Environmental Engineering Reference
In-Depth Information
“polyphosphate”, or “organophosphorus compounds”, which would necessitate structural
identification and would require a specific testing procedure.
The choice of parameters to be monitored is a center point during the design process
of a monitoring program and requires very good knowledge and understanding of the
media to be studied, the available methodology and the study area. It also requires
analysis of the conditions and resources available and ingenuity. This task should be
delegated to experienced staff with proper knowledge of local conditions and resources.
The quality of a water body of just one instant is seldom of interest. Normally,
information is required over a period of time during which the quality may vary. The
times during this period at which samples are collected must be chosen so that they
adequately represent the true quality and its variations. In addition, the number of
samples must usually be kept to a minimum in order to save sampling and analytical
effort. The problem is, therefore, to select the number of samples and the times at which
they are taken, so that information of the required accuracy is obtained with minimum
effort.
In most circumstances, the ideal technical solution would be to use automated
instruments designed for the continuous measurement of the parameters. This approach
can be extremely valuable but it is not generally applicable because suitable
instrumentation for all parameters is not yet available. It could be executed regarding
temperature, pH, dissolved oxygen, conductivity and other limited variables. At present
the bulk of water analysis are made on discrete samples. Whatever the chosen sampling
frequencies, data should be regularly revised to decide whether or not changes in the
frequencies are necessary. In water systems regular cyclic variations of quality may
occur, with periods of one day, one week or one year. For example, diurnal fluctuations
can occur in rivers, lakes and effluents. Persistent cyclic variations with other periods
may also occur (regular variations due to discharges in industrially exploited rivers).
When cyclic variations occur, a biased estimation of quality will be obtained unless
sampling times are carefully chosen. For example, when the dissolved oxygen content of
the river varies diurnally, biased results for the mean daily value will be obtained if
samples are always collected at the same time of the day.
2.5
The sampling process
The choice of a correct technique for collecting samples is most important; otherwise
non-representative samples may be obtained. Some aspects of general importance in any
sampling process are:
• The concentrations of parameters in the water entering the sample should clearly be the
same as those in the water being sampled.
• The concentrations of parameters in the sample must not change during the process of
collection and transport of the sample to the laboratory.
It is very important that the procedure, which will be used for collecting samples, should
be carefully prescribed and followed. This is particularly true if the personnel involved
are relatively unskilled scientifically. Collection of samples may involve hazards to the
person sampling. A good description of such hazards and precautions should be available.
When a laboratory receives samples, they should be preserved and stored, until analysis
