Preliminary Geotechnical Investigation · OPW21/0315

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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.

       7.5.2 Criteria for Evaluation of Acid Sulfate Soils

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). None of the samples tested returned a pHf value of less than
     4.
•    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. Typically
     samples of the natural sands tested returned pHf values of between 4.8 and 5.4.
•    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. Generally the samples of sand fill
     tested returned pHf values of 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. All of the natural sand samples tested
     returned pHfox values of less than 3 and generally with a reaction to peroxide.
•    pHfox between 3 and 4 indicates the possibility of PASS material and a pH fox between 4 and 5
     may indicate the presence of small amounts of sulfide or fine carbonates. The samples of sand fill
     tested returned pHfox values of between 3 and 4.6.

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.
•    The chromium reducible sulfur values (SCR) where greater than 0.01 %S indicates a significant
     level of sulfide, and where greater than 0.03 %S then the soil has a potential acidity level
     exceeding the QASSIT guideline value. Samples of the natural sands tested returned SCR values
     of 0.02% indicating levels of potential sulfur.
•    Where the total actual acidity (TAA) value exceeds 18 mol/t then the soil has an acidity level
     exceeding the QASSIT guideline value. Three of the samples tested returned TAA values greater
     than 18 mol/t indicating the presence of acidity.
•    Where the sulfur in KCl extract values (SKCl) is greater than 0.01 %S, then the soils acidity is likely
     to be a result of sulfide oxidation. None of the samples tested returned a SKCl value equal to or
     greater than 0.01 %S.




Proposed Residence                                                                       95120.00.R.001.docx
33 Wyuna Drive, Noosaville                                                                      October 2020