2.2 Underlying aquifers
The volume presented in the water accounting statements (16,565 ML) represents the sum of the permissible consumptive volume for each underlying aquifer in groundwater management unit in the Melbourne region.
Permissible consumptive volumes were originally announced in 2006 and represent the maximum annual volume that can be extracted from each groundwater management unit. All previous permissible consumptive volumes were amalgamated into a single order in July 2011.
The following table shows the permissible consumptive volume for each underlying aquifer groundwater management unit within the Melbourne region.
|Groundwater management unit||Permissible consumptive volume (ML)|
|Koo Wee Rup Water Supply Protection Area (WSPA)—groundwater||12,915|
|Cut Paw Paw Groundwater Management Area (GMA)
The volume presented does not include groundwater in unincorporated areas outside of groundwater management units. Unincorporated areas have not been evaluated from a groundwater resource perspective and do not have established permissible consumptive volumes. The allocation limit for unincorporated areas in the southern half of Victoria was 71,385 ML. The Melbourne region is only a small part of this area.
For a map of groundwater management units in the Melbourne region refer to Physical information > Groundwater in the 'Contextual information'.
The Victorian Department of Sustainability and Environment (now the Victorian Department of Environment and Primary Industries) determined the volume of underlying aquifers in groundwater management units in order to set a permissible consumptive volume for each aquifer.
Permissible consumptive volumes for the underlying aquifers of groundwater management units were determined as follows:
- Koo Wee Rup WSPA: the permissible consumptive volume is based on numerical modelling and observed response to pumping.
- Cut Paw Paw GMA: the permissible consumptive volume is based on the through-flow method.
Assumptions, limitations, caveats and approximations
- Where the bulk of extraction is from depths greater than 50m, groundwater management units are considered underlying aquifers.
- Permissible consumptive volumes represent the groundwater asset within groundwater management units in the Melbourne region. An estimate of the size of the groundwater asset in the unincorporated areas outside groundwater management units has not been made, making it difficult to estimate the total groundwater resource.
- Koo Wee Rup WSPA includes all geological formations.
- Cut Paw Paw GMA includes all geological formations more than 50m below the surface.
- Cut Paw Paw GMA: the permissible consumptive volume has a low confidence rating due to uncertainty in aquifer parameters and potentiometry.
- Cut Paw Paw GMA: the permissible consumptive volume is based on the assumption that through-flow is equal to the safe groundwater extraction volume.