Acid sulfate soil and acid rock testing

Acid sulfate soil testing

EAL has been involved in research and method development for the identification and management of these soils for almost 20 years. This has culminated in the publication of the following national guidance documents:

• National acid sulfate soils sampling and identification methods manual (pdf)
• National acid sulfate soils identification and laboratory methods manual (pdf)
• Overview and management of monosulfidic black ooze (MBO) accumulations in waterways and wetlands (pdf)

As a consequence, EAL now offers the approved methods in the Net Acidity Suite (AS-PACK-008). Acid sulfate soil reports include Net Acidity and Liming calculations using the new Acid Base Accounting technique (Sullivan et al. 2018a) (pdf).

Adhering to standard sample collection and preservation procedures sample collection and preservation procedures (pdf) ensures the highest level of accuracy. The National acid sulfate soils sampling and identification methods manual (pdf) is available to assist in planning a soil sampling program.

To introduce the new guidance documents EAL, in conjunction with Southern Cross GeoScience, is presenting a series of professional short courses throughout 2019. For more information and updates visit the professional short course page.

Acid Base Accounting for Net Acidity

Net Acidity is a measure of the acidity hazard of an acid sulfate soil. It is quantified using an Acid Base Accounting approach. Prior to the publication of the 2018 National Acid Sulfate Soil Guidance, Net Acidity was calculated as Potential Sulfidic Acidity + Actual Acidity + Retained Acidity - Acid Neutralising Capacity.

In acknowledgment of the fact that available Acid Neutralising Capacity (ANC) methods overestimate the effectiveness of most neutralising materials, the Net Acidity equation has been modified to take into account whether the effectiveness of a measured ANC had been corroborated by other data, and for assessment of the effectiveness of liming for management of an acidity hazard. As a result, there are now three methods of Acid Base Accounting:

• Net Acidity where by ANC has been corroborated by other data
  Net Acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity - Acid Neutralising Capacity

• Net Acidity whereby ANC has not been corroborated by other data
  Net Acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity

• Verification Net Acidity
  Net Acidity = Potential Sulfidic Acidity + Actual Acidity + Retained Acidity - (post treatment Acid Neutralising Capacity - initial Acid Neutralising Capacity)

As a result of the changes in Acid Base Accounting, it is recommended ANC be assessed on any soil that has the potential for acid sulfate soil management - this will allow for verification of the effectiveness of the management approach, post-liming, via calculation of the Verification Net Acidity.

EAL offers archiving of initial ANC data, to be used in post-liming calculations. Clients need only provide an initial job number or date of sample submission to access this information.

Net Acidity - moving away the SPOCAS Suite

Under the National Acid Sulfate Soil Guidance there is a movement away from characterising Net Acidity in terms of suites of techniques, to an emphasis on the components of the Acid Base Account. In this simplified format, there is a reduced number of recommended techniques:

• Potential Sulfidic Acidity - Chromium Reducible Sulfur (S_CR) is the recommended technique. Peroxide Oxidisable Sulfur (S_POS) can be used where the organic carbon content is less than 0.6%, however, it is recommended at least 15% of samples are also analysed by the S_CR method to verify S_POS results.
• Actual Acidity - the presence of Actual Acidity is indicated by a pH_KCl < 6.5. Where this criterion is met, the Titratable Actual Acidity (TAA) method is used for quantification.
• Retained Acidity - the need to quantify Retained Acidity is determined by visual observation of jarosite or a pH_KCl < 4.5. Retained acidity is estimated using the Net Acid Soluble Sulfur (S_NAS) method.
• Acid Neutralising Capacity - may be present when the pH_KCl ≥ 6.5. Acid Neutralising Capacity by Back-titration (ANC_BT) is the routine method of analysis, the Total Inorganic Carbon (C_IN) method is also acceptable. The effectiveness of the ANC can be assessed through Incubation trials.

Acid rock testing

Acid rock testing uses a combination of Net Acid Producing Potential (NAPP) and Net Acid Generation (NAG) techniques for screening acid rock material. EAL offers two alternatives for quantification of sulfur for Maximum Potential Acidity (MPA):

• Acid sulfate rock (AS-PACK-006) using LECO total sulfur, and
• Acid sulfate rock (AS-PACK-010) using Chromium Reducible Sulfur (S_CR).

References:

Sullivan L, Ward N, Toppler N and Lancaster G. 2018a. National acid sulfate soil guidance: national acid sulfate soils identification and laboratory methods manual, Department of Agriculture and Water Resources, Canberra, ACT. CC BY 4.0.

Sullivan L, Ward N, Toppler N and Lancaster G. 2018b. National acid sulfate soil guidance: national acid sulfate soils sampling and identification methods manual, Department of Agriculture and Water Resources, Canberra, ACT. CC BY 4.0.

Sullivan LA, Ward NJ, Bush RT, Toppler NR and Choppala, G. 2018. National acid sulfate soils guidance: overview and management of monosulfidic black ooze (MBO) accumulations in waterways and wetlands, Department of Agriculture and Water Resources, Canberra, ACT. CC BY 4.0.