Case study 7 - Mining groundwater sustainably in the Mallee Region
Background and geographic area
The Mallee Prescribed Wells Area (PWA) is situated in the southwest Murray Basin about 200 kilometres east of Adelaide. It abuts the South Australian – Victorian border.
The Mallee PWA utilises groundwater from the regionally extensive Murray Group Limestone aquifer. This aquifer is the only source of water in the region and is widely used for town water supply, irrigation, and stock and domestic purposes. The main attribute of the Murray Group Limestone aquifer in this region is that it is, on average, more than 100 metres thick, with a maximum of about 140 metres on the South Australian – Victorian border. The aquifer stores more than an estimated 100 million megalitres, although the groundwater moves slowly at only half a metre per year. Fully penetrating bores (about 180 metres deep) yield up to 60 litres per second. The irrigation industry (mainly potatoes by centre pivot irrigation), injects more than $50 million into the region annually.
Most irrigation extractions occur where the limestone aquifer is confined and salinities are low. Consequently, current recharge from rainfall is very low and carbon dating suggests the groundwater is up to 20,000 years old. However, extracted groundwater is replaced by lateral inflows and some inter-aquifer leakage through confining layers. Drawdowns in pressure levels have reached a new equilibrium when extraction levels are stable, with no dewatering of the confined aquifer yet observed on a regional scale.
The current management strategy developed by a joint South Australian – Victorian approach allows for controlled utilisation of the groundwater storage of the larger reserves in storage at a slow rate. In addition to the volumetric limits set by this approach, the Mallee PWA Water Resources Planning Committee set out to achieve the following objectives:
- develop a water allocation plan for the Murray Group Limestone aquifer
- initiate cost sharing arrangements to provide assistance to stock and domestic water users for adjustments of their pumps and wells to irrigation drawdowns
- encourage more efficient water usage by training irrigators and carrying out groundwater monitoring and community education programs
- develop conditions for new extractions to prevent concentrations of use and minimise drawdowns of pressure levels.
Application of a risk–based approach to management
At a regional level, both the Water Allocation Plan for the Mallee PWA in South Australia and the Murrayville Groundwater Supply Protection Area - Groundwater Management Plan in Victoria have extrapolated the methodology used in the border zones to determine permissible annual volumes for their own adjoining areas. In both of these areas, licences and meters are required for irrigation and industrial use, and the management plans determine how the groundwater resources are allocated, ensure adequate monitoring is carried out and encourage efficient use of the resource through metering and education.
Salinity increases have been identified as the main risk to the sustainability of the groundwater resource as a result of extractions and the resultant drawdowns of pressure levels. A salinity risk assessment was carried out which assessed the risk to be low and long term (over decades to hundreds of years).
A groundwater flow and solute transport model was constructed to help determine the salinity risk due to drawdowns from various extraction scenarios.
An adaptive management framework is being adopted whereby extensive water level and salinity monitoring will help determine any changes in allocation levels into the future.
Information requirements to achieve objectives
The key information requirements are appropriate water level and salinity monitoring to establish how the aquifer is actually responding to development, together with accurate metered data of the extraction volumes. This information is essential to ensure effective adaptive management and to ensure the groundwater computer model for the region is well calibrated.
Approach taken in dealing with uncertainty
Uncertainty over information is addressed by using best available science and joint management between jurisdictions. The groundwater resource is robust and slow moving, allowing adequate timeframes for improved monitoring and modelling as more information becomes available.
Approach taken in consulting with stakeholders
Extensive community consultation was carried out for the development of both the water allocation plan and the cost-sharing arrangements (which were funded by a small levy on irrigation allocations).
More infomation, please visit: National Water Initiative