Water pollution discharge assessments

Guidance for industry and practitioners on the relevant statutory, policy, and technical framework to consider when conducting a water pollution discharge impact assessment.

The Protection of the Environment Operations Act 1997 (POEO Act) provides the statutory framework for managing water pollution in NSW. The definition of ‘water pollution’ in the POEO Act sets out general and specific circumstances that constitute pollution. The Protection of the Environment Operations (General) Regulation 2022 (Schedule 5) also includes a list of specific substances (prescribed matter) which, if they are introduced onto or into waters, are automatically taken to constitute pollution of waters.

The main ways that the EPA regulates water pollution include environment protection licences and environment protection notices.

Section 45 of the POEO Act sets out the matters we must consider when exercising our licensing functions, which include

  • the pollution that will be caused and its impact on the environment (section 45(c))
  • practical measures that can be taken to prevent, control, abate or mitigate the pollution and protect the environment from harm (section 45 (d))
  • the environmental values of water affected by the proposed discharge and the practical measures that can be taken to restore or maintain those values (section 45(f1))

The environmental values of water are defined in the Dictionary of the POEO Act as those specified in the Australian and New Guidelines for Fresh and Marine Water Quality 2018 (the national Water Quality Guidelines (WQGs)). The NSW Government has adopted the National Water Quality Management Strategy (NWQMS) and national WQGs as the policy and technical framework to manage and assess water pollution.

The EPA's policy is that water pollution should be avoided. When that is not possible, it is the responsibility of the proponent undertaking the works or activity to conduct an assessment to consider the potential impact on receiving waters. If the impacts are unacceptable, mitigation measures that prevent or minimise impacts on water quality should be implemented.

The level of assessment required will depend on a range of case- and site-specific circumstances.

The EPA recommends consulting the guidance as part of a licence application or licence variation application noting the conditions related to discharge to waters. The practices and principles can also be used to address the relevant environmental assessment requirement for submissions to NSW Planning on major projects.  

Steps in conducting a water pollution impact assessment

flowchart of assessment process described in text below 1 Consider the level of assessment required  2  Assess the likely impact of the discharge then two options 3 Environmental values restored or maintained go to 5 Consideration and negotiation of discharge criteria second option 3 Potential impact to environmental values 4 4. Use mixing zone 5  Consideration and negotiation of discharge criteria

  1. Consider the level of assessment required

The level of assessment should reflect the level of risk to the environmental values of a waterway.

The environmental values of water are the values or uses of the environment that are important for a healthy ecosystem and require protection from the effects of pollution.

The risks to the environmental values will vary depending on the characteristics of the waterway and its sensitivity to change, the extent and duration of the activity, and the pollution likely to be caused.

Where the risk of water pollution is low, a simple qualitative assessment may be adequate to consider potential risks and demonstrate that those risks are being appropriately managed and mitigated.

For example, a water balance could show that by reusing collected runoff for dust suppression, stormwater runoff will remain on site and a discharge is highly unlikely.  Therefore the environmental values of the waterway won’t be impacted by that particular proposal.

Some activities present a higher risk to the environmental values of a waterway and therefore require a more detailed assessment considering one or more of the following

  • a quantitative assessment of indicators and guideline values in the discharge and receiving waters
  • a quantitative prediction of the level of impact a discharge will have on the environmental values  under conditions ranging from typical to worst-case scenario
  • if the use of a mixing zone is proposed, quantification of the extent of the mixing zone and the potential impacts within the mixing zone
  • practical measures that will be implemented to restore or maintain the environmental values of water.

The assessment might also need to consider load related impacts, for example, where discharges are proposed over a longer period and/or to sensitive receiving environments. Biological assessments may also be appropriate.

In all cases, site and case-specific circumstances will need to be considered and the level of assessment and proposed mitigation measures should be commensurate with the risks.

  1. Assess the likely impact of the discharge

  1. Identify the environmental values

The NSW Water Quality Objectives (WQOs) are the community’s agreed values and uses (environmental values) of waterways for all NSW catchments

The environmental values for waterways can be found for each catchment on the NSW Water Quality Objectives website.

Consistent with the national WQGs, stakeholder engagement can also be used to identify community values and identify more specific management goals.

The EPA uses the WQOs to identify the environmental values that apply to a waterway. When conducting the assessment, the guiding principles are that

  • where the environmental values are being achieved, they should be protected, and
  • where the environmental values are not being achieved, all activities should work towards their achievement over time.

Contemporary ambient water quality data can be used to establish if the relevant environmental values are being achieved or if improvements in water quality are required to restore them.

  1. Assign a protection level to the waterway

Different levels of protection are appropriate for different waterways. The national WQGs identify three levels of protection

  • high conservation value
  • slightly to moderately disturbed
  • highly disturbed.

Most waterways in NSW are identified as slightly to moderately disturbed. Examples are waterways in an urbanised or rural landscape.

Waterways that flow through relatively undisturbed National Parks, World Heritage areas, Marine Parks or wetlands of outstanding ecological significance are identified as being of high conservation value. Discharges into these waterways should ensure the waterways remain relatively undisturbed, and there is no change to the natural variability in water quality.

It is not acceptable to allow poor environmental performance or water pollution simply because a waterway is currently degraded.

In a highly disturbed waterway, such as an industrialised port, a reduced level of protection can sometimes be appropriate as a pragmatic short-term goal, with the aim of eventually restoring it to a slightly to moderately disturbed status.

This is the appropriate level of protection where there is a community expectation for this level of waterway health and action can be taken over time to reduce the impacts on a degraded waterway.

  1. Choose appropriate indicators

For each environmental value of water, the WQOs outline common issues or pressures that may result in environmental values not being achieved. Key indicators measure whether the environmental values are at risk. The national WQG lists the indicators.

The choice of appropriate indicators is based on

  • the key threats to the environmental values of the waterway
  • the pollutants likely to be generated by the activity and the likelihood of those pollutants impacting the environmental values of the waterway.

  1. Identify the relevant guideline values

For each indicator, the national WQGs provide default guideline values or guideline ranges but also recognise that waters vary naturally at a local scale and provide a methodology to help derive site-specific guideline values for local conditions.

Site-specific values are the preferred option for physical and chemical stressors such as electrical conductivity, pH and nutrients. For toxicants it is preferable to use local biological effects (including direct toxicity assessments). If local biological effects data are unavailable, the national WQGs provide default guideline values for different levels of ecosystem protection.

Site-specific studies to tailor the guideline values must demonstrate consistency with the methodology in the national WQGs and prior agreement to the approach and study design should be obtained from the EPA.

Where waterways have naturally elevated suspended sediment or metal levels (eg due to the natural geology), site-specific guideline values may be appropriate. It is not appropriate to derive site-specific guideline values where the pollutant is elevated due to human disturbance (eg where metals concentrations are elevated due to urban runoff).

Guideline values represent the level of a pollutant where there is a low risk to the environmental value, however, it should not be inferred that the waterway can be polluted up to this level. Measures to avoid and minimise pollution must also be considered and implemented.

  1. Identify measures to minimise or mitigate impacts to the environmental values

Where a discharge is unavoidable, the EPA requires the licensee to demonstrate that all reasonable and practical measures have been implemented to minimise or mitigate the pollution and restore or maintain the environmental values of the waterway.

The practical measures will vary according to case and site-specific factors, including

  • the type and scale of activity
  • site configuration, size, and topography
  • the local climate (particularly rainfall and evaporation characteristics)
  • the proximity of the work to receiving waters
  • which measures are most cost-effective in each case to meet the environmental obligations.

Practical measures to prevent, control, abate or mitigate water pollution could include

  • disposal of polluted water at a licensed facility or discharge to sewer through a trade waste agreement
  • increased capture such as enlarged sediment basins or wastewater ponds
  • minimising or preventing water ingress to waste stockpiles or processing areas through water diversions and covering areas
  • increased reuse and recycling, including dust suppression or irrigation to land
  • enhanced erosion and sediment controls
  • use of first flush systems or best available water treatment technologies.

The EPA will not authorise, but way of licence conditions, a discharge into waters unless all reasonable and practical measures to prevent, control, abate or mitigate pollution have been undertaken.

The EPA expects all licensees to implement a reasonable level of performance even if this secures higher water quality outcomes than the NSW WQOs. This prevents waters being polluted up to the limits, keeps maximum opportunity for present and future users, and allows adoption of a precautionary approach when there is uncertainty about the impacts of an activity.

  1. Assess the potential impact of the discharge

The assessment should demonstrate that

  • where the environmental values are currently being achieved in a waterway, they will be protected; and
  • where the environmental values are not currently being achieved in a waterway, the proposed discharge will contribute towards their achievement over time.

The impacts of a discharge should be predicted using a technique that is appropriate for the circumstances and level of risk.

For example, where a discharge is to a complex or sensitive waterway, appropriate modelling techniques should be used to predict the ambient water quality outcomes. Where a discharge is likely to contain pollutants at trivial levels and is into a waterway with a substantial volume of water where rapid mixing occurs, a simple dilution assessment could be appropriate.

The time-frame for works and likely frequency of discharges should also be considered. For example, a short-term road construction project that is expected to discharge sediment infrequently would generally be expected to have an impact that is less significant than longer term, more frequent discharges. This should, however, always be considered in the context of the potential impact of users of the waterway.

Where the measured or predicted level of an indicator is below the default guideline value or within the desirable range, the risk to the relevant environmental value is likely to be low. Where an indicator is higher than the default guideline value or outside the desirable range, there may be a risk to that environmental value. 

The numerical guideline values are not directly applied as regulatory discharge criteria, limits or conditions. In some cases, the requirements derived may be the same as the default or site-specific guideline values, for example where there is no dilution available at the discharge point.

  1. Are the environmental values restored or maintained?

Where the environmental values of a waterway are not being achieved and the discharge will not contribute towards their restoration, the assessment should consider the key causes and any operational changes or further practical measures to minimise or mitigate the identified impacts.

The discharge should not prevent the environmental values being achieved in the future.

The nature and extent of practical measures should reflect the level of risk posed to a waterway. For example, a road which is constructed adjacent to a sensitive receiving environment or in areas of high erosion risk could warrant enhanced erosion and sediment controls.

Once all practical management and mitigation measures have been considered and, where reasonable, implemented, the likely impact of the discharge should be reassessed.

  1. Use of a mixing zone

A mixing zone is an area around a discharge where certain environmental values are not protected. The actions of dilution, assimilation or decay of the discharge allow the environmental values to be met at the edge of the near-field mixing zone. The near field mixing zone is limited to the area where dilution is driven by the action of buoyancy and momentum. The use of a mixing zone does not imply that environmental values will not be protected over significant areas of a waterway.

The EPA's policy is that the NSW WQOs should be met at the edge of the area where initial mixing occurs.

A proposal to discharge to receiving waters using a mixing zone should include at a minimum:

  • discharge water characteristics including the quality and rate of the discharge
  • characteristics of the receiving waters including physical characteristics (eg depth of receiving waters), chemical and biological characteristics
  • the environmental values for the receiving waters; and
  • the predicted water quality in the receiving waterway during discharges under typical through to worst case conditions. This should demonstrate that the relevant guideline values and ranges will be achieved at the edge of the near field mixing zone.

The type and extent of modelling required to consider use of a mixing zone should be consistent with the level of risk, including the nature of the receiving environment, the environmental values, and the pollutants being discharged.

The set of model scenarios should include the range of variations in the discharge quality and quantity as well as the relevant range of environmental conditions in the waterway (eg variations due to tides, currents, seasonal temperatures, storm events). The scenarios should describe a set of worst-case conditions (from an environmental perspective) as well as typical or average conditions. The frequency of each of these scenarios should be calculated.

  1. Consideration of the discharge impact assessment

Once the risks are appropriately characterised and the potential impacts understood, the EPA can consider the proposed discharge and, if appropriate, proposed discharge limits for any pollutants that have the potential to cause harm.

The EPA has compiled this Guidance in good faith, exercising all due care and attention. No representation is made about the accuracy, completeness, or suitability of the information in this publication for any particular purposes. The EPA shall not be liable for any damage which may occur to any person or organization taking action or not on the basis of this publication. Readers should seek appropriate advice when applying the information to their specific needs. This Guide may be subject to revision without notice and readers should ensure they are using the latest version.

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