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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

4 Origins of Contaminants in Drinking Water Sources

4.1 Introduction

To determine the effect of a planned activity on the quality of water produced by a drinking water treatment plant, the likely contaminants reaching the plant’s abstraction point need to be identified. To comply with the NES, the consent applicant will have to estimate the extent to which the levels of these contaminants will exceed the levels already challenging the treatment plant.

When assessing the ability of a treatment plant to produce acceptable water, the levels of contaminants in source water already challenging the treatment plant need to be taken into account. If existing levels are high, the expected additional input of a contaminant may be sufficient to exceed the treatment plant’s capabilities and the contaminant’s concentration may exceed its maximum acceptable value (MAV). The existing levels of microbial contaminants may be difficult to assess because of the limited information about their concentrations in source waters. For chemical contaminants, the priority status¹¹ will help in making this evaluation. For a contaminant to be given Priority 2 classification there must be evidence of it exceeding 50% of its MAV (which may include exceeding the MAV itself).¹² Therefore, a Priority 2 contaminant in a water supply is at least halfway to exceeding its MAV and in some cases may already exceed its MAV. If the MAV is not already exceeded, a relatively small increase in the amount of the contaminant in the source water may lead to the MAV being exceeded.

To make these estimates, the applicant will need to know which contaminants the activity may introduce into the environment, and the factors that will influence the contaminant concentration(s) finally reaching the abstraction point.

This section provides some guidance on the contaminants that might be associated with particular activities and advice on what needs to be considered in estimating the contaminant concentrations reaching the abstraction point. The information is generic and is not a substitute for a full assessment of the potential contaminants associated with a particular activity.

4.2 Factors to consider when assessing the likelihood of contamination

4.2.1 Introduction

The presence of a particular activity in a catchment, or recharge zone, does not necessarily mean that the activity will lead to an unacceptable level of pollution of the source water. By taking account of the factors discussed in the following subsections, an indication of which contaminants might reach a water supply’s abstraction point at a concentration of concern can be gained. Figure 1 shows how the factors that influence the concentration of contaminants reaching a treatment plant are linked.

4.2.2 Nature of contaminants the activity may introduce into the environment

As a first step, the contaminants that are likely to arise from a planned activity need to be identified. Table A1 (Appendix 1) provides a guide to possible contaminants from a range of activities. It is not exhaustive with regard to activities or with regard to contaminants from the listed activities, ie, for some activities there may be some additional contaminants not listed. Further, it is possible that some of the contaminants listed for an activity may not arise from it at a level of concern.

4.2.3 Contaminant loads released by the activity

The starting point for evaluating how a planned activity will affect a treatment plant’s source water is to establish the load (weight/unit time) of the contaminant that will result from the activity. This alone will not determine the concentration (weight/volume) of the contaminant reaching the treatment plant’s abstraction point, and other information discussed in the following sub-sections will be required.

For point source discharges, the parameters that influence the volume of discharge and the substances the discharge can be expected to contain will be known and should allow the load of contaminant to be estimated.

Estimation of a contaminant load from non-point source activities is not as straightforward, because the contaminants and the levels they are released into the environment may not be well characterised. Trying to identify the different factors that contribute to the contaminant load will help. For example, an estimation of the contaminant load from grazing animals should be possible on the basis of the number of animals, their daily manure and urine output, and the expected concentration of the contaminant in the manure or urine.

4.2.4 Pathways by which contaminants can be transmitted to the source water

Irrespective of the amount of contaminant released by an activity, it will not cause pollution of the source water unless there is some pathway by which the contaminants can reach the receiving water. The pathway for some landfills, for example, may not exist because of the landfill being lined to stop contaminants reaching the groundwater. There may be situations in which there is uncertainty about the existence of a pathway. In these circumstances, the existence of a pathway should be assumed until there is reasonable evidence that one does not exist. Planned preventive measures, such as lining a landfill, should be reasonable evidence of a pathway having been blocked or removed, unless shortcomings in the measures are evident from the proposal.

4.2.5 Processes that may attenuate contaminants before they reach the source water

Some degree of attenuation may occur as contaminants pass over land and/or through water. The extent of attenuation will be highly variable, depending on the nature of the discharge and environmental conditions.

After pathways have been identified, processes should be considered that may attenuate the concentrations of contaminants as they move along the pathway to the source water. Again, it is unlikely that the effects of these processes¹³ can be quantified, but there are factors that affect them that can help in deciding how important they might be.

The effectiveness of attenuating processes is increased if the time over which they act is increased, or they can act over a longer distance. Greater attenuation of the contaminant therefore occurs as the distance between the activity and the source water is increased. On the other hand, a steep landscape increases water flow into the source water. This reduces the time available for some processes, such as microbial die-off and chemical decomposition and reduces attenuation. Similarly, porous soils and substrata allow water to percolate rapidly into groundwater.

4.2.6 Processes that may attenuate contaminants in the source water

Even when the contaminants have reached the source water, there are still processes acting to reduce their concentration. These include:

• initial dilution by the receiving water (the source water)

• sedimentation as particles, including Cryptosporidium oocysts, settle to the river or stream bottom (Note that micro-organism in the sediment have been found to survive longer than those in the overlying water. Hence a later increase in river flow because of rain can resuspend organisms deposited in the sediment and add to their concentration in the source water.)

• adsorption

• inactivation (this applies to micro-organisms and is the result of the exposure to sunlight)

• dilution by tributaries.

Resuspension of sediment can increase the concentrations of contaminants reaching the abstraction point.

The following may help in estimating the degree of contaminant attenuation that occurs once the contaminants reach the source water:

1. the greater the flow of water in a source (river or stream), the greater the dilution of the contaminant and therefore the lower the contaminant concentration at the abstraction point
2. the more tributaries that enter the source (or if a source is a tributary to a larger river), the greater the dilution and the lower the contaminant concentration reaching the abstraction point
3. more turbulent flows will minimise the opportunity for contaminants to settle to the bottom of the river or stream, which will tend to maintain their concentration in the water
4. the greater the distance between the activity and the abstraction point, the greater the opportunity for processes such as sedimentation, die-off, chemical decomposition and inactivation by sunlight to take place, so reducing contaminant concentrations.

4.2.7 The consequences of rainfall or irrigation

Water, either as rain or from irrigation, is usually the vehicle by which contaminants are moved along pathways to source waters. Some of the consequences of rainfall are noted in section 5.4.4.2.

Rainfall can reduce contaminant concentrations through dilution, but overall rainfall needs to be viewed as a factor that will increase contaminant concentrations in the source water. The increase in concentration will occur for a limited period, but the water treatment plant must be able to produce safe drinking water during this time. The assessment of a new catchment activity needs to consider the ‘worst-case’ level of contamination by trying to evaluate how a rain event will affect pollution of the source water. Where the planning for an activity includes measures to mitigate the effect of rainfall, the risk created by the activity will be reduced.

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