Geophysical, hydrochemical and hydrogeological investigations were
undenaken to evaluate the groundwater prospects in the study area. The factors which
affect/determine the spatial development of ground water in the area was also examined.
The results show that the essential conditions for groundwater availability in hard rock
terrains are satisfied. The average weathering depth is 34 m, rainfall is high and seasonal
and the geological evolutionary sequence included tectonic phases during which suitable
geological structures are expected to have been developed. Analysis of borehole drilling
records revealed that each borehole encountered at least one water strike. The clay
mineral stable in the prevailing physico-chemical conditions is kaolinite, an evidence that
weathering sites are being flushed presumably by flowing groundwater. Available
results of isotope analysis also show that the stable isotope concentration in the
groundwater is similar to that of current meteoric water. It is therefore concluded that the
ground water is receiving current recharge. Infact water balance calculations and results
of baseflow analysis indicate that this recharge is substantial.
The yields of the boreholes varied greatly spatially ranging between 0.7 and 10.9
I/s. This may be low compared with values quoted for sedimentary rock terrains, but in
view of the present low level of water supply in the area, it is considered that exploitation
of this resource would represent a significant improvement on the present situation.
A physical catchment hydrogeological model is presented to explain the observed
spatial variation in yields. In this model, a threshold elevation is considered to exist in
each catchment. At elevations below this threshold, borehole yield is sustained entirely
by fracture flow. At elevations above this threshold, borehole yield is sustained by
storage in the weathered rocks.
Pumping tests on large diameter wells show that groundwater abstration using
these wells would be greatly improved by repeated pumping after every 60 - 80 minutes
recovery. An equation is presented for calculating the number of times a well can be
repeatedly pumped in an operating day.
The drawdown of the boreholes contained a large well loss component. This well
loss was incurred during the early stages of pumping when water had to be withdrawn
from well storage to supplement the aquifer flow in order to meet the pumping capacity.
Field results presented indicated that the drawdown can be minimised by stepping the
discharge rather than imposing it all at once at the onset of pumping.
Field results indicate that the use of well water levels for mapping the water table
may not be valid in weathered hard rocks.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University of Technology.