Securing Broken Hill's Water Supply: Assessment of Groundwater Extraction and Conjunctive Water Supply Options at Menindee Lakes
Professional Opinion No 2011/02
Prepared by Geoscience Australia for the Department of Sustainability, Environment, Water, Population and Communities
2011
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Executive Summary
Broken Hill's water supply is currently reliant on a 110 km pipeline from the Darling River at Menindee. Substantial volumes (~300 GL) of water also need to be retained in the adjacent Menindee Lakes Storages (MLS) in order to secure the town's water requirements (<10 GL/yr) in times of drought. This is because the shallow nature of the Lakes results in the evaporation of a significant amount of water, estimated to be an average loss of 420 GL per year, with a nominal market value of some $420 m. The opportunity cost of this water evaporating each year is realised by downstream irrigators, communities and ecosystems. Moreover, in prolonged periods of drought, even this storage has proven insufficient to provide security of supply of suitable quality.
Changing the management of Menindee Lakes to provide enhanced water security for Broken Hill and reduce these evaporative losses is possible, but Broken Hill's water supply would first need to become less reliant on the MLS. To this end, the Broken Hill Managed Aquifer Recharge (BHMAR) project is tasked in this Professional Opinion with assessing whether a 'groundwater extraction-only' scheme, or a conjunctive use scheme with Managed Aquifer Recharge (MAR) as a key component, are technically and scientifically viable options within the priority target site south of Menindee. More specifically, the project is charged with determining, with a defined level of confidence, whether at least 3 years water supply (~30 GL), at a similar salinity to that already available for Broken Hill would be available at all times through these new arrangements. The following conclusions are drawn:
- Aquifer Suitabilty: The GWMAR1 priority target, and Jimargil sub-area, contains an excellent aquifer (the Calivil Formation), with high storage capacity, very high transmissivities, and significant volumes of fresh groundwater, with a small number of manageable risks with respect to water quality guidelines and potential clogging of wells. The aquifer is sandwiched between variably thick clay aquitards, and over much of the target can be characterised as varying from a confined to a 'leaky confined' system. It is concluded with a high degree of confidence that these excellent hydraulic properties make the Calivil Formation aquifer potentially suitable for groundwater extraction and/or MAR injection, with good recovery efficiencies.
- Option 1: Groundwater extraction only. Significant volumes of good quality (<500mg/l TDS) groundwater in the GWMAR1 target (110 GL) and in the immediate vicinity of Jimargil (~22 GL), as well as 36GL of acceptable water (500-1000mg/l TDS) at Jimargil, have been identified. However, an acceptable extraction rate of this resource is based on the extent of natural aquifer replenishment, rather than aquifer storage. Studies have shown that the Calivil aquifer is recharged primarily during the episodic major flood events. Therefore, it is concluded with a high level of confidence, that the extraction of 10 GL/yr (for 3 years) from Jimargil during drought conditions when recharge is negligible, would be unsustainable even over a 12 month period. Deterioration in groundwater salinity in the borefield, and negative environmental impacts, are forecast in this scenario. Quantification of these impacts requires further groundwater data and modelling. Developing multiple borefields to access a number of fresh groundwater zones in the GWMAR target area would greatly expand the footprint and cost of the groundwater extraction option.
- Option 2: Conjunctive use involving a significant MAR component. Under this option, Broken Hill and Menindee water is supplied from existing surface water arrangements during non-drought times, with a MAR component providing water security during drought events. In the vicinity of Jimargil, there is adequate storage in the Calivil sands and gravels to meet the 30 GL requirement, including an in-situ resource of 58GL of good to acceptable water quality that would provide >90% recovery efficiencies from a MAR scheme. Furthermore, a pre-commissioning semi-quantitative residual risk assessment of 12 hazard types indicates all of the scientific/technical risks for MAR at Jimargil are low. The Jimargil sub-area represents a premium MAR site, and the team have a high degree of confidence that this can be developed into a strategic water storage to secure Broken Hill's potable water supply for 3-years during drought conditions. This option would provide a buffer against future climate variability and change, deliver significant water savings, improve source water quality over time, have minimal environmental impact, preserve some local water amenities for community use, and enable key elements of the engineered MLS to be returned to a more natural condition.
The principal recommendation is that a decision be made to proceed with a conjunctive use scheme involving MAR to provide a secure water supply for Broken Hill. There is sufficient evidence that a MAR scheme based around Jimargil would be viable. Work to finalise the MAR assessment and design of the scheme should be completed, and injection trials carried out using an appropriately treated water source. This work is required to test aquifer hydrodynamics and water quality risks under National MAR Guidelines, and to determine future treatment options.
In summary, scientific and technical risks of delivering at least 3 years water supply to Broken Hill via a conjunctive use scheme involving a significant MAR component are low. Logistic risks are also an issue in the short term, with access to finalise data collection limited by current flooding.
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