Drinking water systems must be routinely monitored to: (1) control operational processes and verify treatment effectiveness and (2) to surveil compliance, ensuring that drinking water meets regulatory standards and protects public health. Regular sanitary inspections are crucial, and sanitary checklists for water supply systems can be a useful complement for monitoring water quality by observation that allows user groups to monitor their own supplies.

Monitoring Parameters

Microbial contaminants: Waterborne diseases are caused by pathogenic bacteria, viruses, and parasites that originate from human and animal excreta. These microorganisms are diverse in their characteristics, fate and transport and may cause acute or chronic health effects. The risk of illness from these organisms depends on the dose and virulence of the pathogen as well as the immune function of the person exposed. Since direct detection of pathogens is costly and technically challenging, the verification of microbial safety relies on indicator organisms, such as Escherichia coli (E. coli) or thermotolerant coliforms. Currently available test kits provide results in terms of presence/absence (P/A), most probable number (MPN), or colony enumeration (in units of colony forming units [CFU]/100 mL), and it usually takes at least 24 hours for the results.

Chlorine disinfection: Disinfection using chlorine is the most common treatment to destroy pathogenic microorganisms in drinking water and to provide residual protection against low-level contamination and bacterial growth in the system. The effectiveness of chlorination depends on the turbidity of the water. Residual chlorine levels should be frequently monitored, as concentrations can vary over a short timescale. Testing procedures are relatively cheap and simple. A common test is the dpd (di-ethyl paraphenylene diamine) indicator that uses a comparator, often found commercially as a simple and cheap ‘swimming pool tester’. The dpd test adds a tablet reagent to a water sample, and the strength of the colour change compared to a standard colour chart determines the chlorine concentration range. Simple test strips are also easy to use and sufficiently accurate for verification purposes.

Chemical and physical contaminants: There is a wide array of chemical constituents that may occur in drinking water supplies. Chemical contaminants can occur naturally (e.g. geogenic contamination such as fluoride or arsenic), originate from human activities (industrial, residential, or agricultural) or stem from the drinking water distribution system itself. Only a small share of the chemicals found in drinking water supplies have an adverse health impact, and usually only after prolonged exposure. The naturally occurring chemical contaminants with the most significant health impacts are arsenic, fluoride, barium, boron, chromium, selenium and uranium. Significant chemical contaminants from human activities or the water system itself include lead, pesticides, nitrate, persistent organic pollutants (POPs), pharmaceuticals and heavy metals. Aesthetic parameters, such as turbidity, colour, odour and taste, are not a health concern but can greatly influence the users’ acceptance of a water supply, and turbidity in particular can negatively affect the efficiency of treatments such as chlorination. Due to the analytical sensitivity and less frequent monitoring intervals required, chemical constituents are usually analysed in a laboratory setting. Field test kits can be useful in regions where known hazards exist or are assumed and where laboratories are not easily accessed. Local water sector professionals are likely to be aware of the main chemical hazards in local drinking water, so it is important to draw on this expertise to prioritise chemical contaminants of concern and develop an effective and resource-efficient monitoring programme. Table 3 presents a summary of common chemical contaminants in drinking water, with examples of field- and laboratory-based testing methods.

System inspections: Water quality monitoring should be supplemented by full-system inspections and should assess the adequacy of source protection measures, structural integrity of the intake, operational status of treatment devices and pressure readings throughout the distribution network. Leak detection and repairs will reduce the risk of infiltration and backflow. Regular inspections can also identify hygienic problems near collection taps that require education or awareness raising among water users.

Operational Monitoring Strategy

Sampling frequency: The frequency of monitoring should be in line with the expected variability of each water quality parameter. Long and short-term variations, such as equipment wear (years), seasonality (months), chemical usage (weeks), filtration cycles (days), weather events (hours) and process control (minutes), all affect water quantity and quality. For example, turbidity levels may change rapidly following rain or the implementation of new treatment processes (e.g. sedimentation or filtration). Especially in intermittent piped supplies, microbial quality may degrade rapidly and by orders of magnitude if impacted by intrusion and backflow or biofilms, loose deposits, and microbial growth. Geogenic contaminants such as arsenic and fluoride typically vary only gradually, although fluctuating groundwater levels due to seasonal variations or abstraction can mobilise contaminants. For most water quality parameters, time lags exist between sampling intervals and between the time of sampling and the analysis of results. These time lags may impede the timely implementation of interventions, leaving consumers exposed to health risks due to poor water quality. Approaches such as water safety plans and sanitary inspections try to address this issue by focusing on problem prevention and identifying problems before they affect water quality (see X.8).

Supporting Infrastructure: In addition to sampling frequency, an effective monitoring strategy must consider sample transport and storage, data analysis, results interpretation and supports for corrective actions. Centralised water supplies will generally follow legal requirements and standard operational procedures for monitoring, and analyses are performed in accredited laboratories that provide reliable results. Operational parameters may be determined with in-line sensors or within an on-site laboratory. Rehabilitation of the water quality monitoring system should be one of the objectives in emergencies. In emergency water supply systems as well as rural and community-scale systems, the frequency and scope of water quality monitoring is typically defined by factors such as road access, material supply chains and availability of technically trained staff. Therefore, an effective and sustainable monitoring program is context dependent and should be tailored to local conditions rather than copying standard protocols from another location. Risk assessment and mitigation approaches, such as those described in the WHO’s Water Safety Plan (WSP) manual, provide a systematic framework for designing site-specific monitoring programs. The WSP approach also encompasses the organisation of the reporting, its interpretation and corrective actions drawing on monitoring data (see X.8).