Environmental Engineering Reference
In-Depth Information
Nine boreholes were drilled. A control borehole was sited outside the Epworth built-
up area, but within the same geological unit, which is the Harare Granite (Baldock et al.
1991). The boreholes were drilled
Table 6.2. Sample size and clusters per ward for
household questionnaire, Epworth.
Ward
Population Size (CSO, 2003)
Weight
Sample size
Clusters required
1
22,976
0.202
77
6
2
13,101
0.115
44
3
3
10,865
0.095
37
3
4
20,253
0.178
68
5
5
17,174
0.151
58
5
6
29,515
0.259
100
8
Total
113 884
1.000
384
30
using a Vonder rig and cased with 6-inch plastic PVC casing. Concrete was used to
secure the PVC casing and a lid made of PVC was used to cover the boreholes.
A sampling campaign was conducted: water samples were collected aseptically from
the nine boreholes, refrigerated with ordinary ice in transit and analyzed within 24 hours.
Coliform bacteria were determined by the membrane filtration method: a measured
volume of water was filtered through a membrane composed of cellulose esters. The pore
size was such that the organisms to be enumerated are retained on or near the surface of
the membrane, which was placed, face upwards, on a differential medium selective for
the indicator organism sought. Volumes were chosen so that the number of colonies to be
counted on the membrane would lie between 10 and 100. Membranes were incubated for
14 hours and at 37ºC to determine total coliforms and separate membranes were
incubated for 4 hours at 30ºC, and then for 14 hours at 44.5ºC. Shiny yellow colonies
were counted. The following formula was used: (Coliform colonies counted/ml of sample
filtered) ×100 and expressed as colony forming units (Clesceri et al. 1989).
Electrical conductivity, pH, temperature, chloride concentration and phosphate
concentration were measured in the field using a Horiba U 20 multi-meter. The
determination of cations was done at the University of Zimbabwe Geology Department
by atomic absorption spectrophotometer (AAS) (VARIAN SPECTRAA 200HT). A 21D
spectrophotometer was used for nitrate determination by the ultraviolet
spectrophotometric screening method at wavelengths of 220 nm and 275 nm on filtered
samples.
