Att 9 - Geotech and PASS report

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8.9    Acid Sulfate Soils

       8.9.1 What are Acid Sulfate Soils

Acid sulfate soils (ASS) are naturally occurring sediments and soils containing iron sulfides or sulfidic
materials, often located over extensive low-lying coastal areas, predominately below RL 5 m AHD.
These soils are generally found close to the natural ground surface, but may also be found at depth
within the soil profile.

ASS includes actual acid sulfate soils (AASS) and potential acid sulfate soils (PASS). AASS and
PASS are often found within the same soil profile, with AASS generally overlying PASS. AASS are
highly acidic resulting from the oxidation of soil materials that contained sulfides. PASS are soils that
contain iron sulfides or sulfidic materials, which have not yet been oxidised.

       8.9.2 Criteria for Evaluation of ASS

The criteria on which the results of screening tests (pH f and pHfox) were assessed as indicative of
possible actual acid sulfate soils (AASS) or potential acid sulfate soils (PASS) were based on the
QASSIT Guidelines as follows:
     pHf of less than 4 indicates oxidation of sulfides has probably occurred in the past, indicating the
      presence of actual acidity (AASS). No samples returned pHf values of 4 or less.
     pHf between 4 and 5.5 indicates the soils are acidic. This may be as a result of limited oxidation
      of sulfides, but may also be as a consequence of the presence of organic acids. Two samples in
      the clayey sand filling from Bore 4 returned pHf values of 4.9 and 5.0.
     pHf of greater than 5.5 indicates the soil has little or no actual acidity; however this can be
      common with PASS materials or marine-influenced samples. The remainder of the samples
      tested returned pHf results greater than 5.5.

The pHf test method does not detect acidity bound within sulfides; however the pHfox test gives an
indication of any potential acid release. Results of the pHfox are summarised as follows:
     pHfox of less than 3, plus a pHfox reading at least one pH unit below pHf, plus a strong reaction
      with peroxide, strongly indicates the presence of PASS. Samples tested below 1 m depth in Bore
      1, and the majority of samples tested in the filling and the natural sands from in Bores 3 and 4
      returned pHfox values of less than 3.
     pHfox between 3 and 4 indicates the possibility of PASS material and a pHfox between 4 and 5
      may indicate the presence of small amounts of sulfide or fine carbonates. Intermittent samples
      from Bores 3 and 4 returned pHfox values of between 3 and 4. Samples tested in the upper 1 m
      from Bore 1 and all of the samples tested in Bore 2 returned pHfox values of between 6 and 7.4.

To provide confirmation of the above qualitative testing, quantitative analytical testing was carried out
on selected samples, generally with the greatest difference in pHf and pHfox readings and strongest
reaction using the Chromium Suite method.




Geotechnical Investigation, Proposed Commercial Development                             90151.00.R.001.docx
11-19 Hilton Terrace, Tewantin                                                                    May 2017