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An Introduction to Drinking Water Contaminants, Treatment and Management

1 Introduction

2 Roles and Responsibilities in the Production of Drinking Water

3 Water Supply Management – Information Sources

4 Origins of Contaminants in Drinking Water Sources

5 Water Treatment

Appendix 1: Activities and the Contaminants That May Contribute to Source Waters

Appendix 2: Treatment Processes and Their Efficacies

3 Water Supply Management – Information Sources

3.1 Introduction

Section 2 has introduced those with major roles in the production and management of drinking water. Each of these players may produce or hold information regarding water supplies that could assist in evaluating whether a new catchment activity will meet the requirements of the NES. This section outlines the nature of the information available and from whom it can be obtained.

3.2 Treatment plant operators

Treatment plant operators undertake the day-to-day running of the water treatment plant, and are a source of detailed information about the operation of their water supply. To obtain information from the treatment plant operators, go through the water supply manager, who should be able to provide additional information about the operation of the water supply. If you do not know who the water supply manager is, call the local authority and ask to be put through to the water supply manager. When the water supply is not operated by a local authority, contact the public health unit and ask to be put through to a drinking water assessor, who might be able to help you contact the water supply of interest.

The operator may be able to provide the following information, depending on their level of training and experience with the treatment plant:

• the monitoring undertaken at the treatment plant, which may include samples taken for compliance purposes (eg, Escherichia coli) and operational monitoring of parameters, such as turbidity and acidity/alkalinity (pH)

• advice on how well the treatment plant copes with increases in turbidity in the source water, and the level of turbidity increase that might be tolerated without compromising the treatment plant’s ability to comply with the DWSNZ

• activities in the catchment that already contribute to the contaminants that must be removed by the treatment plant

• the degree of removal of common chemical contaminants, eg, iron and manganese, provided they have already been identified as a problem and there are treatment processes in place to remove them

• how the flow rate or level of the source water at the abstraction point affects the water quality.

It will be more difficult for operators to estimate the ability of their treatment plant to remove a new contaminant that their plant is not specifically designed to remove, and for which they have not previously needed to carry out treatment. For example, if contamination of the water with cyanide is a possible consequence of a proposed catchment activity, the operator may be unable to estimate the extent to which the existing treatment processes will remove cyanide. In such a situation, an independent consulting engineer could be approached for advice on the likely extent of removal of the specific contaminant.

3.3 Water Information New Zealand

Water Information New Zealand (WINZ) is a national database of information on water supply management and water quality. It is maintained by ESR on behalf of the Ministry of Health. For further information, refer to section 12 of the companion publication A Guide to the Ministry of Health Drinking-water Standards for New Zealand.

3.4 Annual Review of Drinking Water

The Annual Review of Drinking-water Quality in New Zealand provides an overview of drinking water quality in New Zealand. For further information, refer to section 11 of the companion publication A Guide to the Ministry of Health Drinking-water Standards for New Zealand.

3.5 Public Health Risk Management Plans

3.5.1 Introduction

Plans to manage risk to water supplies are called Public Health Risk Management Plans (PHRMPs) by the Ministry of Health. The ministry strongly encourages water suppliers to use risk management planning as the basis for the management of their supplies. Some aspects of compliance with the DWSNZ refer to PHRMPs.

Previously, monitoring water quality has been used as the basis for water supply management. Reliance only on the periodic sampling and analysis of water is a poor approach to ensuring that safe water is always provided to a community. A water sample only provides information about the quality of the water at the particular time the sample was taken; the quality of the water between samples remains uncertain. The aim of the risk management approach to water supply operation is to give the water supplier (and consumers) confidence that the supply can provide consistently safe water. Risk management identifies potential problems and addresses them before poor water quality becomes apparent by water testing. The risk management approach to protecting water quality is proactive. When a water supplier relies solely on water testing to ensure safe water, the approach is reactive, and may result in consumers receiving unsafe water before test results are available. The risk-based approach to water supply management does not dispense with water tests; monitoring water quality is still necessary to check that the steps taken to protect water quality are working.

3.5.2 Useful information associated with PHRMP preparation

The Ministry of Health has prepared a suite of ‘guides’ to help water suppliers identify risks to their supply and determine how these might be managed. One of these guides may be helpful to those who have to work with the NES: s.1.1 Surface and Groundwater sources (Version 2)⁷ addresses the risks associated with catchment activities.

Section 1.1 Surface and Groundwater sources (Version 2) identifies:

• what are termed ‘events’ that may threaten the quality of the source water

• the possible causes of these events

• what can be done to reduce the likelihood of these events occurring (preventive measures)

• checks to ensure the preventive measures are working

• what to do if the events occur despite the preventive measures.

This information, of course, is provided to help a water supplier. It may be of limited value to someone working with the NES. This is partly because the guide was produced before the advent of the NES, and many of the preventive measures contained in the guide are actions now encapsulated in the approach being taken by the NES.

One other aspect of PHRMP preparation that may be helpful for NES implementation is the ‘Improvement Schedule’. The Ministry of Health requires PHRMPs to contain a list of improvements that have been identified through the risk assessment process as being necessary to ensure the safety of the water supplied to consumers. This list is the ‘Improvement Schedule’ and may help those implementing the NES because it will state when any upgrades that may assist in contaminant removal are planned.

3.6 Public health grading of water supplies

3.6.1 Introduction

Public health grading of water supplies has been undertaken since 1993. Its purpose is:

to provide a public statement of the extent to which a community drinking-water supply achieves and can ensure a consistently safe and wholesome product.

Two types of information are needed to do this:

• water quality monitoring data to show that the production of ‘safe and wholesome’ water is being achieved, which requires the grading to take some account of the compliance status of the water supply

• information that identifies risks to water quality and shows how well these are being managed, which helps to assess whether consistently good water quality is likely to be supplied.

The 1993 grading system was revised in 2003. From 1 January 2006, the only grades shown in the Register of Community Drinking-water Supplies in New Zealand (the Register)⁸ are those that have been undertaken based on the 2003 grading system. So far, only a relatively small number of supplies have been re-graded. Only those water supplies with populations of over 500 people are graded, although supplies serving populations down to 25 people are expected to be graded in the future.

The simple way in which the public health grade is expressed makes it readily understood by everyone. Consequently, poor grades in some supplies have given rise to public pressure for improvements to be made. Local authorities can be very concerned about the grade they receive.

3.6.2 How a supply grade is determined

The information used to base the grade for a water supply is collected using three questionnaires: one each for the source, treatment plant, and distribution zone. This information is gathered by drinking water assessors who visit each water supply for which they are responsible, and they reach agreement with the water supplier about responses to the grading questionnaires. A combination of the information from the source and treatment plant questionnaire is used to provide a joint source/plant grade, and a separate grade is determined for the distribution zone.

3.6.2.1 Source/plant grading

The source questionnaire seeks information about the nature of the source and in general terms, the types and level of contamination that may affect it. The information sought is:

• source type (eg, stream, lake, spring)

• security status of groundwater sources (yes/no)

• protection status of the catchment⁹ (yes/no)

• catchment condition (4-point scale)

• degree of human pollution (4-point scale)

• degree of animal pollution (5-point scale)

• degree of chemical pollution (3-point scale)

• median E. coli concentration (where available)

• occurrence of algal blooms (yes/no).

When the supply is graded, the details of catchment activities considered by the drinking water assessor and water supplier are distilled down to brief generic responses for the questionnaire, and are not captured by the grading process.

Environmental Science and Research Ltd (ESR)¹⁰ can provide the information that is captured by the grading process for supplies that have been graded.

Activities in a catchment that are granted consents may influence the responses to the questionnaire and therefore the grade of a supply. Although information about the likelihood of contamination of the source water is presently collected, it makes little contribution to the final grade for supplies that are adequately treated. The likelihood of contamination is only taken into account when the Plant Questionnaire shows an unsatisfactory level of treatment plant performance. Future revision of the grading system is likely to place greater emphasis on PHRMPs and how risks from catchment activities are being managed.

The source/plant grade is determined through information gathered about the treatment plant and its compliance with the DWSNZ. The treatment processes used in the treatment plant are identified, but the grade depends mostly upon the extent of compliance with the DWSNZ at the treatment plant with respect to bacteria, protozoa and chemicals, and the level of control, supervision and record keeping at the plant.

The source/plant grade is designated in capital letters, and will lie in the range from ‘A1’ (top) to ‘E’ (bottom). The descriptions of each grade are given in Table 1.

3.6.2.2 Distribution zone grading

The distribution zone grade is determined by considering many different aspects of the distribution zone. These include DWSNZ compliance and factors that may increase or decrease the level of risk to consumer health. Like the source/plant grade, the grades run from ‘a1’ to ‘e’, and are designated in lower case to distinguish them from the source/plant grade. Their descriptions are given in Table 1. The grade obtained for the distribution zone depends on how well the water supplier has managed the water supply, and on microbiological and chemical compliance with the DWSNZ.

The distribution zone grade can be affected by activities in the catchment. One of the factors that determine the distribution zone grade is compliance with the DWSNZ. If a new activity in the catchment introduces contaminants that cannot be adequately removed by the treatment plant, their presence in the distribution zone may result in non-compliance with the DWSNZ and a reduction in grade.

Table 1: Descriptions of public health grades

3.6.3 Where to find information about public health grading of water supplies

The Register is produced each year by the Ministry of Health and is sent to all public libraries in the country. All registered water supplies are contained in this document and where a supply has been graded, the register lists its source/plant and distribution zone grades.

The source and treatment plant information collected by the grading assessment, which will be helpful in understanding the likely levels of source water contamination and the treatment capabilities of a particular treatment plant, is not recorded in the Register. This information is maintained within WINZ, and can be provided by ESR, or the public health unit with responsibility for grading particular supplies.

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