Environmental Engineering Reference
In-Depth Information
exceed the safe limits in respect to ammonia (< 0.5 mg/l), TP (< 0.5 mg/l), Fe (< 1 mg/l)
and Pb (< 0.05 mg/l). The excessive mean concentration of Fe could be due to the metal
wastes dumped into the river, rather than agricultural activities or background pollution.
This suggestion is in agreement with the relatively high variability of the data set, where
individual sample concentrations varied from 0.1 mg/l to 11 mg/l, which could be
explained with washed metal particles in some of the samples.
The statistical significance test of the mean values between the control point and point
F showed positive results regarding COD, TKN, Cu, TP, Ni and ammonia. The rest of the
parameters were not significantly different due to the high standard deviations of the data
sets. The comparison of the water quality at point F, representing a natural stream
collecting runoff from low-income areas, and the runoff quality described in Chapter 4,
shows a lower level of pollution with respect to COD, the TP concentrations are
comparable, and much higher metal concentrations, which is an indication of a potential
source of toxic pollution from the drained areas, especially with respect to Pb.
Point C represents runoff quality from a drainage ditch in low-income areas with
mixed land use patterns. On certain occasions, the ditch collects overflows from blocked
sewer manholes. One of these occasions was witnessed during the recognisance visits.
The comparison with control point water quality, and with point F water quality, shows
higher pollutants concentrations with respect to pH, conductivity, COD, TKN, ammonia
and nitrates, which reflects a contamination typical for sewer discharges. The relatively
high values of pH are due to two specific sampling occasions, and could be associated
with the discharge of some waste products from the home industry area. The safe limit
concentrations (WWEDR 2000) are exceeded with respect to ammonia, TP, Fe and Zn.
FC were found at both points in selected samples, indicating the presence of faecal
pollution, most probably due to sewer blockages. The test for statistically significant
difference with the control point gave positive results only with respect to TP. This could
be explained by the low number of samples (due to low rainfall events, which generated
runoff), and the considerable variation of the measured parameters. The TP data set
showed a relatively lower variation, and this trend has been observed with respect to the
results presented in Chapter 4 also. At this specific point, the Pb concentrations were
lower compared to point F and comparable to the results presented in Chapter 4, but the
Zn and Ni values were higher than the ones measured in the runoff of the City's central
and industrial areas.
The results of the tests performed at points A and B have been discussed in the light of
the presence of the home industry area and its impact on the natural water quality
(Mvungi et.al. 2003). In general, they show the same trend in the water quality and
identify the same parameters as the main pollutant hazards. The pollutants concentrations
and pollutant loads were higher at point B, but no significant differentiation due to the
home industry site was identified. Point B reflects the total pollution load, including the
areas drained at C and F with a much higher runoff volume. In addition, the pollution
loads calculated, could not be refereed as contributed by the runoff only as they reflect
stream water quality and include the background pollution loads in terms of
concentrations, but exclude the base flow rate in the stream.
In general, the results obtained show that the stream water quality was affected
adversely by diffuse pollution sources. The major pollutant constituents of concern are
TP, ammonia, Fe, Pb and Zn.
