Yanchep National Park Management Plan 1989-1999

6.0 HYDROLOGY
The hydrology of the Park involves a complex interconnecting system between a groundwater
aquifer and the wetlands and caves. (For further information also refer to Sections 9.0 and 15.0.)
Hydrological information was obtained predominantly from the Water Authority Gnangara Mound
Ground Water Resource Environmental Review and Management Programme (ERMP) (1986) and
the EPA's response to the ERMP in Bulletin 295 (August 1987), except where cited.
The Park lies towards the western edge of the Gnangara Mound, an extensive, unconfined ground
water aquifer that provides approximately 20 percent of the public water supply for Perth (Hopkins,
pers. comm.). The Mound is recharged by the direct infiltration of rainfall, mostly between April and
October. The water table generally reflects the topography, and wetlands occur where the water table
is at or near the surface. Beneath the coastal strip the water table has a low gradient for three to four
kms inland then rises very steeply immediately west of the lakes. This occurs close to the contact
between limestone and sand and is believed to mark a ground water cascade where ground water
locally moves through limestone caves.
To cater for public water supplies the Water Authority has proposed a number of schemes, including
the Pinjar and Yeal schemes near Yanchep. Their potential environmental impact is reviewed in the
ERMP. The major constraint in managing ground water resources is the retention of wetlands and
natural water depths in the caves.
The Water Authority (1986) consider that the potential environmental impact on the Park's wetlands
is low because:
,the water table contours rise steeply to the east of the lakes such that lake levels are less
susceptible to reductions in ground water levels to the east" (where the proposed scheme is
situated), "the high transmissivities of the limestone will tend to mitigate drawdowns as water
will rapidly flow into the area, the lake's steep sides and depth means that falls in level affect a
narrower fringe than would be the case of a shallower lake'.
However, the ground water in the caves, which is essential to the maintenance of their environment,
averages only 10 to 20 cms in depth. For the reasons above, abstraction of the ground water some
distance away does not directly reflect a decrease of water depth in the caves. Nevertheless if there is
any 'unnatural' change in depth it can be critical. It is crucial that the normal depth and variation in
the caves is maintained.
The EPA (1987) considered that the environmental impacts of the drawdown predicted in the ERMP
are unacceptable for some wetlands, including Loch McNess and Yonderup Lake. The EPA
recommended that for the Category 1 (Loch McNess) and Category 2 (Yonderup Lake) wetlands
there should be no change in the existing regime of water quality and quantity, and normal climatic
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