Reference Document OPW19 0098 Acid Sulphate Investigation · OPW19/0098

August 2003 -6- 03683042(AA)

 

The following observations are made regarding the results in Table 2:

¢ The residual in situ soils at this site are moderately acidic in their natural state. The pH
drops following oxidation (pHyox) for the samples less than 1.0 unit, apart from the near
surface samples in boreholes BH1, BH2 and BHS. These reductions in pH following
oxidation are. inferred to be due to the presence of organic matter resulting in the release
of organic acid upon oxidation. ,

e The Scg results for nine of the ten samples tested are 0.00%, with one result of 0.01%
(0.25 m depth @BHS). All Scr results are significantly less than the Action Criteria (ie.
0.1%). ‘

e TAA values range from 14 mol H+/t up to a maximum of 244 mol H+/t at 1.0 m depth in
borehole BH2. Eight of the ten samples tested exceed the acidity Action Criteria of
62 mol H+/t.

6.3 Extent and Severity

The results from this investigation indicate that there is a negligible probability that acid
sulfate soils or acidic ground water having the potential to cause environmental harm exist
within the proposed development area.

However, the actual acidity values of the soil, together with the natural pH results, indicate
that the site soils contain existing acidity. Given the geology of the site and lack of sulfidic
content within the soils, the existing acidity is interpreted to be the result of the deep leaching
and weathering of the residual strata, rather than being an indicator of the presence of Actual
Acid Sulfate Soils. ,

Although the site does not contain acid sulfate soils, the State Guidelines require that any
existing acidity above the relevant Action Criteria requires treatment for neutralisation. A
maximum lime treatment at a rate of about 31 kg/m? was calculated for the clay sample
recovered from | m depth in borehole BH2. Existing acidity values (TAA) recorded for the
sand samples are less than the Action Criteria.

6.4 Recommendations

We believe that the results of this investigation confirm that there is a negligible risk of acid
sulfate soils (and acid ground water) occurrence within the proposed development area.
However, due to the presence of existing, non-sulfidic acidity within the clayey soils, lime
treatment of these soils at a rate of about 30 kg/m’ will be necessary where they are
excavated. It will therefore be necessary to include this requirement in the Environmental
Management Plan (EMP) for the development.

Golder Associates