Environmental Engineering Reference
In-Depth Information
with control points, which have water quality characteristics close to background
pollution values. In order to minimize the monitoring efforts, the location of such points
should be chosen carefully and should concentrate on major streams, collecting runoff
from larger areas. In numerous cases, the monitoring stations, which represent large
drainage areas, might include not only the impact of low-income areas but also the
contribution of other types of land use practice and point source discharges as well.
Consequently, the specific contribution of runoff from low-income areas might not be
possible to be identified and separated from the whole load.
The previous chapter discussed the need to introduce the concept of EMC and site-
specific pollutant concentrations with respect to typical storms. In cases of drainage
channels, conveying storm water only, the EMC represents an averaged value for a
selected storm event, and as such defines clearly the contribution of pollution from storm
water regarding a selected parameter. However, when the estimation of EMC of rivers
and streams for a selected storm event is needed, the determined EMC value at a given
location would represent not only the contribution from runoff, but also, the contribution
from:
• Natural sources (background pollution). It would have two components - natural
pollution of base flow from ground water recharge and natural pollution from
materials present in the river bed, which have been contributed from the surrounding
environment or grow naturally;
• Materials dumped into river beds by human actions;
• Effluent discharges from point sources up stream the sampling location.
During the dry season (low flow conditions), EMC would represent only the input from
background pollution and point sources upstream the sampling location, but the term
“event” needs to be well understood. In this case the “event” does not refer to a rainfall
event, but to low-flow events with a corresponding return period e.g. one year return
period would present results regarding the quality of low flows, which could be expected
to occur each year, while 10 years low flow could be expected to occur once per 10 years.
The duration and frequency of the sampling procedure and the return period should be
chosen and planned accordingly to obtain representative results. In most cases it could be
expected that dry weather EMC would be higher than the wet weather EMC at the same
location, with respect to the vast majority of pollutant constituents, except for suspended
solids. These values should be used for comparison with prescribed limits and regulations
or for evaluation of toxicity effects.
The EMC multiplied by the corresponding average flow rate for the event would
represent a pollution flux for this specific event. An accurate estimation of the annual
pollution load contributed by a river or stream, would be the sum of all fluxes calculated
on a daily basis. Such a procedure for the estimation of annual pollution loads would
require extensive data sets regarding flow rates and corresponding EMC, which could be
collected only if specific monitoring stations are available and equipped with automatic
measurement devices, which could provide data in respect to flow rates and the
corresponding pollution constituents. The former could be measured in-situ or by
laboratory tests. This is an expensive option, especially in the case of pollution reduction
and prevention in low-income areas.
