Acid Sulphate Investigation · OPW19/0098

August 2003 -3- 03683042(AA)

 

5.0 ‘GEOLOGY & SUBSURFACE CONDITIONS

Available geological information! indicates that the site is underlain by the late Triassic to
Jurassic age Tiaro Coal Measures comprising “.-sandstone, siltstone, shale and coal”.

The subsurface conditions encountered in the boreholes comprise a consistent profile of
mostly grey silty/clayey sand overlying light coloured, high plasticity residual silty clay to the
depth of investigation. The clay was noted to be tending to weathered siltstone below about
1.5 m depth in boreholes BH2 and BH4. Refusal on cobbles (probably ferruginous) was
encountered at 1 m depth in borehole BHS.

Ground water was not encountered in the boreholes during drilling. Based upon our
experience in similar geology, the permanent ground water table is probably located at least
5 m below the ground surface in the vicinity of the site.

6.0 COMMENTS & RECOMMENDATIONS

The development of Potential Acid Sulfate Soils (PASS) is commonly the result of marine
and estuarine deposition of sulfate and iron bearing sediments in the presence of an abundant
source of readily decomposable organic matter resulting in the deposition of pyrite. This
pyrite is stable within the soil so long as anoxic conditions prevail. Oxidation of this material
produces acidic conditions and oxidation typically occurs as the material is exposed above the
water table, either by excavation or by water table draw-down. :

Previous experience and available guidelines indicate that (PASS) are normally restricted in
extent to recent (Holocene to Pleistocene age) soil horizons deposited in a saline environment
below RL 5 m. The topography of the north-eastern part of this site is consistent with the
above criteria. Our interpretation of the geology (late Triassic to Jurassic residual strata)
indicates a low probability of acid sulfate soils occurrence.

Published information’ classifies the part of the site above RL 5 m as having a “low or
negligible risk of ASS occurrence”, with land below RL 5 m being “where ASS occurs within
5 m of the ground surface”. It should be noted that the available ASS map was based upon
assessment of broadscale information and that no ‘ground truthing’ (ie. drilling of boreholes)
was conducted within the area below RL 5 m north to the shore of Lake Doonella.

6.1 Field pH (pH;) and Peroxide pH (pHrox)

A typical consequence of the oxidation of pyrite in soils and the formation of sulfuric acid is a
drop in soil pH. Measuring field pH (pH;) as soon as possible after sampling and then
measuring pH following oxidation of the soil in suspension with a strong oxidising agent
(hydrogen peroxide) provides a simple, relatively conservative test procedure to detect soils
which are not likely to be acid sulfate soils. The test simulates extreme natural conditions and
pH drops following this test may not be entirely attributable to pyrite oxidation.

' 1:100,000 Geological Map, GYMPIE SPECIAL, Sheet 9445 & Part 9545, DME, 1999
2 1:100,000 Redcliffe to Teewah ASS Map 2, DNR, 1999.

Golder Associates