Water Banking
Water banking (also called Managed Aquifer Recharge or MAR) is a conjunctive technique that seeks to deliberately increase the amount of water stored in an aquifer, which is then recovered by pumping. By storing water underground, loss of water to evaporation can be drastically reduced and the water savings returned to other users such as the environment, towns or irrigators (Table 1). Water banking offers several advantages when compared with the more traditional use of surface water reservoirs (Table 2). The potential for water banking is enormous as the total volume of water that could be stored in aquifers worldwide is several orders of magnitude greater than that available in surface dams.
| Dam name | Potential water evaporated (GL/yr) |
|---|---|
| Lake Victoria, NSW | 1730 |
| Lake Argyle, WA | 1400 |
| Menindee Lakes, NSW | 850 |
| Burdekin Falls Dam, QLD | 300 |
| Fairbairn Dam, QLD | 220 |
| Hume Dam, VIC | 200 |
| Lake Gordon, TAS | 140 |
| Lake Eildon, VIC | 100 |
| Warragamba Dam, NSW | 80 |
| Subsurface Storage | Small dams and surface reservoirs | Large dam reservoirs | |
|---|---|---|---|
| Advantages | Little evaporation | Ease of operation | Carryover capacity |
| Widely distributed | Response to rainfall | Low cost per m3 water stored | |
| Operational efficiency | Multiple use | Multi-purpose (power, recreation, flood control) | |
| Available on demand | Groundwater recharge | ||
| Water quality treatment | |||
| Seawater intrusion | |||
| Limitations | Slow recharge rate | High evaporation rate | High evaporation |
| Groundwater contamination | Relatively high unit cost | Complexity of operation | |
| Cost of extraction | Absence of over-year storage | Siting | |
| Recoverable fraction | Removal of water from the river system | High initial investment cost | |
| Time needed to plan and construct | |||
| Loss of arable land | |||
| Loss of habitat | |||
| Key issues | Rising water levels | Sedimentation | Social impacts |
| Management of access and use | Adequate design | Environmental impacts | |
| Groundwater salinisation | Dam safety | Sedimentation | |
| Groundwater pollution | Environmental impacts | Dam safety |
Water banking has been used successfully in Europe for over 50 years and is increasing in application in the United States. The number and scale of schemes increased by an order of magnitude in the United States since mid 1990s; one scheme in Orange County California now stores about 300 GL per annum (Mills, 2002).
In Australia, most of the focus has been on storing recycled/non-potable water for irrigation and municipal use. Currently there are about twenty small-scale (< 1 GL per year) water banking pilot schemes such as Andrews Farm, Greenfield, Parafield Airport, Mawson Lakes, Angas Bremer and Wilunga Basin located mainly in South Australia. The schemes are to relieve pressure on reticulated water supplies by providing users of large volumes of water, such as market gardens and recreation areas, with a reliable supply of alternative water (Gerges et al., 2002). Investment has been from both the private and public sectors. The oldest water banking scheme in Australia is located in the Burdekin Delta. It is use to replenish aquifers that have been depleted because of long term use for sugarcane irrigation (Charlesworth et al., 2002). In the Northern Territory, pilot schemes are being trialled in remote indigenous communities to provide water security during the seasonal dry periods (Pavelic et al., 2002).
Types of water banking
There are a number of technologies under the banner of water banking. They can be roughly divided (although there is significant overlap) into two broad categories, technologies that are primarily for recharge (Aquifer Storage and Recovery, infiltration pond, rainwater harvesting, underground dams and recharge releases) and technologies that are primarily for water treatment (bank filtration, dune filtration and soil aquifer treatment). Below, is a brief description of the major categories of water banking. Images are from Dillon (2004).
Aquifer Storage and Recovery (ASR) and Aquifer Storage, Transport and Recovery (ASTR)
ASR and ASTR both use bores to inject water into an aquifer. In the case of ASR, the same bore or an adjacent bore is used to recover the water from the aquifer. ASTR uses an adjacent bore to draw the water through the aquifer, which increases the zone of water treatment.
Figure 1: ASR and ASTR water banking
Bank Filtration
Bank filtration makes use of the connected nature of most stream/aquifer systems. Water levels are kept high in streams that lose water to the groundwater causing an increase in aquifer recharge. The water is then withdrawn from the groundwater system from a bore located near the river.
Bank filtered water makes up a large proportion of groundwater supplies in Europe (Tuinhof and Heederik, 2002). It provides water quality improvements compared with water taken directly from the river due to the absorptive capacity of the aquifer. The other benefit is resource security because the capacity of the system is limited only by the ability of the aquifer to process the water and not by the surface water resource, which is generally much larger.
Figure 2: Bank filtration water banking
Dune Filtration
Dune filtration is another type of water banking that is used primarily for water treatment. Pretreated water is pumped into a dune swale and then reharvested at a lower level after gravity transport through the dune. Biological and chemical processes within the dune remove residual organic material, nitrogen and pathogenic microorganisms.
Figure 3: Dune filtration water banking
Infiltration Pond
An infiltration pond is one of the simplest and most effective methods of increasing the amount of recharge into an aquifer. Water is pumped into a permeable area above the target aquifer to allow water to infiltrate by gravity into the groundwater system. In many ways, it is the opposite of a traditional dam, where leakage is kept to a minimum, instead of encouraged. The ponds are highly effective but like most water bank techniques, they suffer from clogging, though this can be managed by mechanical scourers and pretreatment.
Figure 4: Infiltration pond water banking
Rainwater Harvesting
Rainwater harvesting is a variant to traditional rainwater collection using a tank as storage. Rainfall is collected from a catchment surface and stored in an above ground tank. Instead of wasting excess water as in the case of a typical collection system, overflow is stored in an underground percolation tank where it recharges the groundwater system. The water is then available to be exploited from a nearby bore.
Figure 5: Rainwater harvesting
Soil Aquifer Treatment
Soil aquifer treatment also makes use of the natural chemical and biological processes within soil (unsaturated zone) to "polish" treated wastewater. After conventional wastewater treatment, water is pumped into infiltration ponds and then returned via recovery wells. Soil aquifer treatment is most commonly used to remove residual organic material, nitrogen and pathogenic microorganisms (NCSWS, 2001).
Figure 6: Soil aquifer treatment water banking
Underground Dam
Underground dams are a low technology solution to storing water that is well suited to fractured rock terrain or regions with limited resources. A low permeability barrier is introduced into an aquifer either through injection or excavation, which stops the flow of groundwater until the water level rises above the obstacle. Water that would normally drain away is then available to be exploited.
Figure 7: Underground dam water banking
Recharge Releases
Water banking, via recharge releases, makes use of existing (or purpose built) dams that capture surface water during floods and then release for slower infiltration. The water is then reharvested down gradient by production bores. It is a useful technique in steep terrains, where water normally flows too quickly to allow significant infiltration to the groundwater system.
Figure 8: Water banking by recharge releases
Potential Problems of Water Banking
Water banking is not a magic bullet that will cure all the ills associated with water supply. However, use in conjunction with traditional water storage infrastructure and carefully managed it can be very useful tool. This section discusses some of the potential problems with water banking.
Clogging
Much as sedimentation can render dams unusable, clogging in its various forms of an aquifer can make a water banking scheme inoperative. Clogging can affect all types of water banking methodologies and can vastly decrease the amount of water that can be recharged into the aquifer. In the case of ASR, it can also decrease the amount of water that is recovered from the aquifer.
There are four major types of clogging (Figure 8). Surface clogging occurs with the gravity settling of fine-grained sediments sealing the surface of the aquifer. Straining occurs under pressure, when the fine-grained sediment enters the pores of the aquifer. Physical-chemical clogging is due to reaction of the recharge water with the aquifer sediment or matrix, such as the deposition of calcite. Finally, bridging is a special type of clogging, where although the amount of sediment involved is low, silt grains get wedged across the interstices of the aquifer preventing flow of water.
Figure 9: Showing the different mechanisms of clogging in aquifers (McDowell-Boyer et al., 1986)
There are a number of strategies that can be used to manage clogging problems. The primary one is to eliminate the problem either through removal of the suspended material in the water by the use of settling ponds, flocculating chemicals and/or sand filters to decrease the amount of suspended material prior to recharge; or to alter the chemistry of the recharge water so that it does not react with the aquifer. While expensive, such treatment regimes are in use in areas such as Orange County, USA, where the water is both filtered and passed through a reverse osmosis system prior to recharge (Mills, 2002).
The other (often used in conjunction with the first) is to remove the material deposition. This may be accomplished by machines, which scour the bottom of infiltration basins or by flushing aquifers to remove build-up.
Water quality
Although, water banking can be used to treat water for certain water quality problems such as bacteria and some pesticides, it can be vulnerable to contamination if the water is not suitable or it overwhelms the ability of the aquifer to process the waste. This is why water banking schemes need to be strictly monitored.
Potential losses from the system
Building up an artificial groundwater mound in an aquifer can increase the rate of groundwater movement away from the recharge area due to an increase in hydraulic head. Movement can be either within the aquifer, discharging at the surface or between aquifers. Again, proper management of the water banking system can overcome this problem by withdrawing water within the period determined by the aquifer characteristics.
Water accounting/security
A large water banking scheme and associated infrastructure is expensive to develop, so users have to be assured of the cost-benefit of such an investment. Currently, the administrative arrangements needed to develop water banking in Australia are not in place. For example, farmers that harvest water during a major flood for use in a water banking scheme have this water counted against their entitlement despite the surplus of water. As not all of the water from a scheme can be recovered (some is kept in storage and some would be used for environmental/aquifer benefits), it makes more sense for irrigators to only take water when needed. This means that water must be stored in surface storages, transported in channels and flows in rivers must be kept in non-natural states (high flows in summer during the irrigation season), all of which are susceptible to loss of water. Further, in the current situation, irrigators may not be able to withdraw their water because the groundwater cap is exceeded in the catchment.
For water banking to become a real alternative, then true conjunctive water management accounting needs to be implemented. Groundwater levels or surface water flows could be based upon a mandated management period, in which levels/volumes may vary within the period but not between periods. The longer time periods will take into account periods when the full entitlement is not taken from the river, but is instead withdrawn from the water bank scheme.
Case Studies
Potential for water banking in the Murray Basin
Relevant Links
ASR Forum
IAH Commission on Management of Aquifer Recharge
FAO Conjunctive use of surface and groundwater
USGS Artificial Recharge Workshop Proceedings, Sacramento California
References
Charlesworth PB, Lowis B, Laidlow G, McGowan R, 2002. The Burdekin Delta - Australia's Oldest Artificial Recharge Scheme. Management of Aquifer Recharge for Sustainability - Dillon (ed) 2002, Swets and Zeitlinger, Lisse USBN 90 5809 527 4, pp 347-352.
Dillon P, 2004, Engineering Technologies, in Rivers and aquifers: Towards conjunctive water management, Adelaide, Workshop Report, Bureau of Rural Sciences.
Gerges NZ, Dillon PJ, Sibenaler XP, Martin RP, Pavelic P, Howles SR, Dennis K, 2002. South Australian experience in aquifer storage and recovery. Management of Aquifer Recharge for Sustainability - Dillon (ed) 2002, Swets and Zeitlinger, Lisse USBN 90 5809 527 4, pp 453-458
McDowell-Boyer LM, Hunt JR, Sitar N, 1986. Particle transport through porous media, Water Resources Research, 22(13) 1901-21.
Mills W, 2002 The quest for water through artificial recharge and wastewater recycling, Management of Aquifer Recharge for Sustainability, P. Dillon (ed), A.A. Balkema Publishers, p. 3-10.
NCSWS, 2001. An investigation of soil-aquifer treatment for sustainable water reuse. National Center for Sustainable Water Supply. http://www.mines.edu/~jdrewes/WhitePaper.pdf
NLWRA, 2000. Australian Water Resources Assessment 2000. National Land and Water Resources Audit Land and Water Australia, Canberra
Pavelic P, Dillon P, Toze S, Barber C, Yin Foo D, Knapton A, Jolly P, 2002. Water banking in the Australian tropics: results from a trial on South Goulburn Island, Northern Territory. Management of Aquifer Recharge for Sustainability - Dillon (ed) 2002, Swets and Zeitlinger, Lisse USBN 90 5809 527 4, pp 441-446.
Tuinhof A, Heederik JP, 2002. Management of aquifer recharge and subsurface storage: making better use of our largest reservoir. Seminar, Wageningen, 18-19 December 2002. Netherlands National Committee for the International Association of Hydrogeologists.