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Scoring Method

Overview

The Yarra River and Port Phillip Bay Report Card was developed to report on the environmental health of Port Phillip Bay and its associated catchments using water quality indicators. For information on recreational health of waterways see Yarra Watch and Beach Report programs.

Key indicators of water quality, sampled at each of the 102 catchment and 8 marine sites, are used to calculate an overall water quality score for each site in the Report Card. The scores are calculated based on the level of attainment of the relevant indicators against the environmental water quality objectives in the State environment protection policy (Waters of Victoria) (SEPP (WoV)) and its Schedule F6 (Waters of Port Phillip Bay). Water quality scores are determined for financial years from July to June to ensure they include data over a complete summer period.

The water quality indicators used for catchment sites are: nutrients, water clarity (turbidity), dissolved oxygen, salinity (conductivity), pH and metals; with the exception of estuarine sites, where the salinity indicator is not used.

The indicators used for Port Phillip Bay sites are: nutrients (nitrogen), water clarity (total suspended solids), dissolved oxygen, salinity and algae (chlorophyll-a). Metals were also used for the bay sites where available. Due to the influence of freshwater inflows salinity is not used at the Newport site in Port Phillip Bay.

For both the catchments and the Bay, if an indicator is not measured at a site then it is not used in this assessment.

SEPP (WoV) sets statewide environmental quality objective that apply to all catchments in the Report Card. However, the Yarra Catchment also has specific environmental quality objectives for seven different segments within the catchment under Schedule F7 of SEPP (WoV).

For consistency in comparing water quality scores across the sites in all catchments, only the SEPP (WoV) environmental quality objectives were used to assess the sites.

Method Summary

The following steps are taken to calculate the report card scores:

At each site, the individual water quality indicators are calculated from annual monitoring data, using the relevant statistic that applies to each indicator in SEPP (WoV). These results are then compared to the SEPP (WoV) environmental quality objectives that apply at each site.
  • From the above comparison against SEPP (WoV) environmental quality objectives, each indicator is given a rating on how modified it is from reference/natural state.
  • • Some water quality indicators have multiple parameters that can be assessed against environmental quality objectives (e.g. nitrogen and phosphorus for the nutrients indicator). In calculating the above rating, these parameters are combined to give a single rating for that indicator, which is then used in the calculation of the water quality score as below:
    • For the nutrient indicator in catchment sites, ratings are calculated for both phosphorus and nitrogen. The final nutrient rating used to calculate the water quality score is whichever of the two has the worst rating. For the Bay, nitrogen is the only nutrient used to calculate the rating as nitrogen is the controlling (limiting) nutrient for algal growth in the Bay.
    • For the pH indicator at catchment sites, the rating is generated for both the upper and lower limit objectives. The final pH rating used to calculate the water quality score is whichever of the two has the worst rating.
    • Seven metals are routinely measured at most sites in the catchments and the Bay. A single rating is derived for metals based on the total number of times the toxicant environmental quality objectives in SEPP (WoV) are exceeded across all seven metals. For this rating, a worse score is assigned if one particular metal is found to be responsible for the majority of exceedances. This method highlights the one metal that is very toxic as opposed to a spread of slightly toxic metals.
  • The ratings assigned to each indicator are summed and normalised to produce a water quality score out of 10 that corresponds to a rating of Very Poor to Very Good, as illustrated in the following table.
  • The scores are summarised at the catchment and bay level by considering the area of the sub-catchments and marine zones that each site or sites represents and calculating area-weighted average scores.
  • Key of scoring

    Water Quality Index

    To aggregate the ratings from the different indices into the one water quality index for each site the following equation was used:

    Water Quality Index Formula

    where n is the number of indicators at the site and Ii is the indicator value at the site.

    To calculate the water quality index for each sub-catchment area or marine zone, the arithmetic mean of the index from all sites within the area is used. At the catchment or bay level, the water quality index and indicator ratings are the area-weighted mean of the sub-catchments or marine zone.

    Indicators used in Report Card

    Indicator Catchment Parameter Marine Parameter
    Nutrients Total Nitrogen Total Nitrogen
    Total Phosphorus
    Water Clarity Turbidity Suspended Solids
    Salinity Electrical Conductivity Salinity
    Dissolved Oxygen Dissolved Oxygen % Saturation Dissolved Oxygen % Saturation
    pH pH
    Metals Arsenic, Cadmium, Chromium, Copper, Lead, Nickel and Zinc
    Algae Chlorophyll-a

    Further information

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      Program Partners

      Department of Environment and Primary Industries Environmental Protection Agency Melbourne Water