appear, more frequent sampling should be carried out, especially in freshwater resources with poor water circulation that may be overcrowded under those circumstances.
Monitoring a bathing area or a site to reconfirm repeated failure to meet a guideline or poor water quality has little value. Equally, sampling frequency can be reduced when an area is known, through historical microbiological data, to have consistently good microbiological quality and when it is known from the catalogue of basic characteristics that it will not be subjected to pollution influences because potential sources of contamination are absent. In these situations only occasional confirmatory sampling will be required. Such an approach will direct resources to those beaches known to have variable water quality (see Chapter 9).
Resampling and new sanitary inspection, following the detection of unexpected peak values, is essential to establish the cause of the observed peak. An exhaustive investigation, including an inspection of the site and possible collection of additional samples to locate the source or sources of pollution, is also essential where the cause is known not to be due to a sporadic event. The effect of episodic events, such as heavy rainfall, on the water quality of bathing beaches, and the management response to such events, is discussed in Chapter 9.
Sampling in chest depth water, typically 1.2-1.5 m depth, represents areas of greatest bather density although sampling at ankle depth may be appropriate to determine risk to young children.
Microbiological counts from surface samples have been shown to have a tendency to be higher than those beneath the surface (PHLS, 1995), but the epidemiological significance of this has yet to be studied. Therefore samples should be collected from beneath the surface. Precise sampling recommendations vary, for example 30 cm below the surface is indicated by the American Public Health Association (APHA) (APHA/AWWA/WPCF, 1992) and the European Community (EC) Directive (EEC, 1976), while the World Health Organization (WHO) and the United Nations Environment Programme (UNEP) (WHO/UNEP, 1994a) have proposed 25 cm. Every sample within a monitoring programme should be taken as near as possible to the defined sampling location.
Care must be taken to avoid external contamination during sample collection. Sterilised sample bottles should be opened with the opening facing downward and should be held by the base and submerged in the water. At the appropriate depth, the bottle should be turned upwards with the mouth facing the current (if any). After retrieving the bottle, some water should be discarded to leave an air space of at least 2.5 cm to allow mixing by shaking before examination (APHA/AWWA/WPCF, 1989; Bartram and Ballance, 1996). The utmost care must be taken at all times not to touch the top of the bottle during removal or replacement of the cap.