Evaluation of Aquifer Characteristics of Voltaian Sedimentary Rocks in the Brong Ahafo Region of Ghana
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EVALUATION OF AQUIFER CHARACTERISTICS OF VOLTAIAN SEDIMENTARY ROCKS IN THE BRONG AHAFO REGION OF GHANA 1Patrick A. Mainoo, 1Anthony Duah, 1William A. Agyekum and 2Aboagye Menyeh 1CSIR Water Research Institute, P. O. Box M.32, Accra-Ghana. 2Department of Physics, Geophysics Section, Kwame Nkrumah University of Science and Technology, Kumasi-Ghana ABSTRACT Groundwater potential of the sedimentary formation in the Brong Ahafo Region of Ghana has been evaluated using computed transmissivity and specific capacity values. Data on multiple step and constant-discharge tests on twenty (20) boreholes were analyzed. The Cooper-Jacob’s straight line analytical approach was used to determine the aquifer characteristics. Computed transmissiv- ity coefficients of the underlying aquifer ranged widely from 0.82 – 61.27 m2d-1. The specific ca- pacity values obtained varied between 0.59 and 75.20 m3d-1m-1. Total head loses of the boreholes ranged generally between 5.49 and 99.2%. Hydrogeologically, the aquifer systems can be described as fairly heterogeneous in accordance with Krasny’s transmissivity classification model. Ground- water potential of the sedimentary aquifer system could be classified as high to intermediate to yield substantial groundwater resource for domestic and industrial water supply. To secure sub- stantial quantity of water for sustainable water supply in areas underlain by this sedimentary aqui- fer system in Ghana, integrated geophysical approach including 2D-Continuous Vertical Electrical Resistivity (CVES) survey, and other related tools such as Remote Sensing (RS) and Geographical Information Systems (GIS) should be used to enhance the delineation of deeper aquifer zones of high groundwater potential Keywords: Ghana, Brong Ahafo, groundwater, sedimentary rocks, transmissivity, aquifer, sustainable, specific capacity INTRODUCTION live in rural and peri-urban communities, which The Brong Ahafo Region of Ghana has a total are widely distributed within the Region. Ayi- land size of 39,557 km2 and constitutes about botele (1975), indicated that many communities 16.7 % of the total land size of the country and peri-urban towns in the region lack potable (Ghana Statistical Services, 2012). In terms of water and therefore depend on raw water sources size, the region ranks second to Northern Region such as streams, rivers, lakes, ponds, dug-outs of Ghana (Ghana Statistical Services, 2012). and impoundment reservoirs. However, due to The population of the region is estimated to be anthropogenic activities, these traditional 2,310,983, representing 9.4% of the national sources are heavily polluted resulting in water- population. More than 55% of this population borne and water-related diseases such as diar- rhoea, guinea worm, bilharzia, etc. The Brong urban communities in Ghana, sought to provide Ahafo Region of Ghana lies within the semi-arid some selected communities in the Brong Ahafo region where mean annual evapotranspiration Region with sustainable potable water supply. exceed the mean annual rainfall by 49% (Gyau- Consequently, a network of boreholes were Boakye & Timbulto, 1996). The surface water drilled and their combined yield rallied into over resources dry up during the dry season -head water storage tank for onward distribution (November-February) and they severely affect to consumers via standpipes and a distribution domestic and agricultural water supply. In network to residential and industrial establish- Ghana, the recommendations from the Water ments. This paper assesses the groundwater po- Resources Sector Studies (WRSS), which was tential and characteristics of aquifer regimes commissioned by the Government of Ghana which underlie the study area. The boreholes (GoG), recommended making a policy to supply under this study were drilled to supply sustain- potable water through groundwater development able potable water to 14 Small Towns in 6 se- (Nathan Consortium for Sector Studies, 1970). lected Districts of Brong Ahafo Region of The policy recommended that communities with Ghana. Under this project, the potential thresh- population less than 500 are to be helped to se- old for a successful borehole was 5.0 m3/h. cure water supply through digging and construc- tion of hand-dug wells. Water supply to commu- THE STUDY AREA nities with population ranging between 500 and The study area comprises fourteen (14) small 2000 should be through boreholes fitted with towns in five (5) Administrative Districts that hand-pumps; whilst Small Towns and Peri- are scattered throughout the central and eastern Urban Towns with population higher than 2000 part of the Brong Ahafo Region. The geographic are to benefit from pipe-borne water supply location of the beneficiary communities are pre- through mechanized borehole systems. sented in Fig. 1 and Table 1. Geographically, Groundwater resources have some attributes that the areas lie between the boundaries of latitudes 7o 45’ N and 8o 23’N, and longitudes 0o 7’ W make it suitable for water supply in Ghana (Quist et. al., 1988). Firstly, the aquifer systems 3°0'0"W 2°0'0"W 1°0'0"W 0°0'0" 1°0'0"E underlie a large extent of the geographic area of BURKINA FASO the country and can be tapped at moderately 11°0'0"N ± 11°0'0"N shallow depths to meet the water demands of widely dispersed communities and towns. Sec- 10°0'0"N 10°0'0"N ondly, water stored in aquifer systems have proven to be protected from evaporation and 9°0'0"N 9°0'0"N REPUBLIC TOGO OF REPUBLIC thus, borehole/well yields are sufficient to meet COTE I VOIRE D' requirements, providing water security to areas that experience protracted drought. Thirdly, pre- 8°0'0"N Brong Ahafo 8°0'0"N vailing aquifers in Ghana have relatively excel- lent microbial and intrinsic physico-chemical 7°0'0"N 7°0'0"N quality that requires no or minimal treatment to improve portability. Lastly, groundwater devel- 6°0'0"N 6°0'0"N opment can be phased as the population grows rendering cost of development relatively modest 5°0'0"N 5°0'0"N as compared to the traditional surface water GULF0 OF GUINEA100 200 Km treatment. Community Water and Sanitation 3°0'0"W 2°0'0"W 1°0'0"W 0°0'0" 1°0'0"E Agency (CWSA), the official institution man- dated in Ghana for the provision of potable wa- Fig. 1: Map of Ghana showing the Brong ter and sanitation facilities in rural and peri- Ahafo Region Table 1: Geographic location of project towns and geologic setting Geographic Location Town District Geological Formation Latitude Longitude Jema Kintampo south N: 7.89235 W:1.77021 Damango sandstone Amoma Kintampo south N:7.78745 W:1.88054 Damango sandstone Anyima Kintampo south N:7.89286 W:1.86781 Damango sandstone Babatokuma Kintampo North N:8.14189 w:1.69736 Anyaboni sandstone Gulumpe Kintampo North N:8.54986 W:1.57845 Obosom undifferentiated Portor Kintampo North N:8.57768 W:1.56085 Obosom undifferentiated Kadelso Kintampo North N:8.65786 W:1.52397 Obosom undifferentiated Manso Nkoranza North N:7.70260 W: 1.83989 Damango sandstone Busunya Nkoranza North N:7.68677 W: 1.65258 Anyaboni sandstone Nkwabeng Nkoranza South N:7.49062 W: 1.74341 Anyaboni sandstone Ayirede Nkoranza South N:7.48341 W: 1.65312 Afram sandstone Kajeji Sene N:7.78371 W: 0.22656 Tease sandstone Bantama Sene N:7.70914 W: 0.75815 Obosom undifferentiated Akokoa Atebubu N:7.59154 W: 1.15338 Sandstone and 1o 56’ W. The area is bordered to the north the adjoining moist-semi deciduous forest into by Northern Region, to the south by Ashanti interior wooded savannah type (Dickson & Ben- Region and to the east by the Volta Lake and neh, 2004). Baobab, dawa-dawa, acacia and shea Volta Region. butter trees are amongst the most conspicuous adaptable trees of the harsh prevailing climate Physical settings of the project area conditions. The trees are widely scattered and The area falls within the modified Tropical Con- the vegetation is quite open and dominated by tinental or Interior Savannah climate type, which short grasses, which grow in tussocks to an aver- is characterized by double maxima rainy season age height of about 3 m above the ground sur- (Dickson & Benneh, 2004). The major rainy face. Geomorphologically, the study area falls season is experienced within the period of May within the Southern Voltaian plateau, which is to June, whilst the later rains are experienced characterized by layers of sandstone undulating from September to October, followed by a pro- terrain of gently dipping or flat-bedded sand- longed dry season between November and Feb- stone, shales, mudstones (Dickson & Benneh, ruary. The heaviest rainfall occurs in August. 2004). The almost flat and extensive plain has an The mean monthly temperatures vary from about average elevation ranging from 60 - 150 m, 30 0C in March to about 24 0C in August. Rela- whilst the high points rise to elevation of 450 m. tive humidity prevailing in the study area varies The western corridor of the study area is drained from 90 to 95 % in the rainy seasons and it re- by Tankor, Pru, Afram and Sene rivers, which duces to 75 – 80 % in the dry season. The vege- are all tributaries of the main Volta River. The tative cover of the project area is of the Interior Nyemu river principally drains the north-western Wooded Savannah type, which is the largest in part of the study area and empties into the Black Ghana. It is highly envisaged that, the increasing Volta river at a confluence near Bupei in the effect of land use and climate change may turn Northern Region of Ghana. Geological and Hydrogeological Settings Ahafo Region, showing the locations of the pro- Fig. 2 shows the Voltaian sub-provinces in ject towns.