Acid Sulfate Soil/Rock/Mine Drainage

There are National Guidelines for Acid Mine/Rock Drainage (AMD/ARD), Acid Sulfate Soils[1]  (ASS) and Contaminated Land Management (CLM).

In order to assess a site for development you need to know what the scale of site disturbance is going to be before you determine the amount of actual/potential ASS (AASS/PASS) you need to deal with. Ideally if you are not disturbing a site e.g., draining or exposing soil/sediment materials, then if there is AASS/PASS or AMD/ARD, there is no reason to assess the site, however once you drain groundwater or excavate/dredge/mine the surface then the volume and area of air exposure (oxidation of iron sulfide (pyrite) to form sulfuric acid) will determine the level of site assessment.

FeS2(Py/ferrous S) + 3.75O2 + 3.5H20 -> Fe(OH)3(Ferric OH- orange ppt) + 2H2SO4

In the first instance, anyone can carry out a Stage 1 Desktop assessment to gain an indication of the likely (risk) presence of ASS e.g., the local government authority (council) planning scheme provides publicly available maps (overlays) that show likely broad areas of ASS (or see CSIRO ASRIS[2]) maps. If you’re lucky, the surrounding areas may have already been assessed before development and that information may be available on public record. However, it is only with a Stage 2 Site Inspection conducted by a suitably qualified and experienced person (SQEP: qualifications and membership) that a true picture of the site’s potential for ASS can be determined e.g., low lying coastal plain.

An initial site inspection of an unexposed site may simply be to dig a few well-placed holes and see what happens before getting into a proper full-blown soil and water sample collection for lab analysis. You’ll still need a pH meter and electrode (charged and calibrated), 30% hydrogen peroxide (H2O2) pH adjusted to 4.5–5.5 for pHFOX tests and 1 M hydrochloric (HCl) acid for shell presence. After carefully adding H2O2 you’ll get a better indication of ASS or with HCl for buffering by CaCO3. A few initial giveaways are the presence of brightly coloured staining/mottling and ‘rotten egg’ gas. The texture of soils, e.g., clay-silt-sand and peats, is critical in determining net acidity and thresholds.

Stage 3 Detailed soil sampling and testing is required once ASS has been indicated and is the basis for a full-blown ASS Management Plan that in itself is an Environmental Approval in its own right.

But make sure you secure a SQEP before you do anything!

[1] National Acid Sulfate Soils Guidance: National acid sulfate soils sampling and identification methods manual

[2] Australian Soil Resource Information System: Atlas of Australian Acid Sulfate Soils