MCU14/0106 Appendix E - Engineering Report

August 2004 -15- 04682104(AB)

 

7.5 ASS Extent and Severity

The results from this investigation indicate that there is a negligible probability of ASS having a
potential to generate sulfidic leachate where the proposed development extends below RL 5 m
AHD. However, there is generally a slight to moderate existing acidity variably distributed
throughout the soil profile.

The SPP2/02 Guideline requires that the level of treatment for management of excavated ASS is
based on treatment of all existing and potential acidity. The results of the laboratory testing
have been accumulated in an Acid-Base Account to give a Net Acidity Value for each sample in
units of mol H”/tonne as presented in Table C1. This value has been calculated from sulfur trail
potential acidity (Sc, or Spos) plus actual acidity (TAA).

Assuming a bulk density of 1.7 tonne/m’ (for the sandy soils likely to be excavated), and using
the maximum TAA value recorded from test pits GA16 and GA17 (GA18 is located beneath a
proposed fill area) the indicated liming rate for excavated soil is 10 kg.Lime/m’.

It is our assessment that filling for the proposed development should not cause an environmental
impact due to mobilisation of existing acidity. The fill utilised for the development will be
predominantly clayey soils providing a low permeability capping that will restrict rainfall
infiltration and therefore limit the ground water flux.

In summary, the investigation has identified that there is a negligible probability for the
occurrence of PASS material. The existing acidity identified is within both sandy and clayey
soils at a level unlikely to be disturbed by filling. It is therefore considered that there should be
no requirement for specific management of existing or potential acidity within soils in the fill
area. Excavated soil from below RL 5 m AHD (eg. for pond / basin construction) will require
lime treatment to neutralise existing acidity (ref. Section 7.6).

7.6 Recommendations

Based upon the foregoing, the anticipated minimal volume of disturbance and the level of
treatment required indicates that management of existing acidity (not PASS) at this site would
be classified as Category H (high level) treatment in accordance with SPP2/02 Guideline - Table

4 (between 1 and 5 tonne of aglime required),

SPP2/02 Guideline requires that for Category H treatment, the management should ensure a
number of strategies as follows.

> Treat all soil excavated from below RL 5 m AHD ata rate of 10 kg lime/m’.
> Manage site runoff through bunding.

> Ensure that lime is thoroughly mixed with the soil.

Golder Associates