Analysis of pumping tests data of crystalline rock aquifers (a case study of wells at Ashanti Goldfields, Bibiani)
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Date
2001-12-12
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Abstract
The Ashanti Goldfields (Bibiani) Limited is a gold mining company in the BibianiAnhwiaso District of the Western Region of Ghana. Crystalline metamorphic and igneous rocks crop out in the area. As part of its environmental management plan, the company has drilled over twenty boreholes in this terrain. The purpose is to be used to monitor groundwater contamination and as domestic and plant water supply sources.
To address the issue of sustainable yields (since it depends on only groundwater), nine of the wells were pump-tested to determine aquifer characteristics and hence the productivity of these wells.
Step-drawdown test was conducted on two of the wells, (B1 and B3). Constant discharge and recovery tests were conducted on all the nine wells.
Step-drawdown test was analysed by Jacob (1947) and Rorabaugh (1953) methods to determine well losses and aquifer losses, and hence the efficiency of the wells. Well losses were significant. Well loss determined with the use of the Jacob’s method was 8.70 in B1 and 5.78 in BI but aquifer loss was 7.71 in B] and 9.71 in B3. The efficiencies were 49.3% and 63% for B1 and B3 respectively.
Constant discharge was analysed by Cooper-Jacob (1946) straight-line method. The transmissivity (pumping) for the three wells that were pump-tested for 24 hours ranges between 2.985 and 11 .69m2/day with a mean of 7 .649 m2/day. The transmissivity (recovery) ranges between 5.51 and 10.35 m2/day whereas specific capacity (Q/s) ranges between 7.06 and 16.90 m2/day with a mean value of 12 .12 m2/day.
Similarly, the other six wells were pump-tested for duration of 4 hours (240mins). The transmissivity (pumping) ranges between 2.06 and 16.90m2/day with a mean value of 6.80 m2/day. The transrnissivity (recovery) ranges between 2.18 and 12.03 m2/day with a mean value of 6.80 m2/day, whereas specific capacity (Q/s) ranges between 4.22 and 26.35 m2/day with a mean value of 11.11 m2/day.
Logarithmic transformations of specific capacities and transmissivities were correlated positively. Hence productivity which is related to specific capacity is also related to transmissivity. On the basis of the mean values of Q/s and T, well B3 is the most productive and B2 is the least productive, considering the three wells that were pumped for 24 hours. In the same vein, well B6 is the most productive and B7 is the least productive for the six wells that were pump-tested for 4 hours.
Optimal yields were determined by using Jacob’s step-drawdown solution. The values for B1 and B3 were 350 m3/d and 372 m3fd respectively.
Sustainable yields for 7-day specific capacities were determined for B 1, B2 and B3. Well BI, B2 and B3 had values of 333.4, 116.8 and 356 m3/day respectively.
Description
A thesis submitted to the School of Graduate Studies,
Kwame Nkrumah University of Science and Technology in partial
fulfilment of the requirements for the award of Master of Science degree in Environmental Resources Management, 2001