Region: South central PPWP CMA (Mornington Peninsula)
Type areas: Bittern, Devon Meadows, Moorooduc South
Brief description: The underlying bedrock of the Mornington Peninsula comprises Ordovician and Silurian age sedimentary rocks (~ 480 – 420 Ma). Groundwater moves through the fractures in these sandstones and mudstones in local and possibly intermediate systems. The rock underlies the Brighton Group sediments (GFS 10) over most of the central Mornington Peninsula.
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Problem statement: Small areas of salinity associated with this GFS have been mapped in the Devon Meadows, Langwarrin - Cranbourne South, and the Mount Martha – Safety Beach – Red Hill areas. In other areas, salinity in the Brighton Group sediments (GFS 10) and the Quaternary sediment (GFS 1) occur very close to the boundary with the bedrock. The salinity is probably associated with the altered hydrology following the massive changes in land use over the past century or more.
Geology: Ordovician and Silurian marine sediments: mudstones and sandstones (Ol, Ou, S).
Topography: Low hills and undulating plains.
Land Systems:
South Victorian Uplands
3.3 Moderate Ridge – Mornington Peninsula
Regolith: Highly weathered to completely weathered sandstones and mudstones.
Annual rainfall: 750 mm to 1000 mm
Dominant mid-1800s vegetation type: Forest, Scrub and Woodland
Current dominant land uses: Urban development, grazing, water supply catchment (Devilbend Reservoir), conservation area.
Mapping method: Outcrop geology
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Salinity (S1) along a drainage line adjacent to the Nepean Highway, Mount Martha
Aquifer type (porosity): Fractured rock and saprolite (secondary porosity)
Aquifer type (conditions): Unconfined and semi-confined
Hydraulic Conductivity (lateral permeability): Highly variable. The saprolite varies from approximately 10-5 m/d to 10-1 m/d and the rock varies from 10-5 m/d to 1 m/d
Aquifer Transmissivity: Highly variable in the low to moderate range. Estimated to be generally less than 50 m2/d.
Aquifer Storativity: Variable. Estimated to be less than 0.03 for saprolite and 0.02 to 0.05 for the fractured rock.
Hydraulic gradient: Estimated to be low in intermediate systems and locally moderate in local systems.
Flow length: Generally <15 km for intermediate systems and <5 km for local systems.
Catchment size: Small (~<500 Ha) for local systems and moderate (>1000 Ha) for intermediate systems.
Recharge estimate: Unknown. Probably 40 mm to 50 mm annually.
Temporal distribution of recharge: Seasonal (winter and spring), with more recharge in wetter years.
Spatial distribution of recharge: Catchment wide but varies with the depth of regolith, slope and wet areas in the landscape. Where covered by the Brighton Group sediments, continuous recharge would occur.
Aquifer uses: Minor use, mainly for stock water.
Groundwater salinity (TDS): Generally in the range of 1000 mg/L to 8000 mg/L
Salt store: Moderate to high.
Salinity occurrence: Some within this GFS, but may contribute to salinity in the adjacent units (GFS 1 & GFS 12).
Soil Salinity Rating: S1 to S3.
Salt export: Both baseflow to streams and wash-off from surface.
Salt impacts: Both on-site and off-site.
Soil salinity hazard: Low
Water salinity hazard: Moderate
Assets at risk: Streams and rivers, engineering and urban infrastructure, conservation areas, agricultural land.
Responsiveness to land management: Largely unknown, but thought to be moderate for intermediate flow systems and high for local flow systems.
High rainfall (in excess of 750 mm/yr), urbanisation and the relatively isolated instances of salinity will likely result in an emphasis on discharge treatment rather than significant recharge reduction. Salinity adjacent to the margins of this GFS appears more likely to be associated with groundwater flows terminating in neighbouring GFSs (GFS1 and GFS10).
| Options | Treatments | Comments |
|
Biological Management of recharge |
Perennial pastures | Low to moderate impact– rainfall too high for significant impact. Offers some level of run-off and waterlogging control. |
| Crop management | Low impact– cropping is generally absent in these landscapes | |
| Trees/woody vegetation | Moderate impact– in local systems where they can be incorporated into existing land uses. Plantations and belts will reduce gross recharge, run-off, waterlogging | |
|
Engineering intervention | Surface drainage | Low impact– disposal issues |
| Groundwater pumping | Low impact– low hydraulic conductivities make pumping expensive. Disposal issues | |
|
Productive uses of saline land and water | Salt tolerant pastures | High impact– to stabilise and aesthetically improve salt affected areas |
| Halophytic vegetation | Low impact– climate and environs not likely to be conducive | |
| Saline aquaculture | Low impact– discharge sites only minor in extent | |
| Salt harvesting | Low impact– groundwater is not sufficiently saline | |
| Others | See OPUS database (NDSP) |
Management implications given projected land use
On the Mornington Peninsula there are projected increases in urbanisation and intensive horticultural industries (e.g. vineyards) that will have significant implications for landscape hydrology. Under these circumstances, and given the nature of the salinity processes in the GFS, planning may preclude such developments in hazardous areas, and institute specific engineering standards for infrastructure to limit degradation potential.
Dahlhaus P.G., Heislers D.S., Brewin D., Leonard J.L., Dyson P.R. & Cherry D.P. (2004). Port Phillip and Westernport Groundwater Flow Systems. Port Phillip and Westernport Catchment Management Authority, Melbourne, Victoria.