3.5.1 Drinking water quality monitoring
Summary of actions
Determine the characteristics to be monitored in the distribution system and in water as supplied to the consumer.
Establish and document a sampling plan for each characteristic, including the location and frequency of sampling.
Ensure monitoring data are representative and reliable.
Drinking water quality monitoring is a wide-ranging assessment of the quality of water in the distribution system and, importantly, as supplied to the consumer. It includes regular sampling and testing to assess whether water quality is meeting guideline values, microbial health-based targets and any regulatory requirements or agreed levels of service.
Monitoring of drinking water quality should be regarded as the final check that, overall, the barriers and preventive measures implemented to protect public health are working effectively. The purpose of drinking water quality monitoring is different from that of operational monitoring and the two types of monitoring also differ in what, where and how often water quality characteristics are measured. As it is neither physically nor economically feasible to test for all drinking water quality parameters equally, monitoring effort and resources should be carefully planned and directed at significant or key characteristics.
Key characteristics related to health include:
microbial indicator organisms;
disinfectant residuals and any disinfection by-products;
any health-based characteristic that can be reasonably expected to exceed the guideline value, even if occasionally;
potential contaminants identified in analysis of the water supply system (Section 3.2.1) and hazard identification (Section 3.2.3).
In addition to characteristics related to health, those with significant aesthetic impact (e.g. taste, odour) may also need to be monitored. Where these frequently reach unacceptable levels, further investigation may be needed to determine whether there are problems with significance for health.
Sampling locations will depend on the water quality characteristic being examined. Sampling at the treatment plant or at the head of the distribution system may be sufficient for characteristics where concentrations do not change during delivery; however, for those that can change during distribution, sampling should be undertaken throughout the distribution system, including the point of supply to the consumer.
Frequency of testing for individual characteristics will depend on variability, and whether the characteristics are of aesthetic or health significance. Sampling should be frequent enough to enable the monitoring to provide meaningful information. Sampling and analysis are required most frequently for microbial constituents, and less often for organic and inorganic compounds. This is because even brief episodes of microbial contamination can lead to immediate illness in consumers, whereas, in the absence of a specific event (e.g. chemical overdosing at a treatment plant), episodes of chemical contamination that would constitute an acute health concern are rare. Guideline values for most chemical parameters are based on impacts of chronic exposure.
Once parameters and sampling locations have been identified, these should be documented in a consolidated monitoring plan. Monitoring data should be representative, reliable and fully validated (see Box 3.4). Procedures for sampling and testing should also be documented.
Section 9.5.2 provides more information on the monitoring of drinking water quality.
Reliability of data
Monitoring is only as good as the data collected, so every effort should be made to ensure that the data are representative, reliable and fully validated. Appropriate procedures should be in place and the following need to be considered.
Sampling plan:
parameters measured, sampling locations, sampling frequency;
qualifications and training of personnel;
approved sampling methods and techniques;
quality assurance and validation procedures for sampling;
statistical validity.
Analytical testing:
qualifications and training of personnel;
suitability of equipment;
approved test methods and laboratories;
quality assurance and validation procedures (e.g. positive and negative control samples, inter-laboratory comparisons);
accreditation with an external agency such as the National Association of Testing Authorities.
Monitoring equipment:
calibration and inspection procedures to ensure control of monitoring equipment.
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