Environmental Engineering Reference
In-Depth Information
respect to TSS. The concentrations of both parameters are lower than reported EMCs for
residential and commercial areas by Choe et al. (2002). TDS concentrations varied
between 200 and 1000 mg/l with pronounced high values and high standard error at SP2.
TDS concentrations at SP1, SP3 and SP4 are within the safe limit, but at SP2 they present
medium ER. PH values were within the safe limits at all points.
Metals variations (Fig. 4.2c) show that Cu, Zn and Ni concentrations are within the
safe limit. Cd concentrations present low ER, while Pb concentrations present low to
medium ER. Previous research (Mvungi et al. 2003) shows the same trend but Pb
concentrations of runoff in high-density suburbs of Harare were two to three times higher
than the ones measured during this study. Pb and Cd concentrations evaluated in this
study are much higher than the reported EMC of 0.008 mg Cd/l and 0.022 mg Pb/l in
California for similar land use practice (Buffleben et al. 2002).
Nutrients variations (Fig. 4.2b) show that nitrates are within the safe prescribed limit,
ammonia concentrations present low ER, and TP concentrations present low to medium
ER. Comparison with background pollution values in the basin (Hranova et.al. 2002)
show that ammonia concentrations in the runoff are 10 to 20 times higher, nitrates and TP
- up to 10 times higher than the background values.
In addition, mean concentrations of ammonia and TP in the runoff are comparable and
even higher than the same parameters of treated effluents in the two major wastewater
treatment facilities of the City (Hranova et.al. 2002). Choe et al. 2002 has reported
similar EMC regarding TP concentrations for residential, commercial and industrial
areas.
Comparison with previous results from the city of Harare monitoring program
(Chapter 3, Figs. 3.5, 3.6) could be done regarding points SP1 and SP3, as these sampling
locations coincide with the ones referred in Chapter 3 as DR2 and DR1 respectively. The
type of land use practice has not changed considerably since the year 2000. However, the
different approaches and methodologies applied to obtain these results, makes such a
comparison ambiguous. The results in Chapter 3 are mean values of samples collected
throughout the year at different time intervals, and they reflect the flow quality during
both - the dry and the wet seasons. Both channels have shown to have perennial flow
throughout the year. Correspondingly, these results in Chapter 3 represent also the base
flow quality, which might be due to ground water infiltration or to illicit discharges. The
results obtained during this study (Fig. 4.2), present the storm water quality during or
immediately after rainfall events, correspondingly they could be accepted as
representative of runoff quality.
Considering these differences, as well as the different methodologies applied to test
the samples, the comparison should reflect the range of variation of the selected
parameters, but not the specific values. Considering the CBD (SP1/DR2), the phosphorus
variations during this study are comparable and lower than the ones presented in Chapter
3, which could indicate that no substantial increase in these levels have been found.
Ammonia concentrations at SP1 vary between 0.5 mg/l and 1 mg/l, which is considerably
higher compared to median values observed during the period 1995-2000. With respect to
the Coventry Rd. channel (SP3/DR1) phosphorus results are within the same range of
variation, while ammonia show a considerable increase. For both locations nitrates
measured during this study are about 10 times higher than the monitoring data, which
could be associated with a systematic error during the testing procedure.
