Environmental Engineering Reference
In-Depth Information
The need for the assessment of flow rate integrated samples and determination of site-
specific pollutant constituents' concentrations has been shown in the previous sections. In
addition, it would be necessary to determine these qualitative parameters with respect to
typical rainfall events. This would require the use of automatic sampling devices or
automatic monitoring stations, which could generate a massive database. Unfortunately,
automated monitoring stations could not provide information regarding parameters,
which are associated with particulate material. Usually, they are able to measure
dissolved constituents only, such as conductivity, pH and different types of ions. For this
reason, automatic sampling devices at selected locations are an attractive option, while
the testing of the necessary parameters could be done at laboratory conditions.
Toxic constituents, which are important in the determination of annual loads and
estimation of long term toxicity effects or shot term acute toxicity limits, are of
considerable importance as well. During this study, selected metals were tested as
representatives of this group. The specific characteristic of such types of parameters is
their presence at very low concentrations, usually in the micrograms/liter range. In order
to determine the impact of accuracy determination of such parameters on annual loads,
the procedure shown on Figure 4.5b was applied for a mean annual concentration of Cd -
0.05 mg/l as “true value”. Results show that for the same error percentage variation, the
annual loads varied from 10 to 50 t/year corresponding to a variation of Cd from 0.0167
mg/l to 0.083 mg/l. During the study period, a similar variation of the data set was
measured, however, with other metals the range of variation is much higher. This implies
that in the case of micro-pollutants, the accuracy of the testing method applied should be
much higher and a larger data set would be required in order to obtain reliable site-
specific values.
The determination of EMC for each specific land use pattern would require a
considerable effort and cost. For this reason, the monitoring program regarding urban
storm water quality could concentrate on the determination of EMC, associated with
suspended particles, and for the case of the city of Harare, these are the two channels at
SP1 and SP3. The evaluation of EMCs at these locations for a period long enough to
obtain statistically reliable data, together with an adequate record of rainfall data
corresponding to each measured EMC value, would allow to determine accurate site-
specific values and to link them to typical rainfall events with a corresponding return
period. Such information would allow the development of hydrographs representing
typical storms with different duration and intensity at the points under consideration, and
corresponding site-specific EMCs. A typical storm with given intensity, duration and
return period could have different EMCs, depending on the seasonal variation, e.g. if it
has occurred at the beginning, during, or at the end of the wet season. These variations
should also be detected during the monitoring process for each typical hydrograph. It
should be noted that this is a demanding investigation procedure and might be too
complex and costly to be performed by a regular monitoring program. Therefore, the
combination of data from regular monitoring programs, and data obtained by research
projects, having the specific objective to complement the monitoring program, could be a
viable option.
