9.2.1 Monitoring priorities
While the potential contaminants of drinking water supplies are many, there is evidence of human health impacts via drinking water supplies for only a limited number of these. The most significant contaminants are waterborne microbial pathogens: they represent the clearest and most acute risk to drinking water safety, and they can cause outbreaks of illness that affect a high proportion of the community and, in extreme cases, result in death (see Box 3.3, Section 3.4). The single most important monitoring activity is therefore to ensure that microbial contamination does not cross barriers and enter the drinking water supply and hence meeting microbial health-based targets (see Chapter 5).
Chemical or radiological contamination does occur and, in some specific cases, serious health effects via drinking water have been documented. However, illness from such contamination typically arises from specific natural local conditions or from site-specific contamination by humans (distribution cross-connections, inadvertent chemical addition or sabotage). Priority chemicals (arising primarily from natural contamination) include arsenic, fluoride (above concentrations applied for dental protection), selenium, nitrate, lead and uranium. Iron and manganese are also mentioned as frequent sources of aesthetic water quality problems, and these may lead consumers to use alternative water supplies that may not be safe with respect to microbial pathogens (WHO 2008).
Chemicals used in water treatment may pose a risk because of the potential for inadvertent contamination, and they should be monitored accordingly. By-products of disinfection should also be monitored, because of the possible adverse health effects from chronic exposure to these chemicals. However, it remains uncertain whether exposure to disinfection by-products at the levels typically found in drinking water causes human disease, and given the established health risks associated with waterborne microbial pathogens, disinfection should never be compromised.
Most other chemicals, including pesticides and other trace organics, do not warrant the same level of monitoring attention as microbial pathogens or the chemicals of main concern, unless there is evidence or reasonable inference of their potential presence, as determined through site-specific investigation and analysis of the water supply system.
Box 9.1 summarises monitoring priorities based on health risk.
Monitoring priorities based on health risk
Key characteristics related to health include:
microbial indicator organisms and disinfectant residuals to determine if microbial health-based targets are being met;
any known physical and chemical or radiologial characteristics that can be reasonably expected to exceed the guideline value, even if occasionally;
any chemicals used in treatment processes and any by-products that may result from their use;
any potential contaminants identified through the water supply system analysis (see Section 3.2.1) and hazard identification (see Section 3.2.3), even if undetected.
Some characteristics not related to health, such as those with significant aesthetic impacts, should also be monitored. Where aesthetic characteristics (e.g. taste and odour) are frequently unacceptable, further investigation may be needed to determine whether there are problems with significance for health.
Monitoring can be direct, where the characteristic of concern is monitored directly; or indirect, where surrogates or indicators are monitored.
Surrogates are typically quantifiable characteristics that can serve to measure the effectiveness of processes in controlling specific hazards or groups of hazards.
Indicators are physical, chemical or microbial characteristics that are representative of a broader group of related characteristics. Indicators provide an alternative to monitoring for the possible presence of other hazardous substances that are more difficult to monitor.
Effective surrogates and indicators:
directly measure process performance characteristics that are related to the effectiveness of the process in preventing or eliminating hazards;
are amenable to the setting of trigger levels, guideline values and/or target criteria, so that results can be responded to;
provide warning of process performance failures early enough to allow corrective action to be taken before unsafe water is supplied to consumers; and
are low cost and reliable to monitor, and where required, are amenable to on-line monitoring.
Some examples of surrogates and indicators are:
electrical conductivity (EC), which is widely used as a surrogate for total dissolved solids;
turbidity, a widely used surrogate for the performance of media filtration systems;
trihalomethanes (THMs), which, because they are the most common disinfection by-products and occur in the highest concentrations, can be used as indicators for the possible presence of a range of related chlorine-derived disinfection by-products;
faecal indicator bacteria, which are numerous in faeces and serve as indicators for the possible presence of faecal contamination and, by inference, enteric pathogens.
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