Swan Canning Water Quality Improvement Plan

Key findings:
Annually 250 tonnes of nitrogen and 26 tonnes of
phosphorus enter the system. Numerical modelling
was used to quantify nutrient loads by source and
use in the Swan Canning Catchment.
Based on predictive modelling the maximum
acceptable load to the Swan and Canning rivers per
year is 130 tonnes of total nitrogen (TN) and
14 tonnes of total phosphorus (TP).
To meet these objectives the SCWQIP aims to:
• reduce the nitrogen load by 120 tonnes per year
(49%); and
• reduce the phosphorus load by 12 tonnes
per year (46%).
Other key findings of the SCWQIP, based on
predictive modelling, are as follows.
1. The Avon River contributes a large proportion of
the nutrient load (69% TN and 43% TP). While
management of the Avon Catchment is outside
the scope of this SCWQIP, the magnitude of
the nutrient contribution highlights that it is
imperative to reduce this input to improve water
quality in the Swan and Canning rivers.
2. Of the Swan Canning sub-catchments Ellen
Brook contributes the most nutrients with
70 tonnes of nitrogen (28% TN) and 10 tonnes
of phosphorus (39% TP) per year.
3. The main source of phosphorus in the Swan
Canning Catchment is farming activities (33%
TP), predominantly beef cattle grazing in the
Ellen Book sub-catchment.
4. The main source of nitrogen is residential (29%)
and recreational (14%) activity in urban subcatchments,
in particular fertiliser application on
grassed areas and gardens. It is also the second
highest source of phosphorus (22% and 12% TP).
5. Flow from coastal sub-catchments including Ellen
Brook has been equivalent to the Avon River in
recent years. Urban sub-catchments are the main
source of nutrients in summer when agricultural
catchments are not flowing.
6. Septic tanks contribute significant amounts of
nitrogen (18% TN) and phosphorus (8% TP) to
the Swan Canning river system. This percentage
is even higher in some sub-catchments.
7. Increased urbanisation will increase nutrient loads
by 18% TN and 25% TP, due to increased runoff.
8. The effects of climate change, modelled over
a 10-year period on the highest CO2 output,
reduces nutrient load by 15% TN and 31% TP.
Modelled on the lowest CO2 output, the effects
of climate change will reduce phosphorus load
by 5% and the nitrogen load by 3%.

swan_canning_water_quality_improvement_plan_final_dec_2009