Environmental Engineering Reference
In-Depth Information
Another, less expensive option in terms of monitoring efforts, which requires a
continuous flow rate measurement and ordinary approach to water quality monitoring,
could be applied with some compromise in terms of accuracy. It would consider the sum
of seasonal pollution loads, where the different seasons would be determined based on
the determined seasonal variation of EMC. In the case of the South African region, a
significant difference in seasonal EMC variations during the dry and wet season could be
expected. Within the wet season, differentiation could be made with respect to average
values of EMC during the starting period of rains, usually in October and November, and
during typical wet conditions, with respect to typical annual rainfall events. An example
of possible seasonal differentiation could be done as follows:
• Dry season - from May to September with an average seasonal EMC equal to EMC
dry
and a total flow volume = Vol
dry
• Start of the wet season - October - November - with an average seasonal EMC equal to
EMC
s. wet
and a total flow volume = Vol
s. wet
;
• Wet season - December to April-with an average seasonal EMC equal to EMC
wet
and a
total flow volume = Vol
wet
;
Under such an approach, which could be referred as “seasonal” approach for annual
pollution load (PL
annual
) estimation, the load could be calculated as:
(5.1)
Obtaining data for the application of the seasonal approach would require the design of a
monitoring program based on a monthly frequency of the sampling occasions. The EMC
dry
should be determined as the averaged value of the results during the dry months. The
determination of each one of the monthly EMC values during the wet season should be
determined during and/or immediately after rainfall events, catching the variation of
concentrations along the hydrograph. The most convenient approach would be the
collection of flow - integrated samples during the whole duration of the hydrograph,
where the results of the laboratory tests would represent the corresponding EMC. During
the start of the rain season, the frequency of the sampling occasions could be increased to
achieve a representative average value. If results after several years of monitoring, show
no statistically significant difference between EMC
s. wet
and EMC
wet
, the monitoring
program could be re-designed and only two seasonal values could be used for the
estimation of PL
annual
.
The seasonal approach is relatively simple and less expensive. However, the operation
and maintenance of flow rate devices and a regular monitoring program might not be
economically feasible for low-income areas. It would be advisable, to establish and
properly operate such monitoring stations at important sections of rivers and streams,
after major pollution sources or at confluence points to lakes/reservoirs with high water
quality requirements, based on their beneficial use. Under such circumstances, the
monitoring program should include additional water quality monitoring stations, after
major discharges of effluent or storm water channels. A control point, reflecting
background water quality in the upper reaches of the stream, which is relatively
unaffected by human activities is essential. For this specific location, the frequency of
sampling could be lower, as no high variation of the water quality is expected. The
