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Ponds in the northern Botany aquifer typically recharge groundwater. During occasional dry periods, flow directions are reversed so that low pond levels are maintained by groundwater discharge.
Stormwater and treated wastewater are possible water sources for managed aquifer recharge (MAR) in the Botany sand aquifer, located south of the Sydney CBD. MAR using leaky structures and injection bores can improve water quality and enhance water supply security. Aquifers may form part of the treatment train for water reuse, given long residence times and a degree of natural attenuation capacity for specific contaminants.
Advantages to storing water underground
- Boost long term sustainable groundwater yields
- Reduce risk of consolidation & settlement
- No evaporative losses
- Sand filtered water typically improves water quality through natural attenuation
- Lower costs than equivalent surface reservoirs
- Aquifer can serve as a distribution systems
- Protection from water pollution
- Suitable sites may be more common than for equivalent surface reservoirs
- Less expensive to store water at depth than to pump from long distances
- Assist with flood mitigation
- ASR sites are best located where large volume storages are needed
A pre-feasibility assessment of MAR in the north-eastern Botany aquifer was recently undertaken by UNSW and UTS (UNSW Water Research Laboratory Technical Report 2006/33). This rapid assessment that was based on available data concluded that further investigation was warranted, including updating of groundwater hydrographs, geological and groundwater modeling to assess sustainable yields. A comprehensive groundwater quality survey was recommended along with measurements of actual recharge rates. Rough estimates indicated that each MAR system in the north-eastern Botany aquifer could deliver 20 million litres of water year after year. If the feasibility of multiple MAR systems is verified, about 5% of Sydney's annual water usage could be sourced in a sustainable manner.
Recharge and extraction systems in the Botany aquifer can be designed to achieve desired residence times and incorporate protection zones, based on groundwater flow rates of about 150 m per year. The north-eastern parts of the Botany aquifer are at an elevation at least 30 vertical metres above the base of the aquifer near Alexandra Canal and Botany Bay where contaminated groundwater plumes have been identified. Flow paths towards the south and south-west mean that it is not possible for contaminated plumes to be drawn upwards and northwards.
Challenges for MAR
- Ensuring water quality of recharged water is maintained or improved
- Recharged groundwater mound would eventually dissipate so timing is critical
- Turbidity and suspended solids in available water
- Iron bio-fouling of injection and pump bores
- Obtaining licenses for recharge water where surface supplies are fully allocated
- Cost of artificial recharge trials
- Competing demand for surplus surface flows
- Currently no allocation system for crediting ASR inputs to groundwater
- Retrieving the same water, or an equivalent volume of recharged water
- ASR facilities need to be large scale to be viable and effective
Treatment required prior to recharge would depend on source concentrations and attenuation capacity during the recharge process to ensure protection of groundwater quality and prevention of clogging. In the north-eastern part of the aquifer, groundwater would typically meet drinking water quality guidelines with disinfection and removal of iron, while nitrate concentrations are an issue in some areas. There are many urban aquifers and unprotected catchments around the world that with careful management and application of HACCP (Hazard Action Critical Control Planning) are a component of drinking water supply systems.
Sustainable groundwater yield could be boosted by MAR, possibly as part of a 'multiple barrier' approach for indirect potable reuse of purified wastewater. A concept for optimising supplies during dry conditions by low risk pilot MAR tests using stormwater is being developed by UNSW Water Research Laboratory with colleagues at UTS and proponents of MAR.
Recent CWI-WRL team activities for Managed Aquifer Recharge schemes include: