Water Movement Through the Embankments
During and after a rainfall event (1), a number of processes occur:
- 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.
