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8 Compliance requirements for chemicals

8.1 Identification of Priority 2 contaminants

The Ministry of Health funds a programme that identifies the chemical contaminants (Priority 2 contaminants) that water suppliers need to monitor for demonstrating compliance with the DWSNZ. The programme is named The Priority 2 Chemical Determinands Identification Programme (known as the “P2 Programme”) and is administered by ESR. It achieves its purpose through a combination of risk assessment and monitoring.

The P2 Programme assesses all water distribution zones containing more than 100 people for chemical contaminants of potential health concern. The assessment of a particular zone needs to consider all treatment plants supplying the zone, and all water sources feeding these treatment plants. The assessment process for each supply (described below) takes a little over one year, and since 1995, approximately 1050 distribution zones (somewhat fewer supplies, as many have more than one zone) have been assessed. Most distribution zones and their treatment plants have been assessed only once. Repeat assessments are only carried out under the P2 Programme if the drinking-water assessor becomes aware of something having changed in the supply that might affect the contaminant concentrations in the water. Drinking-water assessors are usually reliant on the water supplier to inform them of such changes. Recently, some water suppliers themselves have undertaken reassessments of which contaminants should be tested for in the water, with guidance from ESR.

The first step in identifying possible chemical contaminants is the collection of information about the:

  1. Catchment: Possible sources of natural, agricultural and industrial contamination in the catchment are identified by a questionnaire completed by the drinking-water assessor (or in the past the health protection officer) and the water supplier. The boundaries of the catchment are not defined in the questionnaire; it is left to the discretion of those completing the questionnaire to decide which activities or geological features (such as mineral deposits) need to be considered. No samples are taken from the catchment.
  2. Treatment plant: A questionnaire is used to obtain information about features of the treatment plant that may lead to contamination of the water, eg, the treatment processes and chemicals used, and materials used in the construction of the plant.
  3. Distribution zone: A third questionnaire is used to obtain information about features of the distribution zone and its operation that may result in chemical contamination of the water, eg, materials used in the network of pipes, compounds used to seal pipe joints, and whether back-flow preventers are in place to stop contaminated water being back-siphoned into the water supply.

During this stage of the assessment, a sample is also taken from the distribution zone and analysed for: metals that could arise from corrosion; contaminants that have MAVs in the DWSNZ but for which information about their occurrence in New Zealand waters is limited, eg, uranium; and chemical contaminants for which the hydrogeological factors that influence their occurrence are not fully understood, eg, arsenic.

NES note: With the exception of samples for total organic carbon (a measure of organic matter in the water) and bromide measurements, which are taken from untreated water, all sampling done during the P2 Programme is from treated water. Some of these samples are for the analysis of contaminants that could originate in the catchment, but the concentrations of the contaminants may have been modified by passage through the treatment plant.

Once the possible hazards have been identified from the first stage of the assessment, they are included in a list of chemicals for monitoring. Monitoring samples are collected twice during the assessment of a supply (once in early spring and once in late summer).

Contaminants that exceed 50% of their MAV in any sample taken during an assessment are recommended to the Ministry of Health for assignment to the supply as a Priority 2 contaminant. This means the water supplier is required to monitor them if they are to comply with the DWSNZ. Each water supplier is informed of these recommendations, and given the opportunity to contest any recommendations before the assignments are made official. Priority 2 contaminants are only officially assigned to supplies of more than 500 people,22 therefore it is only these supplies that need to undertake compliance monitoring.

Hard copies of the results of water analyses from the P2 Programme are provided to district health boards. It is standard practice for copies of the test results to be forwarded to the water supplier.

NES note: When identifying possible contaminants that could arise from a catchment activity, particular attention needs to be paid to Priority 2 contaminants already assigned to the water supply. An increase in their concentration from a new activity may result in their concentration exceeding the MAV and the water becoming unsafe, because of the concentration already being close to the MAV. Of course, attention also needs to be paid to contaminants that could be introduced into the water by an activity, but which are present at low concentrations or are not known to be already in the source water.

8.2 Types of Priority 2 contaminants

Priority 2 contaminants are of two types:

  • P2a contaminants: These are contaminants introduced into the water in treatment chemicals and are not relevant to an NES assessment.

  • P2b contaminants: All other chemical contaminants are of this type. Some may originate from the catchment and are therefore important for the NES; others may arise from sources after treatment, such as dissolution of pipe materials, and these are not important for the NES. There are two subcategories of P2b contaminants:

  • Type 1 – chemical contaminants that are unlikely to change in concentration after the water leaves the treatment plant

  • Type 2 – chemical contaminants that may change in concentration following treatment.23

Priority 2 contaminants are assigned either to the treatment plant or the distribution zone; this determines the location at which monitoring samples should be taken. P2b Type 1 contaminants may be assigned to the treatment plant or the distribution zone, and can be monitored at whichever location is more convenient. P2b Type 2 contaminants are assigned to the distribution zone and must be monitored there.

NES note: A check should be made for Priority 2 contaminants assigned to the treatment plant and the distribution zone. Contaminants in the source water may be assigned to the distribution zone rather than the treatment plant. For example, manganese originates in the catchment, but because its concentration may change during distribution (precipitation and settling) monitoring will be required in the distribution zone, not at the treatment plant. Nitrate also originates in the catchment, but it does not change in concentration after the treatment plant, and could therefore be monitored at the treatment plant or in the distribution zone. If the water supplier has other Priority 2 contaminants to monitor in the distribution zone, they may also monitor nitrate there.

From the perspective of implementing the NES, the assignment of a Priority 2 contaminant to the distribution zone only raises concerns about the ability of the treatment plant to remove the contaminant from the water if the contaminant is associated with the source water, ie, it is present in the source water or formed from something in the source water. Except for corrosion-derived metals, contaminants monitored in the distribution system are usually associated with the source water.24

8.3 Compliance criteria for Priority 2 contaminants

With the exception of fluoride, chlorine and the cyanotoxins, all Priority 2 contaminants must be monitored at least monthly. Fluoride and chlorine require weekly monitoring, and the cyanotoxins must be monitored twice weekly during algal blooms. Priority 2 contaminant monitoring is ongoing until 12 consecutive monthly samples have been shown to contain the contaminant at a concentration of 50% of its MAV or less. Once this is shown, the water supplier can ask their drinking-water assessor for the contaminant to be reclassified as a Priority 3 contaminant, and monitoring ceases.

Compliance for all chemical contaminants requires that they not exceed their MAV. As with microbial contaminants, and operational parameters, an MAV transgression can occur without resulting in non-compliance if sufficient samples have been taken in which the contaminant did not transgress the MAV (see section 4.2.3 and Appendix 1). In practice, an MAV transgression effectively results in non-compliance, because samples for chemical contaminants are usually only collected 12 times a year, and a minimum of 78 samples must be taken for a single transgression to be allowed (see Appendix 1).

For most chemical contaminants, the health effect of an individual contaminant is what is considered in assessing whether its concentration represents a threat to health. A comparison of the contaminant’s concentration against its MAV shows whether it is at an unacceptably high concentration. However, there are some chemically related groups of contaminants that have similar health effects, eg, nitrate and nitrite; and several disinfection by-product families, namely trihalomethanes, haloacetic acids and the haloacetonitriles. For these groups, the combined health effect of the group has to be considered. This is done through the following equation:

where A, B, C, ... are members of the group of contaminants being considered, [A] is the concentration of contaminant A, MAVA is the MAV of the contaminant A, etc.

If the summed ratio exceeds 1, the group of contaminants as a whole transgresses the MAV. Taking account of the possible health effects of a family of contaminants as a whole can result in a group transgression, without the transgression of any individual compound.

As with other contaminants, a Ministry of Health-recognised laboratory must undertake the measurements using a method that meets the requirements of the DWSNZ, and remedial actions must be taken in the event of a transgression. An appropriate corrective action must be taken if a chemical transgression occurs. Unlike a microbial transgression, when pathogens may rapidly cause sickness, a rapid response to a chemical transgression is not required unless the contaminant is a cyanotoxin.

8.4 Comments on specific chemical contaminants

8.4.1 Disinfection by-products

Disinfection by-products are formed during treatment through the reaction of the disinfectant, eg, chlorine, with organic matter in the water. Their suspected health effects include cancer, and liver and kidney damage.

Although disinfection by-products are not present in the source water, organic matter in the source water that is not removed by the first treatment processes will react with the disinfectant when it is added to the water. The levels of organic matter in the source water can therefore influence the levels of disinfection by-products in the treated water. Disinfection by-products are not monitored at the treatment plant because they continue to form after treatment25 with the result that their concentrations increase as the water moves from the treatment plant out into the distribution system.

The extent of disinfection by-product formation can be reduced by reducing the concentration of organic matter reaching the point of disinfectant addition in the treatment plant. Organic matter can be removed from the water by some treatment processes, eg, the combination of coagulation/flocculation, clarification and sand filtration.

NES note: Unless a catchment activity is discharging disinfected water, or industrial solvents (several of the disinfection by-products are also used industrially) into the source water, the source will not contain disinfection by-products. Catchment activities are more likely to increase disinfection by-product formation by increasing the loading of organic matter in the water. This can be mitigated to some extent by treatment processes operating before disinfection that remove organic matter from the water. The degree of organics removal by these processes is variable, and dependent on how well the process is being operated, but in the most common process (coagulation/flocculation) removal is about 70% at best.

Naturally-occurring organic substances that result from the decay of vegetation and animal remains are the usual precursors to disinfection by-product formation. These are large complex molecules. Other substances containing similar structures can also lead to the formation of disinfection by-products. As a rule of thumb, activities likely to increase the colour (yellow-brown) of the water or the total organic carbon concentration are likely to increase disinfection by-product formation. An example is activity that increases run-off from a peaty or swampy area.

8.4.2 Heavy metals

The chemical contaminants occurring most frequently in New Zealand drinking waters are heavy metals derived from the corrosion of metals in contact with the water in the distribution system. This problem is widespread because of the low alkalinity and softness of New Zealand’s source waters. The DWSNZ therefore classify all water supplies as “plumbosolvent”, ie, they will dissolve metals, and particularly lead from plumbing fittings.

Monitoring is not required for metals released into the water by the dissolution of consumers’ plumbing fittings because of the water’s plumbosolvency. Metals in the water supplied to consumers that are present at more than 50% of their MAV, however, are classified as Priority 2 contaminants and must be monitored for compliance with the DWSNZ.

NES note: Very few water supplies contain heavy metals originating at the source; plumbosolvency is the main reason for the presence of metals in the water people drink. (A few supplies still have heavy metals assigned as Priority 2 contaminants that arise from corrosion of consumers’ plumbing. These are gradually being identified and reclassified as Priority 3 contaminants so that monitoring is not required.)

Activities in catchments where there have been, or still are, mining operations, metal industries, or operations employing geothermal water could introduce heavy metals into the source water. If their concentrations were high enough they would be classified as Priority 2 contaminants, and their monitoring would be required.


22 The P2 Programme collects information about Priority 2 contaminants from supplies for 500 people or fewer. No official Priority 2 contaminant assignments are made for these supplies, but the monitoring results make smaller supplies aware of chemical contaminants in their water without burdening them with having to monitor official Priority 2 assignments.

23 Some contaminants in the source water, or formed through reaction with constituents of the source water, may change in concentration after the treatment plant. Manganese, for example, may continue to precipitate after the treatment plant and settle out in parts of the distribution system. Disinfection by-products (see section 8.4.1) will tend to increase in concentration throughout the distribution system because of continuing slow reactions between the chlorine and organic matter in the water in the pipes. Where a water supplier has been required to monitor these contaminants, monitoring will have occurred in the distribution zone, not at the treatment plant. However, the results of monitoring in the distribution zone may be relevant to understanding source water contaminants in some situations.

24 The exceptions are metals that could arise from corrosion in the distribution system, chemicals used in the joints or seals of water pipes, chemicals arising from leaking underground fuel tanks in the community, and chemicals that might be drawn into the water supply by back-siphoning from industrial or agricultural operations. Contamination of water in the distribution system from back-siphoning or underground fuel tanks is direct pollution. It does not mean that pollution of the source water is occurring.

25 This occurs because of continuing slow reactions between the disinfectant and the organic matter.