B07056.W-01A Complete - Stormwater [As Published] · 132007.2398

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village



6.0           STORMWATER QUALITY
The introduction of impervious surfaces to the site has the potential to increase the export of pollutants in runoff
and cause adverse impacts to downstream waterways if not managed appropriately. Measures therefore need
to be implemented to minimise any pollutant export from the development.

Reference has been made to Noosa Council’s Cooroy and Lake Macdonald Zone Map ZM3 (February 2006) that
shows runoff from the site flows to a tributary of Six Mile Creek. This tributary then enters Six Mile Creek that
flows to the Mary River Catchment Zone Map ZM5 (February 2006). Please note, despite the site being on the
Cooroy and Lake Macdonald Zone Map ZM3, runoff from the site does not drain to Lake Macdonald.

This section of the report identifies:
      •     key pollutants associated with the development;
      •     appropriate water quality objectives (WQO’s); and
      •     stormwater quality improvement devices (SQID’s) to achieve the WQO’s.

Computer modelling of the adopted SQID’s and presentation of the results are also provided in this section.

6.1                   Pollutants of Concern / Water Quality Objectives
6.1.1                 Regional Water Quality Guidelines

The site appears to have been cleared of most vegetation some time ago and used for agricultural purposes.
Portions of the site have been recently used as an orchard. It is unknown if any pesticides, insecticides or
fertilisers have been applied to the site in the past. However, agricultural purposes and the possibility of
chemical application, leads to the likelihood that higher pollutant loads compared with loads from a natural state.
Due to the fact that the site is not in a natural ‘undeveloped’ condition and other reasoning below, it is considered
that concentration values listed in Queensland Water Quality Guidelines (2006) (QWQG) cannot appropriately be
applied to this development.

Reference has been made to the QWQG to determine the physico-chemical indicators applicable to the water
type of the catchment. Reference to Section 2.5.1.2 of these guidelines shows that the specific waterway for the
subject site is not specified in Table 2.5.1.2. The guidelines then refer the user to the default regional water
quality guidelines for slightly to moderately disturbed waters (Table 2.5.1.1). According to Figure 2.5.1c of the
QWQG, the site falls within the ‘lowland freshwater’ water type. Table 2.5.1.1 outlines the guideline values for
the receiving waters. The guideline values listed in the QWQG are reproduced in Table 6.1 below.

                       Table 6.1: South-East Queensland Regional Water Quality Guideline Values*

          Pollutant Total Suspended Solids (mg/L)         Total Phosphorous (mg/L)       Total Nitrogen (mg/L)

          Objective                  6                                0.050                       0.500
                                            *Concentrations given are median values.

In-situ ambient water quality concentrations (such as those presented in Table 6.1) conventionally relate to the
long term water quality in the receiving water, such as the Mary River which is the main downstream water body.
Concentrations of discharges from the site however, are highly variable, depending on the response of the
catchments to rainfall. Healthy Waterways (2006) argues that selection of pollutant concentrations for
stormwater discharges is therefore problematic. Engineers Australia (2006) states that ambient water quality


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