9.2.2 Principles of monitoring frequency

The frequency of monitoring for each water quality characteristic differs for each characteristic, and depends on the hazard characteristics and risk profile as identified through analysis of the water supply system. In general, characteristics that pose a high level of risk require more monitoring, while those posing a low risk require less monitoring. Typically, the most frequent monitoring is for microbial safety, followed by known or identified high priority and site-specific contaminants, with less frequent monitoring for any contaminants that are not likely to present a risk.

Operational monitoring of preventive measures and barriers throughout the water supply system should be carried out with sufficient frequency to reveal any challenges or failures promptly, so that corrective actions can be taken. Online and continuous monitoring should be used wherever possible, particularly for essential processes identified as critical control points, such as disinfection and filtration (see Sections 3.3.2 and 9.4.3).

Frequency of source water monitoring is determined by the variability and understanding of the challenges present. Monitoring should be more frequent where water quality is more variable or less understood. Depending on the historical data available and the present understanding of source water characteristics, a baseline investigation of contaminants in a water supply may be required to assess hazards and their risk levels.

Frequency of monitoring downstream of treatment depends on how effectively treatment barriers are controlled and the level of understanding of the distribution system. Identified problem areas may dictate increased monitoring frequencies (e.g. where there is difficulty in maintaining chlorine residuals).

Disease outbreaks associated with drinking water supplies are often linked to unusual events. Such events should therefore be recognised as potential triggers for increased challenges and potential sub-optimal performance, and should alert water managers to the potential for problems and the need for increased monitoring of performance throughout the system (Box 9.2). Unusual events include any sudden or extreme change in weather, flow or water quality, as well as power outages, new assets, treatment variations, and maintenance and repairs. The increased monitoring frequency should be maintained until there is confidence that water quality is back within specification.

More detailed discussion on specific sampling frequencies within the water supply system is provided in Section 9.4.5.

Box 9.2

Contamination events

Waterborne disease outbreaks associated with drinking water supplies are almost invariably linked to a significant change in conditions that provides a sudden challenge to a water system; for example, heavy rainfall or run-off from heavy snow melt (Craun et al. 2003, Curriero et al. 2001, Hrudey and Hrudey 2004). Most water treatment processes function best under steady-state conditions, and performance can seriously deteriorate with major fluctuations in water quality or flow. Run-off from heavy rainfall or snow melt can dramatically increase flow and turbidity as well as the concentration of natural organic matter (Logsdon et al. 2004, Hunter 2003). Run-off can also potentially alter the pH and alkalinity of source waters (Logsdon et al. 2004). These fluctuations in flow or quality cause additional stress on water treatment systems and can interfere with the effectiveness of water treatment. The failure of treatment processes to cope with the impacts of heavy rainfall and run-off events is a common theme to many waterborne disease outbreaks.

In addition to changing environmental conditions and their impacts on source water, any changes from the normal operation of a water treatment system can also pose a significant risk of waterborne disease outbreaks. Reviews of outbreaks related to drinking water supplies have frequently identified increased contamination during periods of maintenance of the water treatment plant and storage facilities, plant upgrades, and changes to water treatment processes, as well as during construction or repair of water mains (Hrudey and Hrudey 2004 pp 83-94, Logsdon et al. 1996, Kramer et al. 1996, Craun et al. 2003, Nygard et al. 2007).

Furthermore, reported waterborne disease outbreaks frequently reveal that systems are at most risk when a combination of risk factors coincide; for example, heavy rainfall during plant maintenance and repairs, increased demand for water, and inadequate treatment performance, coupled with old facilities. There are a number of instances where outbreaks have occurred after problems had been revealed but no corrective action had been taken.

The risk of contamination of water supplies with microbial pathogens is always present. While safeguards and multiple barriers may be in place, the historical absence of waterborne outbreaks in a water system is no guarantee that one will not occur in the future unless the effectiveness of the barriers is continuously maintained and verified. Constant vigilance and effective monitoring programs that support understanding of a water supply system – its challenges and capabilities – are of paramount importance in assuring the safety of drinking water.