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Appendix 1: Understanding Determinands and Water Treatment

A1 Introduction to determinands

This appendix gives a basic introduction to determinands and their possible derivation. For more detail, please refer to the ESR report A Guide to the Ministry of Health Drinking-water Standards for New Zealand, which is available from the Ministry for the Environment’s website.

The NES uses the term ‘determinand’ instead of ‘contaminant’. ‘Determinand’ has a specific meaning which is narrower than that of contaminant (as defined in the RMA). The Drinking-water Standards for New Zealand2005 (DWSNZ) define determinand as:

A constituent or property of the water that is determined, or estimated, in a sample, for example:

  • microbial determinand: total coliforms

  • chemical determinand: chloride

  • physical determinand: turbidity

  • radiological determinand: radon.

In the NES regulations, ‘determinand’ specifically refers to the health-related determinands specified in the DWSNZ.

Determinand means a determinand described in Table 2.1, 2.2, 2.3, or 2.4 of the Drinking-water Standard.

There are two different kinds of determinands: those of health significance and those that are not of immediate health significance. Determinands that are not of immediate health significance are those that either:

  • are not damaging to health in themselves and therefore have no specified maximum acceptable value (MAV), but can affect the water’s safety for drinking by leading to the formation of other contaminants referred to as precursors

  • affect the aesthetic properties of the water (taste, odour, appearance).

These determinands need to be considered when assessing the impact of a new activity in a catchment on the drinking water received by a community. The acceptability of a water supply to a community can be as strongly influenced by these determinands as by those with health significance.

An aesthetic determinand is defined in the DWSNZ as:

A constituent or property of the water that can adversely affect the water’s taste, odour, colour, clarity or general appearance, including substances such as manganese and iron compounds that can stain washing and utensils.

In the NES regulations:

Aesthetic determinand means an aesthetic determinand described in Table A2.1 in Appendix 2 of the Drinking Water Standard.

Health and aesthetic determinands have different characteristics, so they are measured in different ways. Determinands of health significance are measured by their maximum acceptable value (MAV), and aesthetic determinands are measured using guideline values (GVs).

The DWSNZ define these terms as follows.

Maximum acceptable value (MAV) – the concentration of a determinand below which the presence of the determinand does not result in any significant risk to a consumer over a lifetime of consumption. For carcinogenic chemicals, the MAVs set in the Drinking-water Standards for New Zealand generally represent a risk of one additional incidence of cancer per 100,000 people ingesting the water at the concentration of the MAV for 70 years.

Guideline value (GV) – the value for an aesthetic determinand that, if exceeded, may render the water unattractive to consumers.

There is no definition of precursors in either the NES or the DWSNZ. However, precursors can be placed in three general categories:

  • nutrients, which encourage the development of algal blooms

  • organic matter, which leads to the formation of disinfection by-products

  • turbidity, which can affect the ability of treatment plants to remove protozoa from the water.

Precursors have no health significance in themselves, but they can affect the potability of water because they can result in treated water containing contaminants that are a health concern.

A2 Water contaminants of health significance

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

  • microbiological

  • chemical

  • radiological.

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

A2.1 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 the 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.

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


The bacterial indicator organism Escherichia coli (E. coli) is used in the DWSNZ to assess the potential for faecal contamination of water. The bacterial quality of treated water is satisfactory if the E. coli concentration is less than 1 organism per 100 millilitres.

Apart from a few strains, E. coli is not a disease-causing organism (pathogen) itself. It is found in very high numbers in the gut of all warm-blooded animals. Fresh faeces almost always contain E. coli, although it may not survive in the environment as long as some pathogens. The presence of E. coli confirms that water has been in contact with faeces, meaning that pathogens may also be present. The types of pathogen in water and their concentration will depend on the nature of the organisms that are infecting the source of the faeces (animal or human) and the number of animals or humans that are infected.


Insufficient information is available to derive an MAV for viruses, or a viral indicator, because a virus suitable to act as a viral indicator (similar to E. coli for bacteria) is yet to be found. Potential candidates have proved unsatisfactory because:

  • they respond differently from viral pathogens to treatment with disinfectants

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

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

Although there is no MAV for viruses in the DWSNZ, this does not mean 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 the presence of viral pathogens. Viruses that cause water-borne disease tend to be enteric (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.


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

The DWSNZ give a MAV for the total concentration of protozoa in treated water of less than 1 organism per 100 litres. Note that the units for protozoa are for litres of water, not millilitres as for bacteria. Giardia and Cryptosporidium are the protozoa of primary concern in drinking waters.

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

Applying the NES for protozoa

Catchment activities that are likely to increase the concentration of Cryptosporidium in a source water could lead to an increase in the log credits a water supply requires to achieve compliance with the DWSNZ with respect to protozoa. Log credits are explained in more detail in Appendix 7 of this guide.

Chemical contaminants

The health effects of greatest concern associated with chemical contaminants are those arising from prolonged exposure to low concentrations. Three notable exceptions, 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; this may happen as the result of accidental spillage.

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

  • metals and metalloids

  • inorganic disinfection by-products

  • disinfectants

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

The second table of chemical contaminants (Table 2.3 in the DWSNZ) lists organic substances:18

  • 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 and blue-green algae)

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

Radioactive contaminants

The final table of MAVs in the DWSNZ covers radioactive contaminants of water. These are seldom a concern in New Zealand, and are expected to arise from natural sources only.

A3 Contaminants that affect the aesthetic properties of water

The taste, odour and appearance of water are collectively called aesthetic properties. Aesthetic properties are important because they determine a water’s acceptability to consumers. Problems with a water’s aesthetic properties are also usually more rapidly evident to consumers than MAV exceedances by contaminants that affect health.

The World Health Organization’s Guidelines for Drinking-water Quality (3rd ed, 2004) state:

The provision of drinking-water that is not only safe but also acceptable in appearance, taste and odour is of high priority. Water that is aesthetically unacceptable will undermine the confidence of consumers, lead to complaints and, more importantly, possibly lead to the use of water from sources that are less safe.

The DWSNZ contain a list of guideline values for aesthetic determinands, using the term ‘wholesome’ to describe water that is potable and acceptable to consumers with respect to taste, odour or appearance.

Guidelines are met at the discretion of the water supplier, and are not a requirement for DWSNZ compliance. This is because it is not always straightforward – and is often expensive – to treat aesthetic determinands. The two most likely reasons for water suppliers undertaking this monitoring are to:

  • address consumer complaints about water quality

  • achieve a high public health grade.

Sampling in response to complaints about the aesthetic aspects of water (eg, taste and smell) occurs more frequently than monitoring for health-significant contaminants. Samples may be taken from the source water and/or following treatment, depending on where the investigation leads. Which determinands are tested will depend on the nature of the complaint received.

A monitoring programme that addresses a particular consumer concern is likely to last only a short while, continuing only until the problem is identified and a solution found. If it is an ongoing problem, which the water supplier has difficulty overcoming, monitoring may continue.

Although aesthetic determinands do not cause toxic effects, health issues can arise if people seek alternative water sources. When alternative sources are used they are often unsafe supplies such as untreated aquifer or river water.

A4 Precursors

Several contaminants of water do not have direct health effects themselves but can have adverse health consequences through reducing the effectiveness of treatment processes or leading to the production of harmful chemical compounds. These contaminants include: turbidity, natural organic matter, hardness, and some major ions such as sodium ions. For more information on the performance of treatment processes, refer to the ESR report A Guide to the Ministry of Health Drinking-water Standards for New Zealand available from the Ministry for the Environment website here


Turbidity (particles suspended in water) needs to be removed to avoid the deterioration in the effectiveness of filtration, disinfection and adsorption processes. If a new activity is likely to result in a major increase in the turbidity of the source water, the treatment plant’s ability to adequately treat water to remove particles must be evaluated.

Filtration processes in water treatment plants can remove particles from the water to some extent, but there is a limit to how much they can remove. Filters will rapidly clog, or particles can break through if the turbidity of the source water entering the filters is too high.

The efficacy of all disinfection processes, chemical and physical, are adversely affected by particles in the water. Microbes can be adsorbed onto the surfaces of particles, which makes them harder to remove using chemical disinfection. Ultra-violet (UV) radiation efficiency is also reduced by turbidity because particles can cause the radiation to scatter.

Some processes (eg, activated carbon adsorption and ion-exchange adsorption) remove contaminants by adsorption; their effectiveness depends on the contaminants of concern reaching the adsorbing surface. Unacceptably high levels of particles in the water will rapidly reduce the available surface area for adsorption and therefore reduce the efficacy of the process.

Natural organic matter

Natural organic matter is made up of large organic molecules formed by the decay of vegetation and animal remains. The efficacy of disinfection, oxidation and adsorption processes is reduced by natural organic matter. Catchment activities that increase the natural organic matter concentration in the source water can therefore reduce the effectiveness of a treatment plant.

Chemical disinfectants (eg, chlorine) are commonly used in water treatment to inactivate microbes, but these disinfectants react with natural organic matter, reducing the disinfectant’s concentration and therefore reducing its disinfection ability. Chlorine and ozone both react with natural organic matter, which reduces their effectiveness in disinfecting water. The disinfectant dose added to the water can be increased to compensate for this, but this also increases the concentration of disinfection by-products,19 which is undesirable because of their possible health effects.

Natural organic matter also reduces the effectiveness of disinfection by UV radiation, because it absorbs light at the same wavelength generated by the UV lamps that are used to kill harmful organisms.

Hardness and other effects of major ions

Water hardness arises from calcium and magnesium ions in the water, and is often the result of the water having been in contact with limestone or marble. High hardness creates problems of scale formation on water heating-elements, and inhibits the lathering of soap.

Any treatment using ion-exchange and UV irradiation is made less effective by waters that are hard or contain high concentrations of other major ions. If the primary use of ion exchange in a particular situation is to remove iron and manganese, an increase in source water hardness may result in the exchange resin removing calcium and magnesium instead. High sodium levels may also compromise the removal of iron and manganese.

The formation of calcium scale on the quartz sleeves in hard waters can also reduce the intensity of the output from UV lamps, which in turn affects their disinfection capability.

18 Organic substances are chemical substances containing carbon, but not including carbon dioxide.

19 Disinfection by-products (DBP) are defined in the DWSNZ as “a contaminant produced in the drinking-water supply as a by-product of the disinfection process”. Examples of these are listed in Table 2.3 of the DWSNZ. Their suspected health effects include cancer, and liver and kidney damage. Note that some disinfection by-products do not have a health effect so are not listed as determinands in this table. However, these are aesthetic determinands and are included in Table 2.3 of Appendix 2 of the DWSNZ.