2.1 Structure of water supplies
The DWSNZ divide water supplies into three components: source, treatment plant and distribution system (Figure 2). All water supplies that have been identified and recorded in the Ministry of Health’s Register of Community Water Supplies (www.drinkingwater.org.nz) have at least one of each of these components. Each is assigned a unique identifying code. The treatment plant and distribution system have different compliance criteria. The following subsections describe each of these components.
There are three main source types:
surface water – eg, streams, rivers, lakes, or reservoirs
groundwater – water drawn from bores or wells
roof catchment – rain water collected on roofs and stored for later use.6
Often water suppliers have limited control over activities in the catchment (or the recharge zone of a groundwater source), and therefore over the quality of the water they have to treat.
2.1.2 Treatment plant
Treatment plants may range from large operations that consist of a series of treatment processes running under automated control, through small plants using a single treatment process with manual control, to a pump drawing water from a source (usually groundwater) without any treatment. Where there is no treatment, the borehead is regarded as the treatment plant by the DWSNZ.
2.1.3 Distribution system
The distribution system carries water from the treatment plant to the consumers. The DWSNZ define the distribution system as the pipes, water storage facilities (tanks or reservoirs) and any other components situated between the treatment plant and consumers’ property boundary. Storage facilities at the treatment plant are considered to be part of the treatment plant.
Distribution systems can be divided into separate distribution zones. Each distribution zone is a part of the distribution system in which consumers should receive water of the same quality.
This diagram shows how water from the "Source" (eg, a river) enters a "Treatment Plant", from which it then enters the "Distribution System". In this figure the distribution system has two "Distribution Zones", which supply different groups of houses.
2.2 Drinking-water Standards for New Zealand
The primary purpose of the DWSNZ is to protect public health by explaining how to assess the quality and safety of drinking water.
They do this by providing two types of information:
Water quality standards,7 which define the maximum concentrations of contaminants that are acceptable in safe drinking water. This is done in the form of maximum acceptable values (MAVs). An MAV is the maximum concentration of a contaminant (microbes or chemicals) in drinking water that will not make consumers ill even if they drink the water all their lives.8 MAVs provide a yardstick by which the safety of drinking water can be judged. Water is safe to drink as long as none of the contaminants it contains exceeds its MAV. MAVs apply to treated waters.
Compliance criteria specify how a water supplier is to show that its water quality standards are being met. It is the responsibility of the water supplier to show that their water supply complies with the DWSNZ.
Compliance criteria make up most of the DWSNZ and can be complicated. The reader is not expected to be familiar with the detail of compliance requirements. The purpose of providing information about compliance criteria is to give an overview of processes associated with the DWSNZ, and to assist understanding of how to apply the NES to a particular consent application.
The DWSNZ can apply to drinking water supplies of any size, irrespective of whether they are public or private. They specify requirements for ensuring drinking water is safe, while minimising unnecessary monitoring.
When this guide was prepared, compliance with the DWSNZ was voluntary. No law currently requires water supplies to comply with the DWSNZ, although the Building Act 2004 (section 123 (c)) does require buildings to be supplied with potable water. Despite this, most water suppliers aim to meet the requirements of the DWSNZ.9
2.2.2 MAVs and source water quality
The concentrations of contaminants in source waters are not required to be less than their MAV (although some groundwaters may be of this quality), but contaminant concentrations in source waters should be minimised as much as practicable. MAVs should not be considered as a “pollute up to” limit. This is an important principle in protecting public health. An increase in contaminant level in the source water, or a drop in the effectiveness of a treatment process, will readily result in the MAV being exceeded if the MAV is the target level for water quality.
6 Roof catchments are mentioned for completeness, but they are rarely a water source that services communities. The catchment activities of concern for this type of source (ie, roof water) are those introducing contaminants into the air, or encouraging the congregation of birds, eg, landfill sites.
7 Although the DWSNZ use the word “standards”, the term does not provide any legal status to these values.
8 MAVs for most chemical contaminants that are suspected of causing cancer represent a risk of one additional incidence of cancer per 100,000 people drinking two litres of water containing the contaminant at the concentration of the MAV for 70 years. MAVs for non-carcinogenic chemicals are based on doses with no observed health effects (animal data for some MAVs and human data for others).
E. coli does not cause sickness. Its MAV is based on the observation that waters with E. coli below the MAV of one organism per 100 mL rarely result in waterborne disease. The other microbial MAV (for protozoa) is set for regulatory purposes. It is not derived from a dose/response relation.
9 As presently drafted, the proposed Health [Drinking-water] Act Amendment Bill will require all water suppliers to take all practicable steps to comply with the DWSNZ.