10.2 Groundwater inflow from outside region at coast
The regional groundwater flow at the coastline into the Adelaide region (2,051 ML) is the sum of flow into 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 the Adelaide Plains (T1 and T2 aquifers).
Coastal groundwater inflow may be either fresh or saline groundwater and is not meant to represent sea water intrusion into the Adelaide region aquifers. However, localised cones of depression due to groundwater pumping have been identified as a potential problem, particularly for saline sea water intrusion, in the McLaren Vale PWA (Martin and Hodgkin 2005) and in the Adelaide Plains (Aquaterra 2011). In the McLaren Vale and Adelaide Plains, salinity monitoring suggests some seawater intrusion may be occurring (Martin 1998; Gerges 1999).
|Groundwater management area||Aquifer||Groundwater inflow at the coast (ML)|
|McLaren Vale PWA||Maslin Sands||175|
|Port Willunga Formation||1,075|
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 Adelaide Plains and the McLaren Vale Prescribed Wells Area 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 below 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 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 blue line at the coast illustrates the area across which groundwater inflow from 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 10.2 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 flow was estimated for the confined and semiconfined aquifers only (i.e. 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 is assumed to be insignificant.
This line item corresponds in part to line item 15.1.1 Regional groundwater inflow reported in the 2010 Account. 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 Adelaide region (reported at line item 10.1 Groundwater inflow from outside region at coast).
- The methodology used to quantify the line item was improved and resulted in a material change in volume: New aquifer properties have been used in the quantification approach.
|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)|
|15.1.1 Regional groundwater inflow||McLaren Vale PWA||Maslin Sands||162||162|
|Port Willunga Formation||750||750|