Acid Sulphate Soils Guideline

Draft Identification and investigation of acid sulfate soils and groundwater

Introduction
Investigations
Using Lab Results
Groundwater
Further Information
References
App. 1: Risk Map
App. 2: Indicators
App. 3: Soil pH

App. 4: Action Criteria

Draft Identification and investigation of acid sulfate soils and groundwater

APPENDIX 4: Texture-based ASS "Action Criteria"

The Action Criteria are based on the sum of existing plus potential acidity, calculated as equivalent sulfur (e.g. s-TAA + S_CR in %S units) or equivalent acidity (e.g. TAA + a- S_CR in mol H+/tonne). The highest laboratory result(s) is always used to assess against the action criteria. For further information refer to Guidelines for Sampling and Analysis of Lowland Acid Sulfate Soils (ASS) in Queensland 1998.

As clay content tends to influence a soil's natural pH buffering capacity, the action criteria are grouped by three broad texture categories - coarse, medium and fine. The criteria are used to define when ASS disturbed at a site will need to be treated and managed.

For projects that disturb ≥1000 tonnes of ASS with ≤0.03 %S or ≤18 mol H+/tonne equivalent acidity, a detailed management plan and development consent will be required.

Texture-based acid sulfate soils 'action criteria'

Type of material Action Criteria if 1-1000 tonnes of materials is disturbed
Existing + Potential Acidity Action Criteria if >1000 tonnes of materials is disturbed
Existing + Potential Acidity
Texture range McDonald et al. (1990) Approx. clay content (%) Equivalent sulfur (%S) (oven-dry basis) Equivalent Acidity (mol H+/tonne) (oven-dry basis) Equivalent sulfur (%S) (oven-dry basis) Equivalent Acidity (mol H+/ tonne) (oven-dry basis)
Coarse Texture
Sands to Loamy sands ≤5 0.03 18 0.03 18
Medium Texture
Sandy loams to light clays 5 - 40 0.06 36 0.03 18
Fine Texture
Medium to Heavy clays and silty clays > 40 0.1 62 0.03 18

The action criteria refer to existing and potential acidity for given volume of ASS. The highest result(s) should always be used to assess if the relevant action criteria level has been made or exceeded; using the average or mean of a range of results is no longer considered appropriate.

Total actual acidity (TAA) is determined by titration of a 1M KCl salt solution to pH 5.5 using NaOH. This is a measurement of the soil's existing acidity prior to oxidation of sulfidic material.

Total potential acidity (TPA) is determined by peroxide double oxidation. This is estimated by titration to pH 5.5 of total acidity after oxidation of the soil with 30% hydrogen peroxide.

When determining lime requirements, subtracting TAA from Total Potential Acidity (TPA) to get Total Sulfidic Acidity (TSA) is acceptable.