Compliance with the DWSNZ is designed to ensure that the concentrations of contaminants in a water supply do not exceed their MAV, and consequently that the water is safe to drink.
As well as health-significant contaminants, source waters may also contain constituents that:
are not damaging to health in themselves and therefore have no specified MAV, but can affect the water’s potability by leading to the formation of other contaminants
affect the aesthetic properties of the water (taste, odour, appearance).
These constituents 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 constituents as by those with MAVs.
10.2 Contaminants that affect potability
At least three types of contaminant exist that have no health significance in themselves, but can affect the potability of water because they can result in treated water containing contaminants that are a health concern. These are:
nutrients, which encourage the development of algal blooms (section 7.3)
organic matter, which leads to the formation of disinfection by-products (section 8.4.1)
turbidity, which can affect the ability of treatment plants to remove protozoa from the water (section 6.2.2).
These have already been discussed in the sections noted.
10.3 Contaminants that affect the aesthetic properties of a water
The constituents of a water that affect its taste, odour and appearance, collectively called the aesthetic properties of the water, are important in determining the acceptability of the water. Problems with a water’s aesthetic properties usually become evident to consumers more rapidly than MAV transgressions by contaminants that affect health.
The World Health Organization Guidelines for Drinking-water Quality (3rd edition, 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 contains a list of guideline values for constituents of water that can influence its aesthetic properties (Appendix 2, DWSNZ). The DWSNZ use the term “wholesome” to describe water that is potable and meets these guidelines. Waters that meet the guidelines are expected to be acceptable to consumers with respect to taste, odour or appearance. Meeting the guidelines is not a requirement for compliance with the DWSNZ; achieving them is at the discretion of the water supplier.
Water can be treated to improve its aesthetic properties, although this is not always straightforward. The removal of cloudiness from water (ie, the turbidity) is achieved by the processes used for removing protozoa (eg, coagulation/flocculation, sedimentation and filtration in combination). Tastes and odours caused by contaminants produced by some micro-organisms (eg, algae and actinomycetes – a type of bacteria) however, are detectable by humans at very low concentrations. Treatment of water to remove these compounds to acceptable levels is difficult and expensive. Other causes of undesirable tastes and odours (eg, sulphides and metals) are easier to treat.
NES note: Awareness of the possible effects of a new catchment activity on the aesthetic properties of a source water is important because of the potential difficulty in making the water aesthetically acceptable, and the impact that an aesthetically unacceptable water will have on those having to drink it. A community cannot be expected to drink water that is unpalatable, even if it is safe.
Monitoring of the aesthetic properties of drinking water is not undertaken as part of the P2 Programme. Any monitoring of this type is done by water suppliers. The two most likely reasons for them undertaking this monitoring are to:
address complaints about the water quality from their consumers
achieve a high (A1) public health grade.27
Monitoring water quality to address consumers’ complaints will occur when a complaint arises. As it is designed to address a particular concern, the monitoring programme is likely to be of a relatively short duration (until the problem is identified and a solution found), unless it is an on-going problem that the water supplier is having difficulty overcoming. Sampling in response to complaints will occur more frequently than monitoring for health significant contaminants, and samples may be taken from the source water and/or following treatment, depending on where the investigation leads. The contaminants included in the testing will be determined by the nature of the complaint.
The public health grading carried out by public health units for the Ministry of Health, is described more fully in ESR Report FW0778 section 3.6. A grade, ranging from A1 at the top to E at the bottom, is given to the source and treatment plant combined. To obtain an A1 grade, the water produced by the treatment plant must meet the guideline values for the aesthetic properties of the water given in the DWSNZ (its table A3.5).
The Ministry of Health publication, Public Health Grading of Community Drinking-water Supplies 2003 – Explanatory Notes and Grading Forms, Appendix B, specifies how a water supplier is to show that the guideline values have been met. A minimum of one sample is taken per year to allow analysis of all constituents. The frequency of any further sampling is determined by the nature of constituents of concern,28 and is given in a table in the explanatory notes.
When this guide was being prepared, only two treatment plants had A1 grades, but others may be collecting information to try to attain the grade.