18.2 Groundwater outflow to outside region at coast
The volume presented in the water accounting statements (121,259 ML) represents the regional groundwater flow out of the Melbourne region through the major sedimentary and basalt aquifers at the coastline. This outflow may or may not specifically represent groundwater discharge to the sea.
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.
Port Phillip Catchment Management Authority groundwater model (Department of Sustainability and Environment 2010b), hydraulic conductivity and aquifer thickness; Victorian Department of Environment and Primary Industries bore locations, groundwater level data and aquifer attribution.
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 Topo 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 five 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 meters Australian Height Datum (mAHD) at 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 impact 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.