A lot of people we surveyed said they're worried about water: rainwater, but also groundwater - or as one participant called it: "Dinosaur water". People are not sure the government is looking after groundwater, and water levels in several places seem to be much lower - especially around Gelorup. We've asked one of our supporters, Peter Lane, for an explainer on aquifers and groundwater. Pete is a geologist and has done much work studying the geology of the Capes. "There is not one, but a number of canaries in the coal mine trying to warn us, and we need to listen." he says.

Here in the south-west, west of the Darling Scarp, we are fortunate to have access to extensive
groundwater resources.
Extensive they may be, but they have to be handled with care.

An aquifer is a rock formation that is both porous (it has holes that can contain water) and permeable (the holes are connected, enabling flow). The most common aquifers are sandstones, but limestones and other rock types which may have voids or be fractured may also be aquifers.
Surficial aquifers can be regarded as a porous/permeable section deep in the soil profile. They are typically sand or karst limestone, of limited extent and water flow is generally ephemeral. Ancient river channels (‘palaeochannels’) consisting of sandstone may also form surficial aquifers. The term ‘groundwater’ refers to water recovered from either ‘true’ or surficial aquifers.

The principle aquifers within the Forrest electorate are sandstones of the Perth Basin. This is the sequence of sedimentary rocks that lies beneath the Swan and Scott coastal plains and the Blackwood Plateau. To its east, and in the south to its west, the basin (green in diagram) is flanked by ancient metamorphic and igneous rocks of the Yilgarn Craton and the Leeuwin Complex (Leeuwin Naturaliste Ridge) respectively. The basin continues offshore to both the Indian and Southern oceans.

Within the area shown, the basin is up to six kilometres deep and contains sediments dating from the early Permian Period, almost 300 million years ago, to the present. The most important aquifer is the Yarragadee Formation, about 160 million years old and the Leederville Formation, some 30 million years younger. As shown in the diagram below, the Vasse Shelf abuts the Leeuwin Complex,
and here the Yarragadee Formation is absent. The deep basin to the east of the Vasse Shelf is known as the Bunbury Troug.

The main aquifer, the Yarragadee, is 160 million years old yet the water it contains ranges from a day to some 40,000 years old. It is a very active system, best explained by the ‘water cycle’: Falling rain evaporates, some is taken up by the soil, some transpired by plants and some flows into streams then to the ocean. Essentially, what remains finds its way to the aquifers, and much of this is eventually discharged into the Indian and Southern oceans. A decline in rainfall and warmer days results in higher evaporation and transpiration and disproportionally reduces the recharge to the aquifer. At these times there is also a greater
natural and human demand for water, and the prognosed continuing reduction in rainfall and planned population growth will sorely test theability for aquifer recharge to replace groundwater lost.

Groundwater is used throughout the south-west. ‘Town water’ for all major settlements in the region is mostly sourced entirely by groundwater, primarily from the Yarragadee aquifer. This includes those towns beyond the Perth Basin, the water for Margaret River, Gnarabup, Bridgetown and Manjimup being sourced from the Yarragadee, and that for Augusta, Yallingup and Dunsborough from the Leederville aquifer. Many farms, nurseries and a range of industries rely on groundwater, again principally from the Yarragadee aquifer.

Should the trends of reduced groundwater recharge and increased extraction continue the environmental impact will be severe.

‘Groundwater dependent ecosystem’ is a scientific classification of streams, wetlands and forests that depend on groundwater for their survival. In essence this includes all streams, wetlands and many forested areas of the Bunbury Trough and Vasse Shelf. Damage has already occurred in some areas, for example in the Bunbury Trough at Gelorup. Here trees have died, and the failure of bores is strong evidence that the decline of groundwater levels is the reason for the deaths. I am also aware of coincidences of the loss of trees and a decline in the water level of bores on the Vasse Shelf.

Poor winter rainfall and the dry summer have also resulted in extensive tree deaths on the Leeuwin Naturaliste Ridge and Yilgarn Craton, especially in areas of shallow soil and of marri trees suffering from marri canker.

As a consequence of extracting too much water, pressure in the aquifer will drop and in places where groundwater has historically flowed to the ocean the flow will reverse. Salt water will flow into the aquifer. This has already been observed in bores at Busselton and Bunbury. Scientists calculate the changes over time of the ‘potentiometric head’ of groundwater to determine if the rate of extraction is sustainable, and indeed there are numerous monitoring bores in the region that provide this information. Yet the above instances alone provide convincing evidence that in some areas the present rate of recharge extraction is unsustainable and that this may be widespread.

There is not one, but a number of canaries in the coal mine trying to warn us, and we need to listen.

The responsible government department is the Department of Water and Environmental Regulation (DWER), “We support Western Australia’s community, economy and environment by managing and
regulating the state’s environment and water resources.” https://www.wa.gov.au/organisation/department-of-water-and-environmental-regulation.

Relevant ministers are Simone McGurk MLA (Water) [email protected], John Carey MLA (Planning) [email protected], Donald Punch MLA (Regional Development)
[email protected] and Reece Whitby MLA (Environment and Climate Action) [email protected]

Photo by engin akyurt Unsplash