View all publications

3 Water contaminants of health significance

For public health purposes, drinking water contaminants fall into three broad classes:

  • microbiological contaminants

  • chemical contaminants

  • radiological contaminants.

This guide focuses on microbiological and chemical contaminants, because they are the most frequently encountered.

Microbiological contaminants

In general, microbiological contaminants are considered to be a greater threat to health than chemical contaminants. This is because they are:

  • fast acting, usually causing sickness in a few days or weeks

  • capable of multiplying within a host

  • transmittable from person to person

  • capable of causing fatal illness.

In New Zealand, exposure to microbiological contaminants is a concern because of our relatively high density of domesticated animals. Conversely, New Zealand’s low level of heavy industry reduces the likelihood of industrial chemical contaminants in source waters.

Chemical contaminants

The health effects of greatest concern associated with chemical contaminants are those arising from prolonged exposure to low concentrations. Three notable exceptions to this, when chemical contaminants can have immediate consequences for health, are:

  • nitrate (specifically for bottle-fed infants)

  • cyanotoxins (the toxins produced by cyanobacteria)

  • copper, which may arise from the corrosion of copper plumbing (if present in high enough concentrations).

Exposure to chemical contaminants can also have immediate consequences when a contaminant’s concentration is very high, as may happen as the result of an accidental spillage.

Section 2 of the DWSNZ provides tables of MAVs (see section 2.2.1). These assist water suppliers and health professionals to assess the health importance of the concentrations of contaminants in a water supply.

3.1 Microbiological contaminants

The DWSNZ recognise three classes of micro-organisms that may cause disease (see Table 2.1 of DWSNZ): bacteria, viruses and protozoa.

3.1.1 Bacteria

The indicator organism Escherichia coli (E. coli) is used in the DWSNZ to assess the bacterial quality of water. The bacterial quality of treated water is satisfactory if the E. coli concentration is less than one organism per 100 ml. Except for a few strains, E. coli is not a disease-causing organism (pathogen). It is found in very high numbers in the gut of all warm-blooded animals. Fresh faeces always contains E. coli, although it may not survive in the environment as long as some pathogens do. When E. coli is detected in water it shows that the water has been in contact with faeces: this means that pathogens may also be present. The types of pathogen and their concentrations will depend on the nature of the organisms infecting the animals or humans that are the source of the faeces, and the number of animals or humans that are infected.

3.1.2 Viruses

There is too little information available on which to base an MAV for a viral indicator or individual viruses. A virus suitable to act as a viral indicator (similar to E. coli for bacteria) has yet to be found. Possible candidates have proved unsatisfactory because:

  • they respond differently from viral pathogens to treatment with disinfectants, or

  • there is no correlation between their concentration and those of viral pathogens in the water, or

  • test methods are unsuitable (incubation time too long, too complex, or too expensive).

Although there is no MAV for viruses in the DWSNZ, this does not mean that they do not present a threat to health. Faecally polluted water can harbour disease-causing viruses (viral pathogens). The presence of E. coli in water, although a bacterial indicator, may also signal that a water contains viral pathogens.

Viruses that cause waterborne disease tend to be enteric viruses, ie, they infect the gastrointestinal tract and are excreted by infected humans. Some viruses that infect animals may also infect humans. Human and animal viruses are highly infective.

3.1.3 Protozoa

Protozoa (eg, Giardia and Cryptosporidium) are among the most common causes of infection and disease in humans and other animals.10 The largest recorded outbreak of waterborne disease in a first-world country occurred in Milwaukee in the USA in 1993 due to Cryptosporidium , with an estimated 400,000 people becoming ill.

Giardia and Cryptosporidium exist as environmentally robust spores outside of a host. Both organisms are resistant to water treatment processes, but Cryptosporidium is more difficult to remove by filtration because it is smaller. It is also more resistant to chlorine.

The DWSNZ give an MAV for the total concentration of protozoa in treated water of less than one organism per 100 L (note that the units for protozoa are “litres” of water not “millilitres” as for bacteria). Giardia and Cryptosporidium are the protozoa of primary concern in drinking waters, so that “total” in the DWSNZ refers to these two protozoa.

3.2 Chemical contaminants

Maximum acceptable values for chemical contaminants, both natural and of human origin, are listed in two tables in the DWSNZ. The first table (Table 2.2 in the DWSNZ) contains chemicals such as nitrate, metals, and chemicals used to disinfect water. These are inorganic chemicals. The classes of chemical contaminants in the table are:

  • metals and metalloids

  • inorganic disinfection by-products11

  • disinfectants

  • a miscellaneous group outside the above classifications: beryllium, boron, cyanide, fluoride, nitrate and nitrite.

The second table of chemical contaminants contains organic substances (chemical substances containing carbon):

  • compounds utilised in industry (including contaminants in water treatment products)

  • agrichemicals (eg, pesticides)

  • substances formed in the water during the disinfection process (disinfection by-products)

  • cyanotoxins (toxins produced by cyanobacteria, blue-green algae)

  • polycyclic aromatic hydrocarbons, PAHs (resulting from incomplete combustion).

NES note: Some readers may be used to using the Australian and New Zealand Environment and Conservation Council (ANZECC) Australian and New Zealand Guidelines for Fresh and Marine Water Quality (2000) when considering resource consents. If you are used to working with the ANZECC guidelines, be aware that some MAVs given in the DWSNZ are lower than ANZECC guideline values. Note also that some MAVs exist in the DWSNZ for contaminants not covered by the ANZECC guidelines.

3.3 Radioactive contaminants

The final table of MAVs covers radioactive contaminants of water. These are seldom a concern and are expected to arise only from natural sources.

3.4 Risk categories for contaminants: priority classes

The DWSNZ use the concept of priority classes for contaminants to ensure that water suppliers monitor the contaminants of greatest health significance for their supply in the most efficient way.

Many contaminants are listed in the DWSNZ. Water suppliers cannot afford to test all of them frequently enough to adequately check the quality of their water. Water suppliers are therefore only required to monitor the contaminants of greatest health concern for their supply.

There are four priority classes. Priority 1 (microbial contaminants) is highest priority, as these contaminants pose the greatest immediate threat to health.

  • Priority 1 contaminants: microbial
    These contaminants are all microbial contaminants: E. coli and the protozoa.12 They are of greatest health significance because they can lead to rapid and major outbreaks of illness. Moreover, because their concentrations in source waters are often very variable, samples for testing need to be taken more frequently than for other contaminants.
    Priority 1 contaminants may arise in any source water and are therefore assigned to all water supplies, ie, all suppliers are required to monitor them, or show that their treatment processes are capable of removing them (discussed in more detail in sections 5 and 6).

  • Priority 2 contaminants: chemical
    In principle, Priority 2 contaminants can be chemical, radiological or microbial,13 but in practice only chemical Priority 2 contaminants have been assigned to water supplies. Since the sources of chemical contaminants can vary among supplies, the Priority 2 contaminants assigned to a supply depend on the situation at each supply. (The process of identifying these contaminants for each supply is discussed later; see section 8.1.)
    Priority 1 and Priority 2 contaminants that have been assigned to the supply must be monitored for a water supplier to show that the water they produce is safe to drink (by complying with the DWSNZ). All monitoring to meet the requirements of the DWSNZ is the responsibility of the water supplier.
    Priority 3 and Priority 4 contaminants are of lower risk than Priority 1 and Priority 2 contaminants and do not have to be monitored, but can be monitored at the discretion of the water supplier. A drinking-water assessor may require monitoring of these lower risk contaminants for public health reasons (see section 8.3 for further explanation). Like Priority 2 contaminants, the characteristics of each supply determine which contaminants are classed as Priority 3 and Priority 4.
    When monitoring of a Priority 2 contaminant no longer shows it to be a potential health concern, perhaps because of improved treatment, the contaminant may be reassigned to Priority 3 status. No official assignments of Priority 3 or 4 status are otherwise made.


10 WHO Guidelines for Drinking-water Quality (3rd edition), 2004, section 11.3.

11 These are compounds formed when a disinfectant reacts with organic matter in the water. They are discussed more fully in section 8.4.1.

12 Viruses are not presently included in this priority class. Compliance criteria have yet to be established for them. When a suitable indicator organism for viruses is found, or the efficacies of treatment processes to inactivate them have been determined, compliance criteria will be set and they too will become Priority 1 contaminants.

13 Specific microbial contaminants that may appear for a limited time in a supply because of an event or events are classed as Priority 2 contaminants not Priority 1 contaminants. The monitoring requirements for them are established when they are assigned a Priority 2 status. They are different from Priority 1 contaminants which require on-going monitoring of the indicator E. coli or the performance of the treatment process (for protozoa).