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
3.0 Stockpiling acid sulfate soils
The risks of stockpiling large volumes of untreated ASS may be very high even over the short- term. Stockpiling small volumes of untreated ASS should only be undertaken as a short-term activity. For example:
- part of a day's extraction of clay and peat may be stockpiled over a weekend before
strategic reburial;
- due to weather conditions that may prohibit treatment or problems with obtaining
laboratory results; or
- required land areas for soil neutralising treatment may not be available as quickly as was anticipated in earthworks strategies, leading to the creation of small stockpiles before changes can be made to earthworks programs.
After soils have been excavated and moved, the soil profile will be mixed. It then becomes difficult to correlate laboratory results with soil profile horizons to identify the specific liming rates needed for those horizons, and consequently, matching the soil horizons with their respective liming rates may become difficult. Impact to the environment may occur if incorrect liming rates are used.
Significant quantities of acid can build up, especially in porous sandy stockpiles, if left in oxidising conditions for even short periods of time. Large stockpiles are difficult to neutralise, primarily due to the earthmoving required. In addition, when determining the treatment options it is important that by-products of oxidation, such as jarosite, are properly characterised and that representative sampling of the stockpile is performed. Sampling rates for stockpiles may need to be double or triple that of an undisturbed profile. Costs of representative sampling of stockpiles are much cheaper if sampling is performed as the stockpile is being created.
4.0 Management considerations
Stockpiling untreated ASS should be minimised by preparing a detailed earthworks strategy that documents the timing of soil volumes to be moved, treatment locations and capacity of those areas to receive the stockpile materials. Stockpiling may mean double-handling and increased earthmoving costs. It is important to account for risk from wet weather and plan for other contingencies.4.1 Short-term stockpiles
The recommended maximum time period for which soils can be temporarily stockpiled without treatment is detailed in Table 1.Table 1 - indicative maximum periods for short-term stockpiling of untreated ASS
| Type of material | Duration of stockpiling | ||
|---|---|---|---|
| Texture range (McDonald et al. 1990) | Approx clay content (%) | Days | Hours |
| Coarse texture Sands to loamy sands | ≤5 | Overnight | 18 hours |
| Medium texture Sandy loams to light clays | 5-40 | 2.5 days | 70 hours |
| Fine texture Medium to heavy clays & silty clays | ≥40 | 2.5 days | 70 hours |
At some sites these figures may be too conservative, and in some circumstances not conservative enough (eg. during hot weather some sands may begin to oxidise within a matter of hours whereas complete oxidation of peat may be longer). Appropriate operational delay times should be determined well before the creation of the stockpile for the specific circumstances. The use of a guard layer under the stockpiles may be warranted in certain circumstances.
The total volume of material that is placed in short-term stockpiles should not exceed 20% of a day's total extraction.
Note: These timeframes do not apply to monosulfidic black oozes. These gels or sediments should not be stockpiled without a risk assessment, and the implementation of strict environmental management protocols.
4.2 Medium-term stockpiles
Situations where it is necessary to stockpile untreated ASS for moderate periods will need to be justified to the DEWCP. Management to reduce the oxidation of sulfides and the collection and treatment of all leachate and runoff water will need to be implemented during the entire stockpiling period. The maximum time period for which soils can be temporarily stockpiled in the medium-term is listed in Table 2.Table 2 - indicative maximum periods for medium-term stockpiling of untreated ASS
| Type of material | Duration of stockpiling | ||
|---|---|---|---|
| Texture range (McDonald et al. 1990) | Approx clay content (%) | Days | Weeks |
| Coarse texture Sands to loamy sands | ≤5 | 14 days | 2 weeks |
| Medium texture Sandy loams to light clays | 5-40 | 21 days | 3 weeks |
| Fine texture Medium to heavy clays & silty clays | ≥40 | 28 days | 4 weeks |
Depending on site specific requirements, a risk assessment should be undertaken if soils are to be stockpiled for longer periods than those listed in Table 2. Neutralisation of the stockpiled materials may be necessary if it cannot be demonstrated there is minimal risk of acidic leachate being generated by the stockpiles. Stockpiling in the medium term should be a contingency measure rather than standard practice.
A guard layer of a suitable neutralising agent MUST be placed under all medium-term stockpiles. In addition, the following management strategies may need to be implemented to manage risk:
- the volume stockpiled should not exceed more than 1 week's volume of extraction;
- all stockpiles will need to be bunded and diversion banks installed upslope to prevent the
ingress of surface runoff into the stockpiles. Bunds and diversion banks should not be
constructed out of untreated ASS or other materials that may be a source of contaminants
to the environment. The bund materials used should have an appropriately low
permeability to avoid infiltration;
- leachate collection and treatment systems should be installed;
- the surface area of the stockpile should be minimised to reduce exposure to atmospheric
oxygen. This may involve shaping the stockpile, and/or capping or lining it with a
material that will minimise drying by wind and sun and prevent rainfall entering the
stockpile. The cap or liner will need to cover the sides of the stockpile as well as the top;
- keeping the surface of the material moist using a spray of water or neutralising solution.
The spray should be carefully managed to prevent over-wetting the material producing
leachate or runoff, and should be a fine-mist to prevent desegregation of the soil from the
stockpile surface; and/or
- construct erosion and sediment control structures.
The amount of neutralising agent used in the guard layer of medium term stockpiles should be based on 0.3 times the average potential and existing acidity of the stockpile/m2 per vertical metre of soil that is to be temporarily placed in the stockpile. For example, if the stockpile is 2 metres high, then twice 0.3 times the neutralising agent required to neutralise the potential and existing acidity should be spread as a guard layer.
For sandy textured soils in environmentally sensitive areas, or in situations where the medium term time frame cannot be met, a higher rate of neutralising agent in the guard layer will be warranted.
4.3 Long-term stockpiles
Any stockpiling exceeding those timeframes provided in Table 2 is considered long-term stockpiling, and an appropriate management strategy is required. The management of such stockpiles would require conducting a risk assessment and obtaining approval from DEWCP. In such cases, an environmental management plan is recommended and should be forwarded to DEWCP for consideration and approval. In addition, the management strategies should include those outlined for medium-term stockpiles. Failure to manage environmental risks posed by long term stockpiling may result in actions being taken under the Environmental Protection Act 1986.4.4 Stockpiles of topsoil
It is a routine practice to scrape the topsoil prior to excavation, and store it until it is required for top-dressing. Some of the management options listed under medium-term stockpiles may be appropriate for managing topsoil stockpiles, especially if they contain low levels of sulfides. Low levels of sulfides may occur in topsoils as a result of 'over-stripping' that have occurred during its collection, or it may be intrinsic to the topsoil.All topsoil should be tested prior to stripping and stockpiling. Neutralisation of the total existing and potential acidity of the topsoil will be required. It may be cheaper and easier to neutralise the topsoil as it is scraped and placed. For example, the appropriate amount of neutralising agent can be spread over the topsoil and, using a reverse scraper, the lime incorporated prior to stripping. Further mixing will occur as the soils are placed into the stockpiles.
