• Evapotranspiration (2) removes water directly from the soil by evaporation, and indirectly from the soil through plants by transpiration. In summer, when evapotranspiration processes are strong, water can be drawn out of the topsoil (4) and Zone H (5) as the soil dries.
• Runoff (3) develops when the rainfall rate is greater than the infiltration rate of the soil.
• Water will move quite quickly into the topsoil, but where the permeability reduces at the contact of the topsoil with Zone H, the rate of infiltration will slow down. The topsoil will become more saturated and more rainfall will run off the surface.
• If rainfall continues, water will infiltrate Zone H. Where water reaches the contact with Zone G (6) (the oxygen and water control layer) the infiltration rate will again be slowed due to the lower permeability of Zone G. A head of water will begin to build at the base of Zone H on top of Zone G and movement of water downslope across the top of Zone G will develop.
• Water which enters Zone G will move slowly, both through this zone and also horizontally back towards the contact of Zone G and Zone H. This is because Zone G is constructed in 250 mm lifts, and the compacted layers result in lower vertical permeability than horizontal permeability. The slow movement through zone G stops air moving into the underlying PAF rock. This is the principal long term control of acid generation.
• Similarly, the stratification,or layering, within Zone D (7) provides preferential movement towards the downstream face of the embankments where water will be collected and diverted into the leachate drain (8).
• Zone A is constructed of selected NAF material to provide a low permeability liner beneath the waste materials. The purpose of zone A is to restrict seepage to natural ground and to divert seepage to the leachate drains. It also serves to reduce upflowing groundwater from moving into the PAF rock in the embankments. The soils which are used to construct zone A have a capacity to adsorb chemical components from the leachate. This provides additional control to seepage movement.
• The ash soils underlying Zone A (9) are relatively permeable and will act as an underdrain diverting any seepage through Zone A and upward rising groundwater (10) to the gully and toe drains (11).
• The weathered surface of the basement rock (12) has lower permeability than the ash and provides containment for water moving through the ash.
• The groundwater flow pattern provides an upwards pressure through the basement weathered zone and provides an additional constraint to any downwards water movement.

Water Movement through the Cap
The performance of the cap as a barrier to water was evaluated using the SoilCover model. The main conclusions were:

• the waste rock cover is an effective barrier to water
• seepage through the cover, from the base of Zone G, is predicted to be very low and less than 1% of annual rainfall
• during most of the year, seepage occurs by unsaturated flow which is much less than saturated flow.
  
  

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