18.2 Groundwater outflow to outside region at coast
The regional groundwater outflow at the coastline of the Adelaide region (1,363 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.
|Groundwater management area||Aquifer||Groundwater outflow at the coast (ML)|
|McLaren Vale PWA||Maslin Sands||13|
|Port Willunga Formation||60|
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 2010–11 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 2010–11 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 throughout the McLaren Vale PWA and Adelaide Plains used in the quantification approach.
Map showing 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 blue line at the coast illustrates the area across which groundwater outflow to outside region at coast was calculated.
Map of 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 semiconfined 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.
This line item corresponds in part to line item 16.1.1 Regional groundwater outflow reported in the 2010 Account. In the 2011 Account, the following changes were made that caused the 2009–10 year value to be restated:
- Groundwater outflow was partitioned into groundwater outflow at the coast and regional groundwater outflow for the rest of the Adelaide region (reported at line item 18.1).
- The methodology used to quantify the line item was improved as new aquifer properties have been used in the quantification approach.
Restatement of the value published in the 2010 Account was made as the difference was material and increased the accuracy of the information provided to the users of the National Water Account. The changes and their respective values are detailed in the following table. The restated comparative year volume is 1,243 ML.
|2010 Account line item name||Groundwater area||Aquifer||Volume at 30 June 2010 reported in 2010 Account (ML)||Value difference to volume reported due to reclassification (ML)||Value difference to volume reported due to change in methodology (new aquifer properties) (ML)||Volume at 30 June 2010 reported in 2011 Account (ML)|
|16.1.1 Regional groundwater outflow||McLaren Vale PWA||Maslin Sands||9||9|
|Port Willunga Formation||239||239|