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1 Introduction

The National Environmental Standard (NES) for Sources of Human Drinking Water comes into force in June 2008. In implementing the NES,1 regional councils and resource consent applicants will need to understand the key factors that determine whether water is safe to drink. The Drinking-water Standards for New Zealand (DWSNZ) are an important resource for doing so. The present guide provides regional council staff and resource consent applicants with the information they need about the DWSNZ, when they are assessing whether a new activity in a catchment may compromise the ability of a water treatment plant to produce safe water.

This guide will assist those who have had little previous involvement with water quality in relation to public health. It is a “beginner’s guide” to understanding those parts of the DWSNZ considered to be most relevant to the implementation of the NES. Reading this guide will not provide an expert understanding of the DWSNZ. The DWSNZ themselves, in conjunction with their companion document, Guidelines for Drinking-water Quality Management in New Zealand, should be consulted if the reader needs a more detailed understanding of the DWSNZ. Drinking-water assessors2 at the public health units of the local district health board can also provide assistance.

The structure of this guide follows the order in which the topics of interest are presented in the DWSNZ:

  • overview of the DWSNZ

  • water quality standards

  • general compliance concerns at the treatment plant

  • how compliance is assessed for the main contaminant groups: bacteria, protozoa, cyanotoxins, chemicals, and radioactive contaminants.

It also discusses contaminants in the source water that could lead to the presence of other contaminants in the treated water, and constituents of the water that can affect its taste, smell or appearance.

The final two sections of this guide discuss two sources of information about supplies: the Annual Review of Drinking-water Quality in New Zealand and the Water Information New Zealand (WINZ) database. These sources provide information about supply water sources, the nature of treatment processes used in the treatment plants, and the extent of their compliance with the DWSNZ.

Where there is an important implication for implementation of the NES, this is discussed in an “NES note” box.

1.1 Water supply management and the multiple barrier principle

Water quality monitoring involving the collection and analysis of water samples, has been the backbone of water supply3 management for many years. Relying solely on monitoring as the basis for water supply management, however, is a poor defence against water-borne contaminants. Unless monitoring is continuous, results always provide historical water quality information because of the time taken to analyse samples.4 Consequently, consumers may have been receiving contaminated water for some time before a water supplier learns of contamination of the supply. Furthermore, each water test provides only a snapshot of the water quality, which is limited to the contaminants tested. The quality of water between testing is unknown.

Recently, the principles and techniques of risk management have been introduced into the management of water supplies. A risk-based approach to supply management requires water suppliers to identify what might go wrong with each part of their supply and to reduce the likelihood of these things happening. Monitoring is not dispensed with and is still necessary to check that the precautions taken by each water supplier are actually working. This approach gives water suppliers much greater confidence that they are providing water of consistent quality to their consumers.

The multiple barrier principle is internationally recognised as a cornerstone in managing risk in water supplies. The use of more than one barrier is encouraged in the DWSNZ. The presence of more than one barrier between water consumers and possible sources of pollution means that consumers are less likely to receive contaminated water. So, if one barrier fails then there are other barriers in place to protect consumers. Key barriers include:

  • protection of source water from contamination, eg, fencing of rivers or streams so that animals cannot get direct access to the water source; this reduces the range and concentrations of contaminants that have to be dealt with by the water treatment plant

  • treatment plant processes:

  • filtration improves water quality by removing particles

  • disinfection follows the particle removal steps and inactivates5 disease-causing micro-organisms (pathogens)

  • protection of the water after treatment so that it is not re-contaminated (eg, ensuring there is some chlorine in all the pipes between the treatment plant and consumers, and undertaking regular checks to make sure there are no leaks in the pipes).

  • protection of the water after treatment so that it is not re-contaminated (eg, ensuring there is some chlorine in all the pipes between the treatment plant and consumers, and undertaking regular checks to make sure there are no leaks in the pipes).

Figure 1: Elements of the multiple barrier principle

This flow diagram illustrates several barriers that should be in place to protect drinking water using the multiple barrier principle. Firstly, at the level of the catchment, various "Measures to prevent contaminants entering source water" should be in place. Water then enters the "Treatment plant" where additional barriers are in place: (1) "Processes to remove particles (including protozoa)" and (2) "Processes to inactivate viruses and bacteria". After treatment, water enters the "Distribution zone", where it is further protected from "Contaminants" by "Measures to prevent re-entry of contaminants".

Protection of the source water is possibly the most important barrier because it reduces the contaminant load that later barriers have to remove. The reason for the development of the National Environmental Standard (NES) for human drinking water sources is to strengthen this barrier.

1 Throughout this guide, the abbreviation NES refers to the National Environmental Standard for sources of human drinking water.

2 Drinking-water assessors are health protection officers who have received additional training in water supply management and treatment, and water supply regulations. Part of their responsibility is assessing the compliance of water supplies with the DWSNZ.

3 A water supply is a system for providing consumers with safe drinking water. It consists of three components: a source, treatment plant and distribution system (section 2.1). Whenever the term is used in this document it includes all three components together.

4 E. coli analysis takes around 24 hours; chemical analyses take minutes or hours, but usually have to be batched for economy, and results may take weeks to be reported. The delay in microbiological analyses is of greater concern because the onset of illness caused by microbes is more rapid than illness caused by chemicals.

5 The term “inactivate” rather than “kill” is used, because disinfection processes may stop an organism from being able to reproduce, without killing it. In this condition they are still viable and capable of revival.