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Hydrogeology and aquifer simulation of the basement rocks of the Kaduna-Zaria area, northern Nigeria
Hydrogeology and aquifer simulation of the basement rocks of the Kaduna-Zaria area, northern Nigeria E. A. Adanu
Department of Geology, Ahmadu Bello University, Zaria, Nigeria M. Schneider
Engineering Consultants, Stettiner Str. 53, D-IO000 Berlin 65, FRG The study area is situated in the central northern part of Nigeria, which experiences a typical semiarid climate. The aquifer consists of weathered and fractured basement rocks, mainly covered by lateritic soil. For rural water supply purposes, more than 40 wells were drilled in an area of about 2500 km 2. Field checked interpretation of pumping tests, geophysical data and soil investigations lead to a fairly precise description of the hydrogeologic situation in the study area. By application of a simplified steady-state groundwater flow model conducted on microcomputer-based systems, recharge conditions were simulated. INTRODUCTION The Kaduna State Water Board is actively carrying out exploration studies in many villages as part of the Groundwater Development Program in the state. The project area which is described in this paper is located in the centre part of Kaduna State between Zaria in the North and Kaduna in the South (Fig. 1). More than 40 deep wells were drilled which are not yet in use (Dec. 87). Up to now groundwater is mainly used for domestic water supply and almost exclusively abstracted by handpumps and shallow wells. Existing hydrogeologic data indicate that the groundwater resources are very limited in this area. For that reason detailed investigations are necessary in order to develop optimal management strategies. GEOLOGY AND G E O M O R P H O L P G Y
inselbergs and domes of resistant basement rocks. A combination of topography and geology more or less control the groundwater occurrence in this area. Similar observation was noted by Chilton and Smith-Carington 2 working in Malawi. 6"
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7" Paper presented at the 1st International Conference in Africa on Computer Methods and Water Resources, Morocco, March 8-1 l, 1988 © 1988 Computational Mechanics Publications
44
Adv. Water Resources, 1988, Volume 11, March
'13'
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;, %
', STATE
12"
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......
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A summary of the geology of the area was given by McCurry 1. The area is underlain by crystalline metamorphic and igneous rocks of Precambrian to lower Paleozoic age. A major part of the rocks is of high-grade metamorphism, mainly gneisses, which suffered intense folding and granitisation and have remained stable for many millions of years. Sediments of late Proterozoic age are found folded and metamorphosed mainly into schists. Intrusive .rocks of late Proterozoic to Cambrian age including synorogenic to lateorogenic granites, minor basic and ultra-basic rock dykes and pegmatites are also found. Fig. 2 shows the geological map of the project area. Prolonged weathering under tropical condition has produced a characteristic topography of peneplain and
9"
Fig. I.
Project area
8
9"
Hydrogeology and aquifer simulation." E. A. Adanu and M. Schneider tt'oo'
30 ,
/,5 ,
:* h :
.
the gross topographic slope extending from the surface divide to the stream. Almost all recharge and discharge are through porous granular material, but some intermediate flow may be in bedrock openings.
tl"oo'
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20"
4"5'
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EXPLANATION []
Medium-coarsegrained granite
[]
Schists
• []
Granitegneiss
[]
Migmatisedbiotitehornblende gneiss
[]
Fine grained biotitehornblende granite
]
Coarse porphyriticbiotitehornblende granite
I ~ Quartz diorite and granodiorite Geological boundary inferred Fault-inferred Fig. 2.
Geological map (Adanu 1987)
HYDROGEOLOGY Fractured-rock regions always have a wide range of hydrologic conditions and a variety of hydrologic related problems. This always poses questions of the proper method to use in studying and solving the problems. An approach similar to that used by LeGrand 3 is used here, which involves identifying a region or setting that has some distinctive characteristics and then developing a conceptual model to which selected specific information can be used. The following are valid statements on this area: []
[]
[] [] []
Igneous and metamorphic rock underlie this area; the overlaying weathered mantle varies in thickness from zero to as much as 60 m. The climate is tropical semiarid; average annual rainfall is about 900mm which is concentrated between April and November; actual evapotranspiration has an average annual value of about 650 mm. Groundwater recharge is seasonal and by the rainfall. There is a natural groundwater discharge through boreholes and large diameter hand-dug wells. The path of groundwater movement is relatively short and is almost restricted to the zone underlaying
Occurrence of groundwater The study area is characterized by a layer of unconsolidated to semiconsolidated material produced by in-situ weathering of the crystalline rock. The degree and depth of weathering depends on tectonic activities, climate and topography. The intensity of alteration decreases progressively downwards until fresh unweathered rock is reached (Fig. 3). This weathered profile is of greater importance as a source for rural domestic water supply than the underlying unweathered but occasionally fractured bed rock. The weathered profile rarely exceeds 50 m in thickness except in valleys and in areas where faults and fractures have permitted the weathering activities to penetrate more freely. The zone of intergranular porosity and permeability usually occurs between 15 and 60m below ground surface. Jones 4 reported this zone to be between 5 and 10 m in thickness but values up to 20 m are sometimes found here. Groundwater levels are generally high. Available well-data show the levels to be usually less than 8 m below ground surface. Along valleys the values are lower (3-5 m) and these levels generally often reflect the topography in a rather suppressed manner 4. Aquifer properties Borehole yields in weathered basement aquifers are generally low. Yields exceeding 21/sec were not obtained here. The lowest yield was 0.21/sec while values between 0.3-0.5 l/sec are common. The highest specific capacity value obtained was 0.39 l/sec/m while the lowest value was 0.015 l/sec/m. Because of the heterogeneous nature of the aquifer standard analyses of pumping test data are usually not very reliable. Data interpretation is complicated in some cases by gravity drainage as the aquifer passes from a semiconfined to unconfined conditions and by the decrease in the saturated thickness which causes a fall in transmissivity as the aquifer is
0
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•
20
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•
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. ~
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- = =- :
'0°.
i e
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°
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son
•
°
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o .
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. •
',
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•
.
.
• •
°
°
o ,
•
.
~m TIT]] Latertte P#o~IA Ilu vium
~z~ Clayey alterite Granular alter/re (D, drilled well
Fig. 3.
Broken bedro~,k ~Unweathered bedrock H, hand dug well )
Aquifer cross-section
Adv. Water Resources, 1988, Volume 11, March
45
Hydrogeology and aquifer simulation: E. A. Adanu and M. Schneider GROUNDWATER MODELLING
100
4m2/d
t 3_E
E t~
T = 1.6m2/d 1
. . . . . . . .
~o
. . . . . . . .
~oo
. . . .
t (min)
Fig. 4.
'
1000
Time-drawdown curves ( Theis" method)
dewatered 2. Transmissivity values obtained using Theis curve matching method (Fig. 4) varies from less than l m2/d to 40m2/d. Aquifer recharge occurs usually seasonally through rainfall, which infiltrates through the weathered body of the rock and fractures.
Groundwater budget For the study area a simplified water balance was established. Assuming no change in groundwater storage, soil moisture content and surface water retention, the water balance equations becomes P=Q+E+G where P is the precipitation, Q the runoff, E the evapotranspiration and G the groundwater recharge. According to long-term rainfall and evaporation data the mean annual rainfall amounts to 943 mm and the mean annual actual evapotranspiration is calculated to be 636 mm. Detailed information on surface runoff in the study area is not available. Within the modelled area with a dimension of 2472 km 2 the inflow (as precipitation) was calculated to be in the order of 74m3/sec. The total outflow from the area is composed of four parts: 1. Losses by evaporation and transpiration; about 50 m3/sec on the basis of the above mentioned data. 2. Water that has moved only over the land surface; ovedand runoff. 3. Water that has int'dtrated into the ground and moved through the aquifer and out along stream channels; groundwater discharge. 4. Abstraction from boreholes and local hand dug wells; artificial discharge. Overland runoff and groundwater discharge are components of the streamflow. Baseflow-analysis of discharge records from a stream-gaging station at the river Tubo gives a value for groundwater recharge of about 20m3/sec (250mm/year) in the study area. Although this value, which is about 26 % of rainfall, seems to be too high, but as first rough estimates it can be used as an initial value for model calibration.
46
Adv. Water Resources, 1988, Volume 11, March
General remarks Without doubt, groundwater modelling today is a very important tool in the management of groundwater resources. With growing groundwater development for drinking water supply and irrigation purposes parallel with growing environmental problems the use of numerical methods in order to simulate different scenarios of groundwater management are of great importance. A numerical model can also be applied in areas with limited hydrogeologic information. In this case even with simple models indentification of missing data, e.g., groundwater recharge conditions, are possible. Today the concept of groundwater flow is well understood and in the last years more and more sophisticated groundwater flow models have been developed. However, for a long time mainframe computers were necessary to run the corresponding programs. The application of groundwater modelling therefore was limited because of the availability of computer facilities and computer experts and financial constraints. With the revolutionary advances in microprocessortechnology during the last decade, low-cost microcomputers are now available with sufficient performance and storage capacity to process even very large data bases. Over the last ten years microprocessor performance has doubled one to two years parallel with a general decrease of computer cost. At the same time also the space required for a certain computing power became smaller that lead to the term 'desktop computer'. Comfortable and user-friendly handling, even for noncomputer experts, is possible by application of modem software products and intuitive user interfaces based on a 'mouse' and pull-down menues. Within the frame of this study it is the objective to test the performance and efficiency of modem microcomputers in the field of groundwater modelling. Therefore it seemed to be convenient to start with simplified boundary conditions. First results are presented.
Hard- and software For regional groundwater flow modelling in the study area it was decided to gather first experiences with microcomputers by using a personal computer with a MOTOROLA 68.000 cpu. Graphic output was executed on a 300 dpi laserprinter. The applied programs were originally written in APPLESOFT BASIC 5, transferred to MICROSOFT BASIC (interpreter) and to PASCAL. The results, which are graphically presented in form of contour lines on the screen are worked up by using interactive graphic software. Model structure and parameters For the solution of groundwater flow problems the finite element method has been applied. The procedure is divided into three parts: input, the actual calculations and output. The input program operates interactively, asking for the required data and stores the dataset on disk. With a separate program the instationary flow equation is solved by the Galerkin method, using the Gauss-Seidel
Hydrogeology and aquifer simulation: E. A. Adanu and M. Schneider state calibration is shown. The effect of recent groundwater abstraction by handpumps for rural domestic water supply is neglected. Under the simplified conditions described above, the calibration was conducted by trial and error. After several steady-state calculations based on diffuse recharge over the entire area, the groundwater recharge is suggested to be in the order of 4 m m / y e a r . More detailed work is necessary to specify the input-data for each nodal area, respectively nodal point. The computations performed with BASIC (interpreter) turned out to be rather slow. Using PASCAL the calculations speed up by the factor 12. For higher performance it is suggested to use an arithmetic coprocessor. CONCLUSIONS
• --620--
------. . . .
Borehole
|
5 km
A !
N
G r o u n d w a t e r level c o n t o u r s (in m a b o v e M S . L . ) Prescribed head b o u n d a r y
Fig. 5. Aquifer configuration
network
and
reference piezometric
algorithm. The same program can be used for stationary flow calculation by setting all storage coefficients to zero and by equating timestep with time level. A special output program is used to present the results of the calculations in graphical form as isolines on the screen. The modelled area, shown in Fig. 5, is divided into a rectangular grid with irregular spacing and each rectanagular cell diagonally into two triangles (560 elements, 315 nodes). The western, eastern and southern boundaries of the area are formed by the rivers Tubo, G a l m a and Kaduna, discharging the aquifer. These boundaries are designed as prescribed head boundaries (first kind boundary condition). In the area of river G a l m a the groundwater level may drop slightly below the river floor. The northern margin of the modelled domain is formed by a stream line which is treated as an impervious boundary where the flux is zero (second kind boundary condition). As a first statement an average value for transmissivity of T = 2 x 10-4m2/sec for each element was chosen and the storage coefficient was set to S = 4 x 10- 3.
Steady-state model calibration Before the numerical model can be applied to simulate different management schemes, unknown or uncertain parameters have to be determined by calibration. This is particularly the case for groundwater recharge. In Fig. 5 the reference piezometric configuration utilized for steady
In the study area groundwater occurs in the weathered basement-zone. In the described geologica formations porosity and permeability are usually greatly reduced which is confirmed by the pumping test data. Diffuse groundwater recharge through the weathered zone exists at a low rate. The aquifer is also recharged through fractures but according to geologic and tectonic studies it is assumed that this mechanism is not dominant. The use of microcomputer-based systems as an additional tool in the fields of hydrogeologic data base management and groundwater flow modelling turned out to be very helpful. The advantages are their extremely good cost/benefit ratio. Benefit in this sense means flexibility, sufficient performance, storage capacity and graphic capability and the availability of user-friendly software packages. ACKNOWLEDGEMENTS The authors wish to thank the Special Research Project Arid Areas (Technical University of Berlin) and the German Academic Exchange Service for their logistic and financial support. REFERENCES 1 McCurry,P. Geologyof degree sheet 21 Zaria, Nigeria - Institute of Geological Sciences, Overseas Geology and Mineral Resources, no. 45, HMSO, 1973 2 Chilton. P. J. and Smith-Carington, A. K. Characteristics of the weathered basement aquifer in Malawi in relation to rural water supply. In Proceedings of the Harare Symp., IAHS public no. 144, WaUingford, 1984, pp. 57-72 3 LeGrand, H. Evaluation techniques of fractured-rock hydrology, Journal of Hydrology, 1979, 43, 333-346 4 Jones,M. J. The weathered zone aquifers of the Basement Complex areas of Africa, Q. J. En#. Geology, 1985, 18(1), 35--46 5 Kinzelback, W. Groundwater Modelling - An Introduction with Sample Programs in BASIC, Elsvier, Amsterdam, 1986, 333 pp.
BIBLIOGRAPHY Adanu, E. A. Some hydrogeophysical characteristics of the shallow basement aquifer in the Zaria-Kaduna area of Nigeria. In Current research in African Earth Sciences, (Eds Matheis and Schanddmeier), Pub. Balkena, Rotterdam, 1987, pp. 451-454
Adv. Water Resources, 1988, Volume 11, March
47
Department of Geology, Ahmadu Bello University, Zaria, Nigeria M. Schneider
Engineering Consultants, Stettiner Str. 53, D-IO000 Berlin 65, FRG The study area is situated in the central northern part of Nigeria, which experiences a typical semiarid climate. The aquifer consists of weathered and fractured basement rocks, mainly covered by lateritic soil. For rural water supply purposes, more than 40 wells were drilled in an area of about 2500 km 2. Field checked interpretation of pumping tests, geophysical data and soil investigations lead to a fairly precise description of the hydrogeologic situation in the study area. By application of a simplified steady-state groundwater flow model conducted on microcomputer-based systems, recharge conditions were simulated. INTRODUCTION The Kaduna State Water Board is actively carrying out exploration studies in many villages as part of the Groundwater Development Program in the state. The project area which is described in this paper is located in the centre part of Kaduna State between Zaria in the North and Kaduna in the South (Fig. 1). More than 40 deep wells were drilled which are not yet in use (Dec. 87). Up to now groundwater is mainly used for domestic water supply and almost exclusively abstracted by handpumps and shallow wells. Existing hydrogeologic data indicate that the groundwater resources are very limited in this area. For that reason detailed investigations are necessary in order to develop optimal management strategies. GEOLOGY AND G E O M O R P H O L P G Y
inselbergs and domes of resistant basement rocks. A combination of topography and geology more or less control the groundwater occurrence in this area. Similar observation was noted by Chilton and Smith-Carington 2 working in Malawi. 6"
7"
~"
" ',
K A I 2 ~. 2 N A
i)
~¢
. o
',
,i
~2"
i
) ) .° /
• ZARIA
..
:"
,,
;
,,
,
i
~1°
i J
. .'
";
I~ A D U N A ,"
t
10'
L.., i i t i
SO
n
50km
~j %.
.
/
,° i
/
".
)
9
f
.-...k-......
7" Paper presented at the 1st International Conference in Africa on Computer Methods and Water Resources, Morocco, March 8-1 l, 1988 © 1988 Computational Mechanics Publications
44
Adv. Water Resources, 1988, Volume 11, March
'13'
"" -
;, %
', STATE
12"
i?."'"
......
),-'"
A summary of the geology of the area was given by McCurry 1. The area is underlain by crystalline metamorphic and igneous rocks of Precambrian to lower Paleozoic age. A major part of the rocks is of high-grade metamorphism, mainly gneisses, which suffered intense folding and granitisation and have remained stable for many millions of years. Sediments of late Proterozoic age are found folded and metamorphosed mainly into schists. Intrusive .rocks of late Proterozoic to Cambrian age including synorogenic to lateorogenic granites, minor basic and ultra-basic rock dykes and pegmatites are also found. Fig. 2 shows the geological map of the project area. Prolonged weathering under tropical condition has produced a characteristic topography of peneplain and
9"
Fig. I.
Project area
8
9"
Hydrogeology and aquifer simulation." E. A. Adanu and M. Schneider tt'oo'
30 ,
/,5 ,
:* h :
.
the gross topographic slope extending from the surface divide to the stream. Almost all recharge and discharge are through porous granular material, but some intermediate flow may be in bedrock openings.
tl"oo'
,o
[] x x
~"
~: ~ x x
x
~"x ~ x
"::'-.
~.~
::"
:::'::':. ,~i~2~
" • ". " • " .
.
.
.
.
•
1
a" 3 ~t~ -
.
. ~'.~
d
•
,o
...;..;-..,.-.
.
.
.
.
.
o
.
.
.
::-':
7"15"
20"
4"5'
7"50'
EXPLANATION []
Medium-coarsegrained granite
[]
Schists
• []
Granitegneiss
[]
Migmatisedbiotitehornblende gneiss
[]
Fine grained biotitehornblende granite
]
Coarse porphyriticbiotitehornblende granite
I ~ Quartz diorite and granodiorite Geological boundary inferred Fault-inferred Fig. 2.
Geological map (Adanu 1987)
HYDROGEOLOGY Fractured-rock regions always have a wide range of hydrologic conditions and a variety of hydrologic related problems. This always poses questions of the proper method to use in studying and solving the problems. An approach similar to that used by LeGrand 3 is used here, which involves identifying a region or setting that has some distinctive characteristics and then developing a conceptual model to which selected specific information can be used. The following are valid statements on this area: []
[]
[] [] []
Igneous and metamorphic rock underlie this area; the overlaying weathered mantle varies in thickness from zero to as much as 60 m. The climate is tropical semiarid; average annual rainfall is about 900mm which is concentrated between April and November; actual evapotranspiration has an average annual value of about 650 mm. Groundwater recharge is seasonal and by the rainfall. There is a natural groundwater discharge through boreholes and large diameter hand-dug wells. The path of groundwater movement is relatively short and is almost restricted to the zone underlaying
Occurrence of groundwater The study area is characterized by a layer of unconsolidated to semiconsolidated material produced by in-situ weathering of the crystalline rock. The degree and depth of weathering depends on tectonic activities, climate and topography. The intensity of alteration decreases progressively downwards until fresh unweathered rock is reached (Fig. 3). This weathered profile is of greater importance as a source for rural domestic water supply than the underlying unweathered but occasionally fractured bed rock. The weathered profile rarely exceeds 50 m in thickness except in valleys and in areas where faults and fractures have permitted the weathering activities to penetrate more freely. The zone of intergranular porosity and permeability usually occurs between 15 and 60m below ground surface. Jones 4 reported this zone to be between 5 and 10 m in thickness but values up to 20 m are sometimes found here. Groundwater levels are generally high. Available well-data show the levels to be usually less than 8 m below ground surface. Along valleys the values are lower (3-5 m) and these levels generally often reflect the topography in a rather suppressed manner 4. Aquifer properties Borehole yields in weathered basement aquifers are generally low. Yields exceeding 21/sec were not obtained here. The lowest yield was 0.21/sec while values between 0.3-0.5 l/sec are common. The highest specific capacity value obtained was 0.39 l/sec/m while the lowest value was 0.015 l/sec/m. Because of the heterogeneous nature of the aquifer standard analyses of pumping test data are usually not very reliable. Data interpretation is complicated in some cases by gravity drainage as the aquifer passes from a semiconfined to unconfined conditions and by the decrease in the saturated thickness which causes a fall in transmissivity as the aquifer is
0
tO
•
20
DD •
•
~mo
. ~
DDO~B ~
- = =- :
'0°.
i e
=-"
30
~o
•
• ° ° °
°
.*° k°°.°
-
son
•
°
•
o .
•
°
°°
,
". •
•
w ,
. •
',
•"
•
.
.
• •
°
°
o ,
•
.
~m TIT]] Latertte P#o~IA Ilu vium
~z~ Clayey alterite Granular alter/re (D, drilled well
Fig. 3.
Broken bedro~,k ~Unweathered bedrock H, hand dug well )
Aquifer cross-section
Adv. Water Resources, 1988, Volume 11, March
45
Hydrogeology and aquifer simulation: E. A. Adanu and M. Schneider GROUNDWATER MODELLING
100
4m2/d
t 3_E
E t~
T = 1.6m2/d 1
. . . . . . . .
~o
. . . . . . . .
~oo
. . . .
t (min)
Fig. 4.
'
1000
Time-drawdown curves ( Theis" method)
dewatered 2. Transmissivity values obtained using Theis curve matching method (Fig. 4) varies from less than l m2/d to 40m2/d. Aquifer recharge occurs usually seasonally through rainfall, which infiltrates through the weathered body of the rock and fractures.
Groundwater budget For the study area a simplified water balance was established. Assuming no change in groundwater storage, soil moisture content and surface water retention, the water balance equations becomes P=Q+E+G where P is the precipitation, Q the runoff, E the evapotranspiration and G the groundwater recharge. According to long-term rainfall and evaporation data the mean annual rainfall amounts to 943 mm and the mean annual actual evapotranspiration is calculated to be 636 mm. Detailed information on surface runoff in the study area is not available. Within the modelled area with a dimension of 2472 km 2 the inflow (as precipitation) was calculated to be in the order of 74m3/sec. The total outflow from the area is composed of four parts: 1. Losses by evaporation and transpiration; about 50 m3/sec on the basis of the above mentioned data. 2. Water that has moved only over the land surface; ovedand runoff. 3. Water that has int'dtrated into the ground and moved through the aquifer and out along stream channels; groundwater discharge. 4. Abstraction from boreholes and local hand dug wells; artificial discharge. Overland runoff and groundwater discharge are components of the streamflow. Baseflow-analysis of discharge records from a stream-gaging station at the river Tubo gives a value for groundwater recharge of about 20m3/sec (250mm/year) in the study area. Although this value, which is about 26 % of rainfall, seems to be too high, but as first rough estimates it can be used as an initial value for model calibration.
46
Adv. Water Resources, 1988, Volume 11, March
General remarks Without doubt, groundwater modelling today is a very important tool in the management of groundwater resources. With growing groundwater development for drinking water supply and irrigation purposes parallel with growing environmental problems the use of numerical methods in order to simulate different scenarios of groundwater management are of great importance. A numerical model can also be applied in areas with limited hydrogeologic information. In this case even with simple models indentification of missing data, e.g., groundwater recharge conditions, are possible. Today the concept of groundwater flow is well understood and in the last years more and more sophisticated groundwater flow models have been developed. However, for a long time mainframe computers were necessary to run the corresponding programs. The application of groundwater modelling therefore was limited because of the availability of computer facilities and computer experts and financial constraints. With the revolutionary advances in microprocessortechnology during the last decade, low-cost microcomputers are now available with sufficient performance and storage capacity to process even very large data bases. Over the last ten years microprocessor performance has doubled one to two years parallel with a general decrease of computer cost. At the same time also the space required for a certain computing power became smaller that lead to the term 'desktop computer'. Comfortable and user-friendly handling, even for noncomputer experts, is possible by application of modem software products and intuitive user interfaces based on a 'mouse' and pull-down menues. Within the frame of this study it is the objective to test the performance and efficiency of modem microcomputers in the field of groundwater modelling. Therefore it seemed to be convenient to start with simplified boundary conditions. First results are presented.
Hard- and software For regional groundwater flow modelling in the study area it was decided to gather first experiences with microcomputers by using a personal computer with a MOTOROLA 68.000 cpu. Graphic output was executed on a 300 dpi laserprinter. The applied programs were originally written in APPLESOFT BASIC 5, transferred to MICROSOFT BASIC (interpreter) and to PASCAL. The results, which are graphically presented in form of contour lines on the screen are worked up by using interactive graphic software. Model structure and parameters For the solution of groundwater flow problems the finite element method has been applied. The procedure is divided into three parts: input, the actual calculations and output. The input program operates interactively, asking for the required data and stores the dataset on disk. With a separate program the instationary flow equation is solved by the Galerkin method, using the Gauss-Seidel
Hydrogeology and aquifer simulation: E. A. Adanu and M. Schneider state calibration is shown. The effect of recent groundwater abstraction by handpumps for rural domestic water supply is neglected. Under the simplified conditions described above, the calibration was conducted by trial and error. After several steady-state calculations based on diffuse recharge over the entire area, the groundwater recharge is suggested to be in the order of 4 m m / y e a r . More detailed work is necessary to specify the input-data for each nodal area, respectively nodal point. The computations performed with BASIC (interpreter) turned out to be rather slow. Using PASCAL the calculations speed up by the factor 12. For higher performance it is suggested to use an arithmetic coprocessor. CONCLUSIONS
• --620--
------. . . .
Borehole
|
5 km
A !
N
G r o u n d w a t e r level c o n t o u r s (in m a b o v e M S . L . ) Prescribed head b o u n d a r y
Fig. 5. Aquifer configuration
network
and
reference piezometric
algorithm. The same program can be used for stationary flow calculation by setting all storage coefficients to zero and by equating timestep with time level. A special output program is used to present the results of the calculations in graphical form as isolines on the screen. The modelled area, shown in Fig. 5, is divided into a rectangular grid with irregular spacing and each rectanagular cell diagonally into two triangles (560 elements, 315 nodes). The western, eastern and southern boundaries of the area are formed by the rivers Tubo, G a l m a and Kaduna, discharging the aquifer. These boundaries are designed as prescribed head boundaries (first kind boundary condition). In the area of river G a l m a the groundwater level may drop slightly below the river floor. The northern margin of the modelled domain is formed by a stream line which is treated as an impervious boundary where the flux is zero (second kind boundary condition). As a first statement an average value for transmissivity of T = 2 x 10-4m2/sec for each element was chosen and the storage coefficient was set to S = 4 x 10- 3.
Steady-state model calibration Before the numerical model can be applied to simulate different management schemes, unknown or uncertain parameters have to be determined by calibration. This is particularly the case for groundwater recharge. In Fig. 5 the reference piezometric configuration utilized for steady
In the study area groundwater occurs in the weathered basement-zone. In the described geologica formations porosity and permeability are usually greatly reduced which is confirmed by the pumping test data. Diffuse groundwater recharge through the weathered zone exists at a low rate. The aquifer is also recharged through fractures but according to geologic and tectonic studies it is assumed that this mechanism is not dominant. The use of microcomputer-based systems as an additional tool in the fields of hydrogeologic data base management and groundwater flow modelling turned out to be very helpful. The advantages are their extremely good cost/benefit ratio. Benefit in this sense means flexibility, sufficient performance, storage capacity and graphic capability and the availability of user-friendly software packages. ACKNOWLEDGEMENTS The authors wish to thank the Special Research Project Arid Areas (Technical University of Berlin) and the German Academic Exchange Service for their logistic and financial support. REFERENCES 1 McCurry,P. Geologyof degree sheet 21 Zaria, Nigeria - Institute of Geological Sciences, Overseas Geology and Mineral Resources, no. 45, HMSO, 1973 2 Chilton. P. J. and Smith-Carington, A. K. Characteristics of the weathered basement aquifer in Malawi in relation to rural water supply. In Proceedings of the Harare Symp., IAHS public no. 144, WaUingford, 1984, pp. 57-72 3 LeGrand, H. Evaluation techniques of fractured-rock hydrology, Journal of Hydrology, 1979, 43, 333-346 4 Jones,M. J. The weathered zone aquifers of the Basement Complex areas of Africa, Q. J. En#. Geology, 1985, 18(1), 35--46 5 Kinzelback, W. Groundwater Modelling - An Introduction with Sample Programs in BASIC, Elsvier, Amsterdam, 1986, 333 pp.
BIBLIOGRAPHY Adanu, E. A. Some hydrogeophysical characteristics of the shallow basement aquifer in the Zaria-Kaduna area of Nigeria. In Current research in African Earth Sciences, (Eds Matheis and Schanddmeier), Pub. Balkena, Rotterdam, 1987, pp. 451-454
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