PAF and NAF Rock
It is important to manage the placement of Potentially Acid Forming (PAF) material within the embankments to ensure that acid drainage is controlled both during the short term while the tailings storage facilities are being constructed, and in the long term. Non Acid Forming (NAF) material must also be managed carefully as sufficient NAF material must be available for the cover (zones G, H) and for the liner (zone A).
Details of the short and long term controls on acid drainage follow. The design strategy is to prevent generation of acid leachate as far as possible and as a contingency to provide for containment, collection and monitoring of any leachate that might be generated.
Short Term Controls
While the embankments are being constructed, the most important issue is the water quality of the runoff from the embankment structure. The following controls are used to prevent a deterioration in water quality:
- the application of crushed limestone to all PAF rock to slow the rate of sulphide oxidation and prevent the development of acid conditions while the rock is exposed during construction of the embankments
- placement of 0.25 m thick intermediate sealing layers to minimise rainfall/runoff contact and limit oxygen entry to the underlying PAF rock during construction of the embankment zones C, D and F
- placement and compaction of material; for example, in the bulk fill zone the material is placed and compacted in 1.0 m lifts
- installation of leachate drains to allow water within the PAF rock to be actively removed and monitored, providing a direct measure of the performance of the embankments
- limiting the active working areas of exposed PAF rock.
Long Term Controls
Controlling the rate of oxygen movement from the atmosphere to the reactive sulphides in the PAF rock is a key factor in controlling the oxidation of the sulphide minerals. Long term control of sulphide oxidation within the PAF material is achieved by the cap which has low porosity and maintains a high degree of saturation which limits oxygen movement into the embankment structure.
The performance of the cover (the zone G cap, subsoil and topsoil) in limiting oxygen movement has been verified by field-testing and from theoretical modelling. The SoilCover model has been used to evaluate performance by examining water movement and moisture concentrations within the cover itself. The SoilCover model was run for cases of an average rainfall year (2,183 mm), a low rainfall year (1,424 mm) an extreme drought year (90 mm) and a wet year (3,976 mm).
Main conclusions relating to the performance of the cover layer in providing long term oxidation control are:
- the soils used to form the cap retain water effectively
- during an extreme dry year, although Zone H dries, Zone G remains at more than 90% saturated and will continue to perform as an effective oxygen barrier
- this indicates that the cover design is extremely robust and is able to withstand more than one year without rainfall, without affecting the performance of the cover as an oxygen barrier
- the model demonstrates that the cover design at Waihi will provide long term oxidation control under wet, average and very dry years as well as under extreme drought conditions.
Field measurements of oxygen movement in Zone H and Zone G have been collected over a number of years. The measurements show that the oxidation rates are very low and confirm the results of the SoilCover modelling work.
For further information see CN, pH and Acid Drainage