The sample volume should be sufficient to carry out all the required tests. In practice, 300-500 ml are adequate. If Salmonella, Vibrio cholerae or enteroviruses are to be analysed, as required by some authorities or under certain circumstances, greater volumes of water will be necessary (1.5 litres, 10 litres and 10 litres respectively). Bottles of borosilicate glass or suitable autoclavable plastic (PHLS, 1994; Bartram and Ballance, 1996) are recommended. They should have screw caps that withstand repeated sterilisation at 121°C or 180°C. Quality assurance procedures, as described in Chapter 4, should be followed. All sampling bottles should be correctly labelled with the reference of the sampling point. Additional information of the time of collection, temperature of water and other observations should be recorded on sample record sheets designed for this purpose.
8.2.2 Sample storage
There is little published information available that gives a consensus on the time limit for storage of samples to avoid changes in the concentrations of indicator organisms (Gameson and Munro, 1980; Tillet and Benton, 1993). Storage times should be as short as possible and it is recommended here that samples should be analysed as soon as possible, preferably within 8 hours of collection. If samples cannot be analysed within 24 hours field analysis should be considered. Immediately after collection, the samples should be stored in insulated boxes with cooling packs (prefrozen packs) and/or ice. Samples should be kept in the dark and the temperature of the cooling box maintained below 10°C where possible (APHA/AWWA/WPCF, 1995). This temperature may be difficult to reach and so in practice samples should be kept as cold as possible, but not frozen. In practical terms these storage conditions can have at best only a limited effect on reducing variations in bacterial populations. It is generally accepted that changes in microbial populations in water samples will begin to occur around 2 hours after collection; within 6 hours the samples are likely to have altered significantly particularly if no cooling mechanism was available and the samples were exposed to light. The key factor to consider in storage and transport of samples is time between collection and analysis rather than the time between collection and receipt at the laboratory. Ideally, the temperature of the insulated box should be controlled and recorded, as should the storage time. This information should be considered in the interpretation of results. Storage under these conditions should be as short as possible, and samples should be analysed promptly after collection.
8.3 Index and indicator organisms
Natural waters are subject to important changes in their microbial quality that arise from agricultural use, discharges of sewage or wastewater resulting from human activity or storm water runoff. Sewage effluents contain a wide variety of pathogenic micro- organisms that may pose a health hazard to the human population when the effects are discharged into recreational waters. The density and variety of these pathogens are related to the size of the human population, the seasonal incidence of the illness, and dissemination of pathogens within the community (Pipes, 1982). Appropriate indicators of faecal contamination under various conditions are discussed in Chapter 9 and in the Guidelines for Safe Recreational Water Environments (WHO, 1998).
Many waterborne pathogens are difficult to detect and/or quantify and the specific methodology to detect them in environmental water samples has still to be developed