Draft Treatment and management of disturbed acid sulfate soils and acidic ground and surface waters
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Draft Treatment and management of disturbed acid sulfate soils
and acidic ground and surface waters
5.0 Neutralising acid sulfate soils
All levels of treatment and management of acid sulfate soils should be provided to DEWCP and should include details of preliminary investigations, disturbance dimensions, volume calculations, field soil test results and laboratory analysis results. Factors that will influence the level of treatment include the nature of ASS disturbance, the soil characteristics, (e.g. the variability of sulphide concentrations, bulk density, physical characteristics such as texture and inherent neutralising capacity), the surface and subsurface hydrology, the sensitivity of the surrounding environment and the past history of the site.Table 3 provides the estimated level of aglime (CaCO3) required to treat the total weight of disturbed ASS based on the soil analysis and the amount of aglime required to neutralise the total existing and potential acidity of the ASS soils. Aglime has a pH of about 8.2 and is the safest neutralising agent that poses least environmental risk. Because of the difficulty in mixing lime with acid sulfate soils and the low reactivity of even the fine lime, safety factors of at least 1.5 to 2 times the theoretical lime required will apply.
| TABLE 3 Existing treatment levels and aglime required to treat the total weight of disturbed ASS based on soil analysis developed by Ahern et al 1998a. |
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The tonnes of lime required for treating the total mass of ASS are provided in Table 3 at the intersection of the mass (tonnes)(row) and the existing plus potential acidity (converted to equivalent S% units)(column). Potential acidity can be determined by Chromium Reducible Sulphur (Scr), Peroxide Oxidisable Sulfur (SPOS) and Total Oxidisable Sulfur (STOS). For samples with pH less than pH5, the existing acidity must also be determined by appropriate laboratory analysis e.g. Titratable Actual Acidity (TAA).
Other neutralising agents besides aglime can be used to treat ASS. However factors to be considered when choosing neutralising agents include pH, solubility, neutralising value, particle size, purity of the agent, method of application and transportation costs. If other neutralising agents are used, the figures in Table 3 will need to be adjusted accordingly.
Soils with jarosite or other similar insoluble compounds have a less available existing acidity and will require more analysis. Please refer to the Queensland Acid Sulfate Soil Technical Manual available on www.environ.wa.gov.au website.
5.1 Calculating the quantity of neutralising agent for acid sulfate soils
It is important to provide adequate neutralising material to reduce the potential for environmental harm or damage. The amount of neutralising materials required is based on the calculated percentile of oxidisable sulphur from the soil analysis. Table 4 provides an indication of the maximum acid that can be produced and the financial feasibility of managing the disturbance of ASS.The rate of application must be calculated according to the neutralising value of the neutralising materials. The fineness of the neutralising agent will influence the effectiveness and reactivity of the agent. Another factor to be considered is the coating of the neutralising agent by low solubility gypsum, insoluble iron or aluminium compounds can also limit the effectiveness of the materials. A minimum safety factor of 1.5 only applies for good quality fine aglime with neutralising value of 100.
| Lime required (kg CaCO3/tonne materials | = kg H2SO4/tonne of materials x safety factor = (oxidisable S% x 30.59) x 1.5 |
Table 4 - acid sulfate soil conversions (based on 1 mol pyrite producing 2 mol sulfuric acid and corresponding neutralising rate)
| Oxid. S% | moles H+/kg (S% X 0.6237) | Moles H+/t or moles H+/m3 (S% X 623.7) | kg H2SO4/tonne Kg H2SO4/m3 (S% X 30.59) | kg lime/tonne soil or kg lime/m3 Safety factor = 1.5 | Est. lime cost / tonne soil or cost / m3 of soil $ | Cost/ha/m depth of soil @&50/t of lime $ |
|---|---|---|---|---|---|---|
| 0.02 | 0.0125 | 12.47 | 0.61 | 0.94 | 0.05 | 468 |
| 0.03 | 0.0167 | 18.71 | 0.92 | 1.4 | 0.07 | 702 |
| 0.06 | 0.0374 | 37.43 | 1.84 | 2.8 | 0.14 | 1,404 |
| 0.1 | 0.0624 | 62.37 | 3.06 | 4.7 | 0.23 | 2,340 |
| 0.2 | 0.1247 | 124.7 | 6.12 | 9.4 | 0.47 | 4,680 |
| 0.3 | 0.1871 | 187.1 | 9.18 | 14.0 | 0.70 | 7,020 |
| 1.0 | 0.6237 | 623.7 | 30.6 | 46.8 | 2.34 | 23,410 |
| 5.0 | 3.119 | 3119 | 153.0 | 234.0 | 11.7 | 117,000 |
Note - Assumes a bulk density of 1.0g/ cm3 or 1 tonne/m3 (bulk density can range from 0.7- 2.0 and as low as 0.2 for peat). Where bulk density is >1g/cm3 or 1 tonne/m3 then the correction factor for bulk density will increase for lime rates/m3 soil (eg. If BD=1.6, then 1 m3 of soil with 1.0% SPOS will require 75 kg lime/m3 instead of 47 kg).
