Proposal for Perth Future Water Plan by Dr. Ramsis Salama (2007)
Perth does not need to import groundwater from the South West or surface water from the Kimberley. The present and future climate change consequences and the delicate balance of fauna and biota along the coastal plain, can no longer tolerate the abstraction of large quantities of water from confined aquifers or mega engineering projects for importing water. With proper management and design of smaller creative schemes, the local resources are more than adequate to supply present and future Perth water requirements.
Appropriate management of the Gnangara mound, Reduced usage, Recycling and Desalination are viable sources for Perth present and future Water supply. The resources gained from proper management can supply Perth with additional 100 GL/yr within 5 years and can be progressively increased to 275 GL by 2050. An additional desalination plant will increase the rainfall independent resources to 90 GL/yr and will increase the total additional resources to 145 GL/yr and 320 GL/yr respectively.
Gnangara Mound Management Plan
The Gnangara Mound is the most valuable source of fresh water in the Perth Region. The sustainability of the resource and its ecological and socio-economic uses are under threat because water levels have fallen across most of the Mound since the mid 1970s. The Water Dept seems helpless or unwilling to design a sustainable management plan for the Gnangara Mound due to pressures from the water Corporation and the other users who use their political influences to strangle all appropriate measures required for a meaningful plan to be formulated.
Present water use in the Gnangara Mound
The Banksia native vegetation is the major consumer of water from the Gnangara Mound, this is followed by the pine plantations, and together they use more than 500 GL/yr. This is followed by several other users including the Water Corporation (150 GL/yr), the licensed users and horticulturists (224 GL/yr), and the unlicensed private gardens and private house bores (105 GL/yr).
The long term sustainable utilisation of the Gnangara Mound will depend on the conservation of the unconfined aquifers from collapsing due to extensive drawdown which can diminish the aquifer capacity of recharge and discharge. It is also essential to remember that the past and present regime of pumping will take between 30 to 1000 years to be evident. That means we need to compensate the millions of litres of water which was withdrawn in the past. The long term management plan for the Gnangara will depend on capturing more recharge and more discharge.
Applying these measures will only reduce the environmental impacts and not eliminate them, because the moment an aquifer is developed as a water source, even taking into consideration all the environmental water requirements, ecologically sensitive areas will be altered. The delicate balance between the groundwater supply and demand by different users cannot be achieved without securing additional recharge for the over-utilised parts of the aquifer or by reducing groundwater discharge (aquifer outflow). Nevertheless, it is very difficult to achieve this balance, especially with changing weather patterns, increasing water demands and the relatively low infrastructure cost of groundwater abstraction that encourages further development to satisfy demand during drought periods. It is therefore essential that the environmental criteria, which control sustainable yield of the aquifers, be sensibly based to take into account these factors (Salama et al., 2003).
I) Reduce water usage
The first and most important management option which needs to be followed by all Perth residents is to reduce water usage. Our water supply problems would be substantially decreased if we reduced our demand from a world high of 300-475 m3year-1 per household to an OECD average of less than half this amount. Our enormous water wastage occurs in the home gardens, where nearly 200 m3year-1 per house is used to keep our lawns and gardens green. This reduction in water use applies first and foremost to the owners of bores in their homes. Although there are no restrictions for any one to drill a bore hole in his backyard, this largesse is causing the depletion of more than 100GL/yr of groundwater from the Gnangara Mound. It is quite clear that a system of licensing is required for home bores and an annual fee to be applied. Millions of dollars will be spent each year to replace the amount of water pumped by private bores to maintain the sustainability of the Gnangara Mound. The annual fee structure will need to be increased as the distance of the bore from the Ocean decrease to reduce usage near the Ocean and maintain the sea water at bay which is already invading several areas.
II) Capture more recharge:
Proper Management of Vegetation in the Gnangara Mound
a) Remove the pines
Removing the pines will reduce evapotranspiration, eliminate interception and the uptake of groundwater from the aquifers, all this will increase recharge by 146 GL/yr if replaced with native bush and by 194 GL/yr if replaced with pasture.
At the moment there are plans to remove the pine plantations within the next 10-15 years, which is far too long and the pines will continue to lower the water table in the Gnangara Mound even if all the trees are removed today. CSIRO modelling results (Salama et el., 2003) show that removing the pine plantations is an effective solution to recover the water levels in the Gnangara Mound to their original levels of 1975. Pumping in the superficial aquifer can still be achieved at the same rates as before. To respond to the increasing demand, pumping in the Leederville can also be continued as long as the groundwater source (flow from north east to south-west) is being protected. If that groundwater flow would decrease, the levels in the superficial aquifer would start to be seriously endangered by the pumping in the Leederville aquifer. The results indicate beyond doubt that the pine plantations need to be removed at an accelerated rate as no further drawdown can be tolerated in the aquifer with the present land use and pumping regimes (Salama et al., 2003).
b) Modify the Banksia
a) Improving the management of the Banksia by opening wider fire lines, managing the undergrowth and encouraging the growth of more draught tolerant species similar to the varieties in North Perth
b) Sensitive management of water levels under native bush, and removal of some of the groundwater dependenent phreatophytic vegetation which uses groundwater directly and is sensitive to cyclical water levels by encouraging non-phreatophytic
These measures would possibly increase recharge by over 100 GL/yr.
c) Control of water use in irrigated horticulture: Beside the high water consumption required by vegetables and strawberries, some of these farms use water three times per day just to cool the plants during hot days. Plants which need cooler climate should not be planted in the Gnangara.
A possible scenario for the above suggested management options is applied to the southern region of the Gnangara Mound defined from the Swan southward to 55.2 km north and from the ocean westward to 26 km east with a total area of about 1,435 km2. The results indicate that it is possible to obtain addition 100 GL/yr in the first 5 years and about 275 GL/yr by 2050.
III) Capture groundwater discharge:
Due to the hydro-geological nature of the Gnangara mound, with flow emanating from the Mound, groundwater flow is from the east to the west and southwest. Natural groundwater discharge takes place along the Ocean front, the Swan and to the east towards the Ellenbrook. There are also several lakes which receive groundwater discharge from the east and discharge back to the aquifer along the west margins after losing substantial amount through evaporation and evapotranpiration. It is estimated that about 200 GL/yr is discharged through the coastal areas. There are several possible alternatives to capture most of this discharge.
a) Redistribution of Storm Water injection along the coast to reduce groundwater discharge to the Ocean
Although some of the inland suburbs in Perth use compensation basins and infiltration galleries for stormwater drainage, most of the rainfall drainage in the coastal areas is discharged directly to the ocean; this can be diverted further inland into filtration ponds to recharge the coastal aquifers which are under threat from sea water intrusion. For example 400-900 ML/yr from Hemingway pumping station is discharged directly to the Canning River and to the Ocean as it is not required for minimum flow during winter. Similarly stormwater flows are discharged directly to the ocean in most of the Northern suburbs i.e. Ocean Reef.
Decreasing the natural groundwater discharge by extensive pumping from production wells along a line parallel to the ocean as is taking place in Whitford and Neerabup production lines, will eventually induce sea water intrusion and contaminate most of the private wells in the coastal suburbs. However, discharge of fresh groundwater water from the aquifer to the ocean can be reduced through the recharge of storm water along a line of compensation basins parallel to the ocean and further downstream from the production bores. It is estimated that an additional 45 – 100 GL of groundwater can be abstracted from the coastal areas of the aquifer (Salama et al., 2001). A new state of dynamic equilibrium cannot be achieved unless the natural discharge which will be decreased by pumping is balanced to keep the saline sea water – fresh water interface within safe limit. It is important to keep the amount of groundwater discharge to the minimum required level which keeps the interface zone within the shore boundary.
b) Injection of Waste Water along the coast to reduce groundwater discharge to the Ocean
Treated wastewater is currently a wasted resource over 100 GL/yr is discharged to the ocean. The Western Australian Government is planning to achieve its target of 20 per cent increase in the reuse of treated waste water by 2012. The Water for a Healthy Country Flagship is researching how to safely recharge used water in aquifers on the Swan coastal plain. The project is looking at knowledge gaps in managed aquifer recharge for treated drainage, storm water and waste water. This target of 20% by 2012 is too far away, an accelerated program is required. Due to the ff factor and the non acceptance of the population to drink recycled water, it is suggested to use the treated water for substituting the natural groundwater discharge.
Treated wastewater can and is used for a number of different applications depending on the level of treatment. These include; groundwater recharge and storage (secondary/tertiary level), irrigation: forest plantations and food crop (secondary/tertiary level). Discharge of primary treated wastewater directly into the ocean might cause some harm to the marine ecosystem. A better solution would be the injection of the wastewater into the aquifer a small distance from the ocean; this will be naturally cleansed and the discharge into the ocean will be more beneficial to the marine ecosystem and will replace the better quality groundwater discharge which takes place naturally. Accelerated programs must be put in place to start using wastewater at large scales.
IV) Conservation of wetlands through recycling
Prior to 1900, most of Perth lakes were wetlands, due to the development and increase of surface runoff from urban areas; some of the wetlands were transformed to lakes. The decrease of rainfall caused some of these lakes to return to their original wetland status and some other wetlands to dry up. To maintain the viability of these drying ones it is possible to supplement there water supply with waste water injected 20 m upstream of the wetland, that will guarantee maximum cleaning and purification process before the water enter the wetland..
V) Desalination
Water scarcity poses the biggest threat to Western Australia from climate change. All the planned water resources for Perth are dependent on rainfall. GMC models suggest decrease in rainfall in the South West. Perth is already having a desalination plant which produces 45 GL/yr; this is less than 1/5 of Perth water requirement. Although the various suggested management options will be sufficient to satisfy the water needs for Perth, due to the uncertainty of rainfall; the delayed response of the aquifer to the past abstractions which can occur within the next 30 years and increase the drawdown below the calculated levels which might cause the cessation of pumping from parts of the aquifer, it is therefore highly recommended that the level of security of supply be increased by constructing another desalination plant to increase the output from non dependent rainfall sources to 90 GL/yr.