraction. It is expected that at some time in the future, intervention would be required to keep abstraction to acceptable limits. It is hoped that the results of an early study such as this would provide a basis upon which a rational policy could be formulated. This paper presents the results of the preliminary stage of the study. Groundwater chemistry was used in an attempt to assess qualitatively whether or not the shallow regolith aquifer is being recharged. Resistivity measurements were used to evaluate the depth to the bedrock and the aquifer characteristics were determined from pumping test results.

THEORETICAL BACKGROUND

Crystalline rocks are formed by interlocking silicate minerals such as quartz, feldspars, micas, hornblende, pyroxenes, olivine and a host of minor accessories. Chemical weathering involves the dissolution of these minerals resulting in the formation of both soluble as well as solid phase products.

203 Hydrogeology in weathered crystalline rocks

The dissolution process has been described by Oilier (1984) as involving the replacement of the cations in the tetrahedral structure with hydrogen ions and can be schematically represented by the equation:

MSi04 + H+ [H+]Si04 + M+

mineral hydrogen ion solid phase product cation in solution

Most minerals are aluminosilicates, and the solid phase product is generally a clay mineral. Thus if water is the reagent, we have:

MAlSi04 + H20 - HAlSi04 + M+ + OH"

This may be illustrated by the weathering of potash feldspar to produce illite:

6KAlSi3Og + 4H20 + 4C02 - 4K+ + K^Al^SigALjOgjXOH),

+ 4HC03- + 12Si02

The stability of clay minerals depends on the concentration of silica (present as colloids rather than as ionic species) and the cation to hydrogen (M+/H+) ratio in the system. The stability relations for gibbsite, kaolinite, montmorillonite, muscovite and feldspar at 25°C and 1 bar as determined by Tardy (1971) are presented in Fig. 1. Using the sodic feldspar albite to illustrate, the initial alteration product is gibbsite. In a closed system, further dissolution causes the concentrations of silica and the Na+/H+ ratio to increase until the equilibrium boundary of kaolinite is reached after which subsequent alteration products are kaolinite and the previously formed

log Si(OH),, rr— log Si (OH)., , log Si(0H)4

1 10 100 1 10 100 1 10 100

S i0 2 mg/^ Si02 mg/<? S i 0 2 r n g / /

Fig. 1 Clay mineral stability diagram at 25"C and 1 bar (from Tardy, 1971).


gibbsite is converted to kaolinite. In a similar manner montmorillonite is produced until eventually albite becomes the only stable mineral under the prevailing concentration of Si02, Na+ and H+. In a groundwater system, "a long time and sluggish flow conditions are required for water to achieve equilibrium in relation to feldspar. The nature of the clay-mineral weathering products can therefore be dependent on hydrodynamic and hydrochemical conditions as well as on mineralogical factors" (Freeze & Cherry, 1979).

PRESENT STUDY

The study was carried out in Ilesha, an urban city in southwestern Nigeria with an estimated population of about 300 000. The location is shown is Fig. 2. The town is underlain by quartzites, amphibolites and granite gneiss with the latter being the most abundant. The climate is tropical with characteristics as presented in Table 1. Ilesha was selected for case study because of its central location within the basement complex of southwestern Nigeria and the fact that the rocks are fairly typical of the region.

Fig. 2 Generalized geological map of Nigeria; showing location of Ilesha.

HYDROCHEMISTRY

Water samples for chemical analyses were collected from shallow large diameter wells from the sites shown in Fig. 3. The well depths ranged between 1.4 and 15 m with diameters averaging 1.2 m. The pH, temperature and conductivity were determined in the field while the major ions were determined in the laboratory of the Institute of Ecology, Obafemi Awolowo


Table 1 Mean climatic characteristics at Ilesha, southwestern Nigeria

Rainfall mm Temperature °C Actual Evapotransp. mm

January

February

March

April

May

June

July

August

September

October

November

December

Annual

8.4

23.9

82.0

122.7

164.3

176.5

178.1

91.9

191.3

200.7

69.9

10.2

1319.5

26.1

27.2

28.3

27.2

26.7

25.6

24.4

23.9

25.0

25.6

27.2

26.1

8.4

23.9

82.0

122.7

137.5

118.3

99.5

91.9

108.6

118.3

69.9

10.2

991.2

University, Ile-Ife, Nigeria. Calcium and magnesium were determined by EDTA titration. Sodium and potassium were determined by flame photometry.

Chloride concentration was determined with a Marius chloro-o-counter. The principle is similar to silver nitrate titration except that the titration is automatically achieved and the number of chloride counts is read off at the end-point. A standard curve was used to convert the counter reading to chloride concentration. Sulphate concentration was determined with the same instrument. Iron, silica, phosphate and nitrate were determined colorimetrically while alkalinity was determined by titrating against sulphuric acid. All the analyses were carried out according to standard procedures (APHA, 1975; Hutton, 1983). The chemical analyses were intended to satisfy two objectives. The first was to assess whether groundwater recharge is taking place or not. The other objective was to be able to assess the groundwater quality especially in relation to drinking water standards and in view of the proliferation of pit latrines and septic tanks. The concentrations of sodium, silica, chloride and the pH are presented in Table 2. The same information is presented graphically as histograms (Fig. 4) to illustrate the variation in the analysis. In pursuance of the first objective, a plot of log[Na+]/[H+] vs. Si02 was prepared as shown in Fig. 5 in order to determine the stable clay weathering products. The analyses plot in the kaolinite stability field indicating that dissolution products are being flushed away from the reaction sites. This flushing can only be flowing groundwater since abstraction is at the moment negligible. At this stage of the study, the kaolinite weathering product provides only a qualitative indication that recharge is


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LEGEND

6 lS Q u a r t z i t e Am A m p M b o l i t e GG C r a n t t e g n o i s : ; °iei Sampled we 1 1 •v.« VES s iCC —- Township major roads --* Approximate geological

boundary

^ suspected pollution from cattle manure

Fig. 3 Map of Ilesha showing the location of sampled wells and vertical electrical sounding.

taking place. Further work is required to quantify this recharge but isotope studies at the Obafemi Awolowo University campus, Ile-Ife (about 15 km south of Ilesha) gave a rather low value of 20 mm per annum (Loehr.cii, 1980).

The presence of contaminants in groundwater is best monitored by using elements whose concentrations are not altered by processes other than hydrodynamic dispersion. Chloride and nitrate are conservative elements which have been successfully used as tracers in studies of contaminant transport (Fillham & Webber, 1969; Sudicky et al, 1983). Using chloride as a tracer it would be noticed from results presented in Table 2 that abnormally high values are recorded in a few places. The high chloride values recorded in wells 132, 133, 134, 135 are probably attributable to leachate from cattle manure. This pollution appears to have migrated some distance downstream as suggested by the high chloride values in samples from wells 131, 136, 130, 128, 129, 122, 123, 117, 401. The 25-year old cattle market is located on a hill-crest (local groundwater divide) and the pollution seems to have spread with a circular plume centred about well 134. Further study is required to map this pollution in detail in both lateral and vertical directions. From the available evidence, it would appear that the groundwater has been polluted to a depth of at least 15 m. Wells 134 and 131, 13.4 m and 15 m deep have


Table 2 Chloride, nitrate, sodium, silica and pH results

II no

101

1 0 2

1 0 3

1 0 4

1 0 5

1 0 6

1 0 7

1 0 8

1 0 9

1 10 1 1 1

1 1 2

1 1 3

1 1 4

1 1 5

1 1 6

1 1 7

1 1 8

1 19 1 2 0

1 2 1

1 2 2

1 2 3

1 2 4

1 2 5

1 2 6

1 2 7

1 2 8

1 2 9

1 3 0

1 3 1

1 3 2

1 3 3

1 3 4

1 3 5

1 3 6

2 0 1

2 0 2

2 0 3

2 0 4

2 0 5

2 0 6

2 0 7

3 0 1

3 0 2

3 0 3

3 0 4

3 0 5

3 0 6

3 0 7

3 0 8

3 0 9

3 1 0

4 0 1

4 0 2

4 0 3

4 0 4

4 0 5

4 0 6

4 0 7

4 0 8

4 0 9

4 1 0

41 1 4 1 2

4 1 3

4 1 4

4 1 5

4 1 6

,4 17

swl mbg

14.1 1 0

7 . 6

12.9 6 . 1

0 . 9

10.9 12.8

10.5 14.2

9 . 4

7 . 9

12.7 5 . 4

6 . 3

10.5 7 . 0

0 . 7

7 . 5

5 . 5

10.8 6 . 7

8 . 2

8 . 6

2 . 5

7 . 7

6 . 7

14.9 8 . 3

5 . 4

13 .2 5 . 9

1 .2 2 . 5

7 . 3

7 . 8

6 . 6

4 . 4

2 . 3

10.9 7 . 9

12.6 7 . 4

1 .4 5 . 4

6 . 4

4 . 5

9 . 8

10.7 7 . 5

5 . 6

8 . 2

9 . 7

0 . 9

1 .0 1 .1 9 . 6

10.5 8

9 . 1

1 0.5 5 . 5

1 0.9 9 . 7

13.0 13.4

9 . 1

welldeplh. m Chlor

1 4 .9 10 .7

8 . 4

8 . 0

1 3 .8 8 . 3

1 .6 1 1 .7 1 2 .9 1 2.4 1 0 .7 1 4 .6

9 . 9

9 . 0

1 3.3 6 . 1 6 . 4

1 0 .8 7 . 7

8 . 0

8 . 5

5 . 8

1 1 .3 7 . 0

9 . 8

9 . 3

2 . 8

9 . 1 8 . 3

1 5 .0 1 0 .4

6 . 5

13 .4 7 . 2

4 . 2

3 . 2

8 . 1 8 . 1 7 . 9

4 . 9

3 . 0

1 2 .5 9 . 3

1 2 .8 8

1 . 8 6 . 5

8 . 1 5 . 0

1 0 .3 1 1 .8

8 . 7

5 . 7

8 . 5

1 0 . 5 1 .4 1 .8 1 .8

1 0 . 7 11.7 8 . 4

1 0 .0 1 0 .6 6 .7

1 1 .2 1 0.2 13 .1 1 3 .8

9 .5

ide mg/l

12.4 4 . 2

8 . 1

14.5 2 . 1

35.4 26.9

4 . 2

16.6 72.7 12.4 29.0 16.6

8 . 2

8 . 2

6 . 2

2 . 0

41.5 56.0 47.7 27.0 12.4

0 . 0

29.0 60.2 99 .6 87.2 47.7

105.9 124.6

95.5 93.4

130.8 49.8

0 . 0

0 . 0

0 . 0

0 . 0

56 .0 31 .1 70.9

0 . 0

29.7 64.5 18.7 42.5

0 . 0

45.7 0 . 0

79.0 16.6 31 .1 53.8

' 62.3 35.4 56.0 14.5

0 . 0

56.0 85 .0

0 . 0

10.2

12.4 0.0

Nitrate mg/l

0 . 5

0 .2

2 . 3

0 . 2

0 . 2

2 . 7

0 . 6

: .6 0 . 2

6 .9

0 .6

2 .8

3 . 2

0 . 2

4 . 0

0 . 3

: .3 0 . 2

5 . 2

3 : .8 • o . o •0.3

4 . 9

0 . 3

•0.2 0 .4

32.3 28.8 27.7 39.7

S 3 .

71 .1 48.5 25.4

2 . 5

0 . 5

0 . 3

0 . 5

9 . 5 7 . 1

10.6 3 . 4

0 . 6

5 . 5

2 . 5

2 . 0

7 . 3

0 . 3

1 .4 0 . 6

24.2 3 . 1

10.8 20.7

8 . 1

9 . 6

9 . 3 7 . 6

1 .2 40.8 29.5

0 . 3 0 . 4

2 . 6

0 . 6

Sodium mg/l

3 . 2

2 . 2

5 . 4

2 . 3

2 . 0

15.4 24.0

6 . 3

1 .1

23.1 3 . 2

8 . 3

20.7 1 .4 5 . 4

1 .7 2 . 9

1 .4 20.3 60.9 43.9 13.1 10.0

1 .4 15.5 43.9 53 .9 48.9 26.2 82 .9

102.9 68.9 85.9 90.9 1 1 .1

2 . 9

2 . 0

2 . 0

25.4 26.2 18.6 79.9

4 . 3

24 .6 5 . 9

10.6 25.4

1 .4 24.4

1 .4 50.9

3 . 4

17.4 53.9 50.9 20.7 41 .9 16.0

3 . 4

19.4 43 .9

1 .7 4 . 3

9 . 4

3 . 2

Silicate mg/l

2 . 3

5 . 2

3 . 6

3 .4 4 .4 6 .0

1 8.7 1 .5 0 .9

2 .3 2 . 8

3 3 4 . 2

1 .9 4 .4 6 . 6

5. 1 3 . 6

5 . 1

4 .8 2 .4 3 . 7

3 . 6

4 . 1 3 . 6

1 4 .0 3 . 4

3 .7 4 . 6

2 . 8

5 . 7

4 . 9

4 . 2

1 0.1 5 . 1 4 .0 7 . 7

4 . 2

4 . 1 8 . 6

4 . 3

3 . 1

2 . 9

4 . 2

5 . 3

3 . 6

4 .0 4 . 2

5 . 8

4 . 2

4 . 1 2 . 7

3 . 6

5 . 6

12.4 3 . 6

4 .6 6 . 8

6 . 8

6 .0 3 . 6

3 . 6

4 .0

5 .2 4 .6

pH

5 . 3 5 . 5

5. 1

6 . I 5 . 7

5 . 9

6 . 6

5 . 3

5 . 0

5 . 8

5 . 6

5 . 6

5 . 6

5 . 2 5 . 4

5 . 3

6 . 5 5 . 8

6 . 6 4 .5 5 . 2

5 . 0

6 . 1 5 . 6

6 .5 6 . 5

5 .8 6 . 5

5 . 0

5 . 9

6 . 0

6 . 7

4 . 9

6 . 7

5 . 3

6 . 0

6. 1 5 . 4

5 . 3

5 . 3

4 . 7

6 . 0

5 . 9

5 . 7

5 . 2

5 . 3

5 .7 5 . 5

5 . 7

5 . 5

7 . 0

5 . 2

5 . 7

4 .5 7 . 3

5 . 1

6 . 0

5 . 4

5 . 7

5 . 1 6 . 2

6 . 3

5 . 5

5 . 2

5 . 3

Ayodele O

woade et al.

208

r"

o .o

c

o

1.

.' • !

1 1

§

•«

S S

U

5: O

c-5

I t


./~

.r

•&B-

« *

d®

r .1 1 Si02mg/1 10 100

Fig. 5 Plotting positions in the clay stability diagram of the chemical analyses of shallow groundwater in the weathered regolith of southwestern Nigeria showing kaolinite is the stable clay (see Fig. 1).

96 mg l"1 and 46 mg I"1 chloride concentrations respectively compared with a background mean chloride value of 15 mg l"1 i.e. when the polluted areas have been excluded. The overall chloride average is 31 mg l"1. High chloride concentrations are also recorded around the abattoir - see wells 113, 201, 112. Apart from a few isolated sites, the chemical water quality appears generally good and in conformity with WHO (1983) drinking water guidelines. The nitrate values in Table 2 indicate past pollution. It will be necessary to consider the bacteriological quality of groundwater in this area, especially near latrines. Some of the wells are open and unprotected and future studies need to consider improved quality brought about by improvements such as covering the wells, installing pumps and other sanitary measures.

Sampling from the wells as they exist may not be representative of the water body as the water already in storage in the wells could have undergone some chemical changes, and there is also the possibility of contamination due to ingress of surface water and by buckets. Future studies will require the contents of the wells to be pumped out two or three times to allow water from the aquifer to be more properly sampled.

This preliminary survey has shown promise in its methodology and a more scientific approach. More detailed studies involving multi-level sampling, repeat analyses, and bacteriological testing will be needed to allow a more complete assessment to be made of the water quality and geochemistry.

GEOPHYSICAL SURVEY

The depth to bedrock was investigated geophysically using vertical electrical sounding (VES). The Schlumberger array was used and the field results were interpreted by computer modelling. Eighteen soundings were made at the sites indicated in Fig. 3 - numbers prefixed by V e.g. V103. The


<l 709.001 , -; i npi n HODII

3?g4.43(— !

24 Ï5 .02 j~

1934.32 -

i 5 4 9 . 3 0 ~

1240.«2 -

553,92 -

f + + + T + f l ( I ! Q

408

900

3200

14

S7

796.08 r~ 1 laje v ° z

637.63b--"" resist iv i ty data: * observed; calculated; __ node!

510.714 1,00 1.88 3.55 6.68 12,59 23,71 44,67 34.14 158,49

Schluitberger array spacing flB/2 met res

Fig. 6 A sample of the electrical resistivity results and the modelling interpretation.

current electrode spacing used was 200 m - enough to ensure the bedrock was reached. A typical field result is plotted in Fig. 6 together with the model interpretation. Typically four geoelectric layers are indicated: from the surface, there is a low resistivity thin layer, (mean thickness 0.9 m), probably moist top soil (the field work was undertaken in August, when the soil moisture was probably at field capacity); next is a high resistivity layer of variable thickness, the unsaturated zone, the base of which (ranges between 2 and 16 m, mean 7.2 m) corresponds with the water table; the saturated zone itself is characterized by low resistivity (compared with the unsaturated zone); and this geoelectric layer terminates at the bedrock which is marked by a sharp rise in the resistivity with thickness to infinity. The bedrock varies between 14 and 60 m with a mean of 34.5 m. This interpetation of the observed geoelectric layering is shown in Fig. 7.

0 -moist top soil 192ohm-m

0.9

dry regolith 1650ohm-m 7.2 --- (not to scale, depths in metres)

saturated zone 473 ohm-m

34.5

BEDROCK 1029 ohm-m

Fig. 7 Mean characteristics of geoelectric layers.

HYDROGEOLOGY

The objective of the geophysical survey was to assess the viability of the weathered regolith with respect to groundwater storage. This viability is dependent on the development of adequate weathering in depth, which in


turn depends on climatic, environmental and lithologie characteristics. The essential requirements have been summarized by Faniran (1974) as: (a) readily weatherable rocks; (b) tropical to sub-tropical humid climates with seasonally-incident rainfall

or alternating wet and dry periods; (c) relatively open vegetation; (d) low relief surface which minimizes dissection and the removal of the

weathered mantle by channel processes; (e) stable geological or structural conditions; and (f) a fairly long period during which the above conditions remain stable.

The environmental requirements for the development of deep weathering are largely satisfied in southwestern Nigeria and one would therefore expect the development of thick regoliths. This expectation is confirmed by the results of the vertical electrical soundings. The computed depths to bedrock at the various VES sites are given in Table 3.

Table 3 Interpretation of results from vertical electrical soundings (VES)

VES Location

V101 V102 V103 V104 V105 V106 V201 V202 V203 V301 V302 V303 V304 V401 V402 V403 V404 V405

mean

s.dev.

geoelectric layer

Lay

a

0.7 0.7 1.0 0.9 0.8 0.3 0.9 0.5 0.5 0.9 0.9 1.2 1.1 2.0 0.9 2.0

0.9

0.9

0.5

mois so

îr 1

b

220 400

50 110 200

60 120 220 120 270 410 550 100 94 62 42

250

193

142

t top il

Layer 2

a b

4.0 14.0 10.0 10.0 4.0 2.0 4.0

2.0

10.0 16.0

10.0

1800 4700 1500 1900 1600 100

1600

700

1700 900

3 data not ana

7.8

4.6

1650+

1153

dry regolith

Layer 3

a b

25.0 37.0 39.0 22.0 30.0 18.0 50.0 40.0 14.0 35.0 50.0 60.0 50.0 22.0 28.0 30.0

lysable 6.0

34.4+ +

12.8

1080 900 300 200 400

48 100 550

60 1600 300 560 760 205

32 54

900

474

432

saturated regolith

Layer 4

a b

infinity " " " " " " " " " " " " " " "

" "

1400 3200 700

1200 600 800 450

1500 700 700

1200 700

1700 800

1000 200

1900

1103

686

fresh bedrock

a = depth to base of layer, metres + = excludes V403 b = layer resistivity, ohm-m ++ = excludes V405

The VES results indicate that the bedrock depth varies spatially. The mean depth is 34 m with a standard deviation of 13 m. The thickest depths


are on the higher topographic locations. The computed water table depths have an average of 7 m, again with considerable spatial variation. The spatial variation in hydrogeologic characteristics in the study area has been described by Owoade & Moffat (1989). The low depths to bedrock in the valleys were attributable to removal by erosion, with consequent opening of the fractures thereby resulting in higher yields in the valleys.

The VES were carried out in August and the regolith aquifer can be reasonably assumed to be at full capacity. The results thus indicate an average saturated thickness of 26 m. The mean porosity and specific yield for southwestern Nigeria are respectively 0.39 and 0.21 (Omorinbola, 1984).

The hydraulic properties of the regolith were investigated by pumping out some water from well number 106 whose diameter and depth are 0.8 and 8.3 m respectively. Because of the large diameter, there is well storage which violates an important assumption in the conventional method of well test analysis (Theis, 1935; Cooper & Jacob, 1946). Some quantity of water was pumped out of the well and the recovery was then monitored for two hours. Two methods were used to analyse the results. These were the piezometer test of Hvorslev (1951) (described in Freeze & Cherry, 1979, p. 340) and the auger hole seepage theory of Boast & Kirkham (1971). These methods were less restrictive in the assumptions underlying their derivation and both produced permeability values of 40 and 60 mm h"1 respectively. These values are considerably larger than those obtained from laboratory permeameter tests on spot samples by Omorinbola (1984) who reported average values of about 2 mm h"1 from investigations in the middle Oshun valley, Ife area, and the middle Owena valley. The geology and weathering characteristics vary spatially and the differences in reported values indicate the need for caution in extrapolating results from the laboratory to the field. For this study, an average of 50 mm h"1 is taken for the permeability of the weathered regolith derived from the granite gneiss in the Hesa area.

CONCLUSION

The results presented above indicate the availability of appreciable quantities of groundwater in the weathered regolith of the study area. From hydrochemical and hydrodynamic considerations, the stable clay mineral is kaolinite and this suggests that weathering products are being regularly flushed and transported away from the reaction centres. This is proof that recharge is currently taking place which is in accordance with the findings from stable isotope studies (Loehnert, 1980; Ogunkoya, 1986). Loehnert (1980) reported that the soil moisture is. subjected to evaporation before percolating to join the groundwater. Such evaporation would result in the percolated water being slightly enriched in tritium in comparison with current meteoric water. Ogunkoya (1986) analysed the tritium contents in the baseflows of some rivers in the Owena River catchment which adjoins the present study area. The average rainfall tritium content during his samplings was 20 TU which was slightly lower that the values (8 to 34 TU) obtained from the stable isotope analysis of the baseflow waters. Evaporation is


thought to have enriched the baseflows prior to sampling; and the conclusion that the water represents a much earlier period when tritium values in the atmosphere were much higher is probably erroneous.

The storage capacity as represented by the specific yield is quite high but the permeability is rather low and only moderate demands such as for household needs could probably be met. For maximum yield, the entire thickness of the regolith should be penetrated but this calls for improved digging methods as opposed to the manual axe-and-hole methods currently in use. Also previous studies (Owoade, 1988) have shown that the use of larger diameters would improve yield.

The results of the chemical analyses hint at pollution of the groundwater at some locations. On-site sanitation does not seem to pose much danger probably because of the low loading rates generally involved and the fact that the pit latrines and soak-aways do not reach the water table. However the 25-year old cattle market in the Ibosin area (about the centre of the town) appears to be a source of considerable pollution - this is reflected in samples 136, 135, 134, 133, 121. The pollution could be traced for distances beyond 1 km away. The cattle market is located on the groundwater divide and its relocation is recommended to prevent further pollution. The abattoir at Omieran also appears to be introducing contaminants into the groundwater (samples 112 and 114) though not as severe as those recorded in the cattle market area.

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