Adelaide
18.2 Groundwater outflow to outside region at coast

Supporting information

The regional groundwater outflow at the coastline of the Adelaide region (921 ML) was the sum of flow out of the region at the coast through the major sedimentary aquifers in the McLaren Vale prescribed wells area (PWA) (Maslin Sands and Port Willunga aquifers) and Adelaide Plains (T1 and T2 aquifers).

Regional coastal groundwater outflow, itemised by management area and aquifer type, is shown in the following table.

 

Regional groundwater outflow at the coast broken down by management area and aquifer in the 2011–12 year 
Groundwater management area

Aquifer

Groundwater outflow at the coast (ML)
McLaren Vale PWA Maslin Sands 11
Port Willunga Formation 56
Adelaide Plains
T1 740
T2 114
Total 921

Quantification approach

Data source

South Australian Department of Environment, Water and Natural Resources (DEWNR): Drillhole Enquiry System; Aquaterra 2011.

Provided by

Bureau of Meteorology (the Bureau).

Method

Coastal groundwater inflow to the Adelaide region was calculated using a simple GIS approach based on Darcy's Law (Bureau of Meteorology 2010). Groundwater levels measured at monitoring bores were interpolated to a groundwater-level grid for each season during the 2011–12 year using the ArcGIS Topo-to-Raster tool. Seasonal groundwater-flow grids were then derived from these groundwater-level grids along with aquifer thickness and hydraulic conductivity data using a modification of the ArcGIS Darcy velocity tool. Groundwater inflow was subsequently calculated across selected coastal boundaries in the McLaren Vale PWA and the Adelaide Plains using a simple GIS analysis. Seasonal inflow volumes were summed to determine the total volume for the 2011–12 year.

The following two maps are presented to aid interpretation of the methodology used to quantify this item. The first map illustrates the location and concentration of bores used in the quantification approach.

Map showing the location and concentration of bores used in the quantification approach
Map showing the location and concentration of bores used in the quantification approach

The second map identifies the lateral groundwater flow boundaries used in the quantification approach. The green line at the coast illustrates the area across which groundwater outflow to outside region at coast was calculated.

Map of the lateral groundwater flow boundaries in the Adelaide region
Map of the lateral groundwater flow boundaries in the Adelaide region

Assumptions, limitations, caveats and approximations

  • Flow across boundaries not included in the estimates for line item 18.2 was assumed to be negligible on an annual basis either due to limited flow (e.g. in fractured rock) or could not be estimated because limitations of the method prevented its application.
  • Regional groundwater outflow was estimated for the confined and semi-confined aquifers only (T1, T2, Maslin Sands and Port Willunga Formation). These productive aquifers are considered to be the most hydraulically conductive units and flow in other units was assumed to be insignificant.

Uncertainty information

The uncertainty estimate was not quantified.

Comparative year

Compared to the 2011 Account, a different bore data set was used for the 2012 Account corresponding to data available at the time of compiling the report. Applying this new bore data set to the 2010–11 year resulted in a material difference (33%) with the volume quantified for the 2011 Account. Consequently, the 2010–11 year volume has been restated from 1,363 ML in the 2011 Account to 919 ML in the 2012 Account as shown in the following table.


Restatement of comparative year information for line item 18.2 Groundwater outflow to outside region at coast

Groundwater management area

Aquifer

2012 Account volume for the 2010–11 year (ML)

2011 Account volume for the 2010–11 year (ML)

Additional information

McLaren Vale PWA

Maslin Sands

12

13

new bore data set

Port Willunga Formation

60

60

new bore data set

Adelaide Plains

T1

745

722

new bore data set

T2

102

568

new bore data set

Total

919

1,363