10.2 Groundwater inflow from outside region at coast
The volume reported (1,562 ML) represents regional groundwater inflow at the coast to the Adelaide region during the 2012–13 year, 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 PWA 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||163
|Port Willunga Formation||1,054|
South Australian Department of Environment, Water and Natural Resources: Drillhole Enquiry System, groundwater level data from online groundwater database; Aquaterra 2011.
Coastal groundwater inflow to the Adelaide region was calculated using a simple Geographic Information System (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 2012–13 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 2012–13 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 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 this line item 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.