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Groundwater and the environment in Port Phillip & Western Port

Groundwater and the environment in Port Phillip & Western Port

Groundwater interacts with the surface environment when it occurs at or near the surface. In this region the upper aquifers interact most closely with surface water environments such as streams, the bays and other Groundwater Dependent Ecosystems (GDEs) as they occur near the surface across the entire region. The middle aquifers rise to the surface between between Moorabbin and Boneo where they are thought to interact with surface features such as the Edithvale and Seaford Wetlands. The lower aquifers rise to the surface in the highlands and provide baseflow to streams around Deep Creek near Lancefield and around the Yarra Valley region.

Where is groundwater most likely to interact with the environment?

To view areas where groundwater is most likely to interact with the environment on a map visit the Port Phillip and Western Port region map and select the 'Environment' layer from the drop down box in the top right corner.

What do we know?

West of Melbourne

Deanside Wetlands at Rockbank and Kororoit Creek on the volcanic plains west of Melbourne are home to numerous threatened and significant species of plants and animals. Streams and wetlands in this area depend on surface water and groundwater.

Deanside Wetlands is a freshwater marsh permanently connected to a low salinity shallow basalt aquifer layer.

Kororoit Creek occurs within close proximity of Deanside Wetlands however it is connected to the deeper, saltier basalt aquifer layers while its tributaries are connected to the fresher shallow layers. Interaction depends on groundwater levels that rise and fall with rainfall so the creek gains groundwater in some areas and seasons and loses it in others. Saline groundwater contributes to high salt levels in the lower reaches of Kororoit Creek and the plants in these areas are salt-tolerant.

Large old red gums in this area are also thought to use groundwater in dry periods. Groundwater interactions in these areas are more complex due to the multiple basalt layers. Each basalt layer can function as a separate aquifer with a very different flow rate and salinity. It is therefore common for fresh and saline ecosystems to exist very close to each other.

The Highlands

Near Lancefield spring flows from the fractured rock basement and Older Volcanics aquifers contribute to streams, including Deep Creek. Groundwater discharge to these waterways also contributes to drought refuges and supports endangered species such as the Yarra Pygmy Perch. Melbourne Water has mapped these drought refuges and their groundwater catchments.

Around the Yarra Valley region river valleys, streamflow is most likely to be affected by groundwater pumping in dry periods when rainfall and river flows are low and pumping levels peak. Where bores are located close to the river this reduction in river flow can happen with very little lag time. However, the impact tends to be localised because high yielding, low salinity groundwater generally exists in small, isolated pockets.

In the Woori Yallock catchment (which includes most of the Wandin Yallock GMU), the volume taken from streams is actually greater than that taken from bores (RMCG, 2011), however groundwater supply is more reliable than the surface water supply from streams. However, if in summer all licensed groundwater entitlements and registered D&S bores entitlements were used, the total depletion could be as high as 11% of the stream’s flow (SKM, 2009).

Evapotranspiration is difficult to quantify directly, however it can sometimes be observed indirectly. An example of this occurred in the days following the Black Saturday bushfires in 2009. Locals observed significantly increased flows in the upper reaches of Diamond Creek south of Kinglake. A study commissioned by Melbourne Water found that the sudden and dramatic decrease in evapotranspiration due to the destruction of trees and plants in the bushfires resulted in an increase in discharge of groundwater to waterways. It is also likely that there was a significant increase in run-off after the bushfires that also contributed to increased stream flows. There is no available long-term data or analysis of stream flows in the area during the period of bush regrowth and regeneration since 2009.

South-East of Melbourne

Between Moorabbin and Boneo the middle aquifers are unconfined and occur at or close to the surface. These aquifers may interact with the Edithvale and Seaford Wetlands and with Western Port. Discharge volume to coastal waterways and wetlands in this area has been estimated at 33,000 ML/yr (Parsons Brinckerhoff, 2010).

There is potential for sea water from the bays to enter the middle aquifers if pressure falls below sea level. To protect against this groundwater salinity is monitored in the Moorabbin and Koo Wee Rup GMUs (no sea water intrusion has been detected to date). The Ramsar-listed sea grass beds in Western Port are potential GDEs, however we do not have a good understanding about the flow of groundwater into and under Western Port. Further research is being conducted.

The Ramsar-listed Edithvale and Seaford Wetlands are potential GDEs. They may be supported by groundwater from the middle aquifers that discharges upwards through the overlying upper aquifers.

Tootgarook Swamp is a dune swamp found on the Mornington Peninsula and is a site of State biological significance. It is the largest remaining natural freshwater swamp in the Port Phillip & Western Port region and supports numerous endangered, threatened and vulnerable plants and animals as well as migratory birds. It also helps to limit flooding in nearby areas by providing some storage of stormwater. The swamp interacts with the upper aquifer and plants in the swamp directly access the shallow water table. The aquifer is recharged by rainfall and stormwater run-off through the surrounding sandy soils and dunes. It also discharges groundwater to Chinaman’s Creek (an artificial waterway also known as Boneo Drain). The shallow groundwater maintains the swamp and its dependent habitat. It also prevents toxic discharges by preventing potential acid sulfate soils from drying out. Tootgarook Swamp may be sensitive to a number of issues including over-extraction of groundwater, urban development (which would reduce local recharge), poor quality and potentially contaminated stormwater run-off and irrigation in the area that uses recycled water. Mornington Peninsula Shire Council is currently developing a management strategy for Tootgarook Swamp together with professional partners and stakeholders.

How do we manage this?

Southern Rural Water takes into account the needs of the environment when assessing licence applications for new entitlements and to trade existing entitlements. Also, bore construction is not permitted immediately adjacent to streams or rivers.

Learn the basics

To find out about how groundwater supports surface water features such as lakes and rivers visit our How does groundwater interact with the environment page.

To find out about who manages groundwater visit our Who manages groundwater page.

Page last updated27th May 2015
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