10.2 Groundwater inflow from outside region at coast
The volume presented in the water accounting statements (1,113 ML) represents regional groundwater flow at the coastline through major sedimentary and basalt aquifers. The inflow includes fresh and saline groundwater.
In general, the dominant direction of groundwater flow in the Melbourne region is from the recharge area in the fractured rock aquifers in the north, northwest and northeast towards the Port Phillip Bay and Western Port in the south. However, localised groundwater pumping has been identified as a potential problem for saline seawater intrusion, in the Koo Wee Rup area (Longley, T., Turner, A. and Lawson, J., 1978.) and Werribee Delta sediments (Elmahdi et al., 2005; Melbourne Water - Southern Rural Water, 2004).
Groundwater inflow from the coast is less than for the 2009–10 year (2,683 ML). This is attributable to the high rainfall during the period that resulted in groundwater recharge, as well as reduced groundwater abstraction.
Groundwater flow was calculated using a simple geographic information system (GIS) approach based on Darcy's law. Groundwater levels were interpolated for each season using the ArcGIS top-to-raster tool from reduced groundwater levels measured at monitoring bores. Seasonal groundwater flow grids were derived from groundwater level grids, aquifer thickness and hydraulic conductivity using a modification of the ArcGIS Darcy Velocity tool. Groundwater flow across selected flow boundaries (see the following figure) was then calculated using a simple GIS analysis and seasonal values were aggregated to reporting year.
Groundwater flow boundaries
Assumptions, Limitations, Caveats and Approximations
Regional flow was estimated for layers 1 to 5 of the Port Phillip groundwater model (Department of Sustainability and Environment 2010b), which represent the sedimentary and basalt aquifers with the exclusion of the basement (layer 6). These productive aquifers are considered to be the most hydraulically conductive units and flow in other units is assumed to be insignificant.
Consistent with the groundwater model report (Department of Sustainability and Environment 2010a) groundwater flow in the Nepean Peninsula was calculated only for layer 1. It was modelled as a fresh water lens in the highly permeable unconfined aquifer using the same GIS tool.
Groundwater levels were estimated by assuming that all 5 hydrogeological layers within the Port Phillip groundwater model region are hydraulically interconnected. This assumption facilitated the interpolation of a groundwater potential surface from groundwater level measurements, as these measurements were limited in number. Groundwater levels were also assumed equal to 0 metres australian height datum (mahd) the coastline. These assumptions were used to generate seasonal groundwater level surfaces across the sedimentary area.
Bore locations and the through-flow boundaries considered in the estimations are indicated in the previous figure. Flow across the remaining boundaries was assumed negligible on an annual basis because aquifer properties like hydraulic conductivity limit flow (e.g. fractured rock basement), the northern and northeast boundaries represent a groundwater divide with no through-flow, or groundwater flow was approximately parallel to the boundary (e.g. western boundary).
The uncertainty in the field measured data (e.g. groundwater levels, hydraulic conductivity) was unspecified and unknown; the impacts of such uncertainty on the groundwater flow was not estimated.
The regional flow estimations were based on the interpolated groundwater level grids produced using ArcGIS Topo to Raster tool. Use of different interpolation methods may impact the values of the groundwater level grids and hence the estimated regional flow.
Groundwater flow was estimated for a simplified boundary constructed from a series of line segments. Groundwater flow across this boundary was calculated using the method described above. The uncertainty surrounding this simplification was not analysed.
This line item corresponds in part to line item 15.1.1 Regional groundwater inflow reported in the 2010 Account. In the 2010 Account, this line item was unquantified. In the 2011 Account, the following changes were made that caused the 2009–10 year value to be restated:
- The presentation of the line items has changed since the 2010 Account: Groundwater flow was partitioned into groundwater inflow at the coast and regional inflow for the rest of the Melbourne region (reported at line item 10.1 Groundwater inflow from outside region).
- In the 2011 Account, a suitable quantification approach was available to estimate groundwater inflow from outside the region at the coast.
The restated comparative year volume is 2,683 ML.