– By Russell Martin –
10 August 2009 – The need to integrate all of our available water sources is the only means by which cities and towns can provide security of supply to meet the demands of predicted population increases within the next 10 to 20 years. In response to Australia’s ongoing drought, most large metropolitan cities have opted for engineering solutions such as desalination and treated wastewater reuse to ease water restrictions.
Urban stormwater runoff is still largely untapped and in many cases equals the amount of water the same cities use for drinking, irrigation and industry. Urban stormwater can potentially be used to substitute potable water where the intended use can accept water of lesser quality, eg irrigation of open recreation spaces or third pipe schemes for human non-contact uses.
It is acknowledged that stormwater runoff has an important role to play in flushing environmental systems, but the untapped potential to capture and recycle stormwater currently being lost could be used to deliver large quantities of water for urban and agricultural purposes. Whichever water source is used there is still a mismatch between availability and demand. In order to meet peak summer demand, storage must be provided during winter.
Most metropolitan cities have limited opportunities to create new or expand existing storage infrastructure and therefore aquifers present a perfect solution.
As part of the National Water Commission projects, “Facilitating Recycling of Stormwater and Reclaimed Water via Aquifers in Australia”, and Managed Aquifer Recharge – an Introduction (Waterlines Report Series Number 13) Sinclair Knight Merz, in collaboration with Australia’s Commonwealth Scientific and Industrial Research Organisation has carried out assessments of subsurface storage opportunities for selected regions in Australia and provided input into costs associated with the feasibility investigations and installations for various operational schemes.
Managed Aquifer Recharge (MAR) with stormwater offers great promise in urban environments providing a “new”, low cost option compared to desalination of seawater. MAR presents a significantly smaller carbon footprint than a desalination plant and the levelised unit cost per kilolitre is A$1.12 compared to A$2.45 to A$3.76 for seawater desalination (NWC 2009).
However, there is a relatively high up front cost in assessing the potential for schemes due to the limited availability of information on the subsurface hydrogeological properties of aquifers.
MAR is the process of intentionally injecting or infiltrating water into an aquifer and then extracting the water for use at a later date.
Aquifer Storage and Recovery (ASR) is a specific type of MAR that involves injecting harvested source water (stormwater or recycled water) via one or more wells, into an aquifer with subsequent extraction for reuse via the same wells. ASR is predominantly a means of storing water, but may also provide further water treatment as a result of the biogeochemical processes within the aquifer (Dillon et al. 2008).
In general, the selection of an ASR site has four requirements:
- A demand for water of the quality that can be recovered
- Access to a source of water, such as from a stormwater drain or recycled water pipeline
- Sufficient land available to build a detention storage and/or treatment system
- An aquifer with suitable storage capacity and water quality, and allowing adequate rate of injection and recovery.
The method for a regional ASR assessment is based around the concept that the achievable recharge volume is determined by the interrelationship of three key factors:
- ASR potential – areas considered preferable based on hydrogeological criteria (eg hydraulic conductivity, aquifer thickness and available storage)
- Source water – stormwater runoff from open drains and creeks (rural) and closed or lined drains (urban), or water reclaimed from sewage treatment plants
- Open space – to harvest and treat stormwater from creeks and drains.
Spatial analysis of aquifer parameters and operational constraints is used to determine the annual injection rate per bore for a given area; with an ASR potential map for each distinct aquifer system produced.
The next step is to highlight suitable areas of open space as potential locations for detention storage to harvest and treat the stormwater.
The identified areas are then integrated with the ASR potential (using a spatial overlay approach) to identify areas that coincide with suitable aquifers. It should be noted that where open space is limited mechanical treatment options can be employed but that will increase the unit cost per kilolitre of water and any social benefits associated with a wetland in an urban setting are lost.
The ASR potential represents the volume of water that could be injected through a single bore that fully penetrates the aquifer and is operated continuously for 180 days (Dudding et al 2006). Aquifer properties in conjunction with the operational constraints (eg maximum allowable impressed head, well efficiency, period of operation, and velocity across the well screen) were used to determine the annual injection rate per bore, in four categories ranging from very low to high.
Table 1: Categories for estimated ASR capacity
Category Average Injection Rate Annual Injection Volume (ML/yr)
High – >1 ML/day > 200
Moderate 0.5 – 1 ML/day 100 – 200
Low – 0.1 – 0.5 ML/day 20 – 100
Very Low – <0.1 ML/day < 20
In broad terms, the categories described above reflect the potential magnitude of an ASR project. Sites with very low ASR capacity may still be adequate for small-scale operations, however larger scale operations are much more likely to be economically attractive (Dillon and Pavelic, 1996).Russell Martin, is Principal Hydrogeologist, Sinclair Knight Merz