Environmental Engineering Reference
In-Depth Information
corresponding data sets reflect more closely the true characteristic of the water quality in
both channels.
Despite the distinct seasonal rainfall pattern in the region, where no rainfall or
extremely rare rainfall occasions are recorded during the months of April to November,
water was flowing and samples were collected throughout the year at both sampling
locations. This indicates the presence of informal discharges into the drainage system or
drainage of infiltrated ground water. The most probable source of perennial flow in both
channels could be associated with informal discharges because the groundwater table in
both areas is relatively low during the dry season. Thus, a clarification needs to be done,
that the term “storm water quality”, used in this section as well as in Chapter 4, refers to
the flow in man-made urban drainage channels, which are not always associated with
direct surface runoff, but could convey wastewater from informal discharges as well.
Temporal variations of the nutrients concentrations in urban storm water show a
general trend of increase during the period. The ammonia concentrations at DR1 (Fig.
3.6) show very high mean values for the period 1996 to 1998, but the median values are
within or near the prescribed limit, except for 1998, when the median value exceeded it 3
times. Background pollution values, which are 0.05 mg/l of ammonia for wet season
conditions and 0.2 mg/l for dry season conditions (Hranova et al. 2002), have been
exceeded by the median values of DR1 throughout the study, which indicates pollution
regarding this parameter. Results regarding phosphate variations at DR1 with respect to
both-mean and median values exceed the prescribed limit of 0.5 mg/l, with peak mean
values during 1996 and 1997. However, it should be noted that the variability of the data
set for these two years is very high, resulting in high standard error values (Table 3.5).
Phosphates variation (median values) during the study period was between 0.6 and 1.7
mg/l, except for the year 1996, when a peak of 3.6 mg/l was found. Background pollution
values for the city of Harare area, regarding phosphorous, was reported to be ranging
between 0.1 and 0.4 mg/l with no seasonal variation (Hranova et al. 2002).
The annual median values for phosphate, observed during this study, exceed both
background pollution values and prescribed limits up to several times. Similar results
regarding high phosphate concentrations at this drainage channel have been reported as
well by Jarawaza (1997).
At DR2 (Fig. 3.6) ammonia variations show low values regarding this parameter for
the period 1995-1997. During 1998 to 2000, occasional high values were measured but
the median values are within
Table 3.5. Statistical characteristic of the data sets.
pH
Ammonia
Nitrate
Phosphate
DR1
DR2
DR1
DR2
DR1
DR2
DR1
DR2
1995
Standard Error
0.09
0.067
0.353
0.033
0.02
0.004
0.644 0.535
Count
12
12
12
12
12
12
12
12
1996
Standard Error
0.191
0.050
3.851
0.069
0.023
0.004
2.333 0.788
