Environmental Engineering Reference
In-Depth Information
sampling procedure and flow rates calculated based on the Manning's equation
(Chadwick & Morfett 1993), assuming n = 0.014.
Laboratory analyses of water samples were performed at the water quality laboratory
of the Civil Engineering Department of the University of Zimbabwe, within 4 hours of
sampling. Laboratory analytical procedures were executed following the Standard
methods (1989). COD was determined by the open reflux method. Nitrates (NO 3 ), pH,
and ammonia (NH 4 ) were measured by selective electrodes (WTW pMX3000/ Ion
meter), and standard curves with respect to each parameter have been prepared before the
sample collection. TP was determined using the Vanadomolybdophosphoric acid
colorimetric method after nitric acid-sulphuric acid digestion, by a Spectronic 21D
spectrophotometer (wavelength of 450 nm). The metal concentrations were determined as
total metals at the laboratory of the Geology department of the University of Zimbabwe.
Samples were digested using the nitric acid-sulfuric acid digestion method. The analyses
were performed by a “Varian techtron spectra 50b” atomic absorption spectrometer,
employing an air-acetylene fuel.
Statistical data analysis was performed by the use of standard EXCEL statistical tools.
The evaluation of a statistically significant difference between two data sets, with respect
to each one of the measured parameters for the respective sampling point, was executed
by the application of the two-sample t-test, assuming unequal variances at 95%
confidence interval. Correlation and regression analyses between TSS and selected
parameters were executed by the regression analysis tool at 95% confidence level, based
on all measurements for the study period.
3.3 Water quality assessment and environmental risks
A comparison between historic data about monthly rainfall volumes for the period 1980-
2002 and the total monthly rainfall volume during the study period is shown in Table 4.1.
MAR value for the study period (840 mm) is close to the historic MAR value of 865 mm,
with lower rainfall volumes during the months of December and January and higher
values in March. Therefore the results obtained during this study could be assessed as
representative for an average wet season.
The evaluation of measured parameters (Fig 4.2) is based on a comparison of the mean
values obtained for the different sampling points with the standard effluent discharge
regulations in the country (WWEDR 2000). Fig. 4.2a shows variations of TS, TSS, TDS
and COD concentrations. COD varies from 100 to 200 mg/l, which presents medium to
high environmental risk (ER). The same risk has been evaluated in
Table 4.1. Characteristic monthly rainfall volumes
(mm) for the study area.
Sep
Oct
Nov
Dec
Jan Feb
March
April
May
June
Av (1980-2002)
2
40
80
180
210
190
120
30
15
0
Max (1980-2002)
80
170
250
420
480
470
300
170
200
10
This study
0
95
90
105
110
190
210
40
0
0
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