ResponsetoApplicantReport

4. Water quality management particularly addressing potential
impacts from use of shed waste and composting material for
fertilising the orchard and any associated run off.

Comments by Peter Edmiston, Bio Track Pty Ltd

The effect on runoff water quality can be estimated as:

Loading (kg nutrient)/year    x   export proportion in runoff = kg nutrient/year
in runoff

There is 1030 m3 with a density of ~0.9 = 900,000 kg compost

If the compost has 2% nitrogen (need to measure this to confirm the value) then
the N load is 18000 kg/year. (Published range (attached) is 2.4-4.2% N so the
total nitrogen load may be up to double this value).
The disposal area is 38 ha so the average loading rate is 473 kg/ha/y and 4 times
this rate if applied to 25% of the disposal area.


Sugar cane farms use a lot of nitrogen (300-400 kg/ha/y). The rule of thumb is
1/3 is lost (drainage, runoff, volatilisation). Sugar cane has a high biomass
growth per year with a lot of crop either exported as green cane or burnt with the
stems used for sugar. Avocado crop removal is modest by comparison.

Therefore a loss of 1/3 for fertilised avocados on sloping ground in a high
rainfall area is reasonable.

The third can be partitioned into drainage losses, runoff and volatilisation.

Assuming the soils are not saturated for very long and the N release is gradual
then volatilisation losses will be modest, 20% would be an upper estimate.
Depending on the slope, placement and rainfall intensity runoff losses will be in
the range of 40% to 60% with the remaining lost as deep drainage.

So:

0.33 lost x 0.6 x 0.8 runoff x 18,000= 2850 kg/y runoff loss & 1900 kg/y drainage
loss
0.33 lost x 0.4 x 0.8 runoff x 18,000= 1900 kg/y runoff loss & 2850 kg/y drainage
loss

The drainage loss is to the groundwater. This forms base flow to local waterways.

The application area is thought to be 38 ha and the runoff losses are estimated to
be in the range of 93 to 62 kg N/ha/y with deep drainage losses in the same
range.