Proceedings of the International Symposium on Efficient Water Use in Urban Areas


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<Proceedings of the International Symposium on Efficient Water Use in Urban Areas
- Innovative Ways of Finding Water for Cities ->

Aquifer Storage and Recovery in Urban Areas
- Technology, Risks, and Implementation Issues

Peter Dillon
Centre for Groundwater Studies and CSIRO Land and Water Adelaide,
South Australia

SUMMARY

An underground water banking technique known as Aquifer Storage and Recovery (ASR) has emerged as a means of expanding urban water resources by harvesting waters that would otherwise be foregone. Where peri-urban areas are endowed with suitable aquifers, even those where groundwater may be saline, these present opportunities for generating new water resources. Injecting stormwater or treated effluent may restore saline aquifers so that they become underground reservoirs for irrigation supplies. With appropriate pretreatment these supplies can even be made potable. In South Australia, five years of experience with stormwater recycling has led to the development of an ASR trial for reclaimed municipal effluent for recovery as an irrigation supply in the dry season. Some of the issues to be addressed are; can the quality of groundwater be adequately protected?, will the quality of the recovered water be fit for its intended use?, how can we prevent clogging of the injection well?, how much can we store? The research project at Bolivar near Adelaide is currently underway to answer these questions, and to assess technical viability, environmental sustainability, and commercial/economic feasibility at this site, and where possible in a generic sense. Injection of potable water to aquifers in years of plenty could add even more value to our natural urban water infrastructure below ground, by buffering seasonal peak demands that exceed the capacity of water treatment plants, and providing emergency or drought supplies of drinking water. This paper briefly describes the motivations for these developments, the types of artificial recharge methods available, some experience with stormwater ASR and some of the research being performed to address the risks identified in ASR with recycled water. Legal, social and economic aspects of implementing ASR with reclaimed waters are also mentioned.

1. INTRODUCTION

Many cities and agricultural areas rely on conjunctive use of surface water and groundwater. However for some cities which depend solely on surface water, there comes a time when the most economic next source of supply, taking into account the environmental cost of increased diversions or new dams, is groundwater. Where the groundwater quality is unsuitable for supply, artificial recharge of water in times of excess surface water, can produce new groundwater supplies of suitable quality.

Two developments are occurring that will enable more flexibility in future urban water supply. (1) Wastewater treatment processes are improving and reducing in cost, and in some locations, reclaimed effluent is now more economic than developing new resources for some classes of use, such as irrigation. (2) Utilities that differentiate the demand for water across several water quality types can economise on treatment costs. For example only a small fraction of the use of reticulated water is for drinking and human contact, so that dual reticulation systems, especially at smaller than traditional scales of water supply and effluent collection and treatment systems, may be commercially attractive. This flexibility presents opportunities for more holistic urban water management, recycling more water and reducing water imports and discharges of polluted water.

CSIRO, in sympathy with the Council of Australian Governments (COAG) water reform agenda (Thomas et al, 1997) which encourages competition within the water industry, and in partnership with members of the Water Services Association of Australia, is providing research support for implementation of full scale experimental urban water management systems. Quantifying actual costs, risks, operating requirements, public acceptance, and financial and environmental benefits through such trials, will encourage appropriate innovation by infrastructure investors. This also gives opportunities for small-scale investors to profit from provision of alternative water supplies, while generating environmental benefits. Artificial recharge and recovery of stormwater, reclaimed effluent, and mains water are vehicles, among others, to realise system flexibility, augment water resources, expand supply capacity, improve the efficiency of use of water infrastructure, and reduce adverse environmental impacts of urban water systems.

2. ARTIFICIAL RECHARGE METHODS

There are various methods for storing water in aquifers, known collectively as artificial recharge.

Aquifer storage and recovery involves injecting water into a well and later recovering it from the same well. This can be performed even where the native groundwater is not fit for the intended use, such as where aquifers are saline or suffering from relic pollution, so long as the injected water after a period of storage is of suitable quality for its intended use.
Injection and recovery from different wells has the advantage of filtration provided by passage through the aquifer. However this is usually only used when the native water in the aquifer is of suitable quality for reuse, and injection helps to maintain the supply.
Pond infiltration is used when water can be stored in an unconfined aquifer, and leakage from the pond through the unsaturated zone recharges an aquifer, which is subsequently pumped to provide a water supply. Percolation through the unsaturated zone provides relatively rapid attenuation of some contaminants in comparison with passage through the aquifer. Soil aquifer treatment (SAT) is a form of intermittent pond filtration, in which recycled water undergoes alternate nitrification and denitrification beneath a leaky pond.
Induced infiltration describes pumping of groundwater from aquifers that are hydraulically connected to ponds or streams. The hydraulic gradient induces seepage from the surface water into groundwater and provides filtration of the water en-route to the water supply well. This is commonly used in alluvial aquifers in Europe for purifying water supplies.
All forms of irrigation are unintentional artificial recharge. Salt is more concentrated in recharge water than in the irrigation water, and therefore this is not a preferred way to achieve artificial recharge.

The best technique to use depends on local needs, conditions and contraints. However the rest of this paper focuses on aquifer storage and recovery (single well systems), as

this makes the least demanding assumptions on the ambient quality of groundwater,
recovery from the injection well helps to keep it from clogging, and
this has the smallest requirement for land area, which can be an important factor in urban areas.

A number of the risks and implementation issues for ASR are common to the other techniques.

3. ASR WITH STORMWATER, RECLAIMED EFFLUENT AND MAINS WATER

Artificial recharge ponds have been used extensively throughout the world, including Australia, and injection of water into aquifers using wells is much less common. However the number of operational ASR sites has grown remarkably in recent years (Pyne, 1995). In an international context, use of urban stormwater for ASR is relatively unique. A review of international experience in ASR (Pavelic and Dillon, 1997) identified 45 case studies, including 70 known sites in 12 countries, with published information on; site characteristics and recharge techniques; operational problems such as clogging, and means to resolve these; and monitoring of impacts on groundwater quality or the quality of recovered water. Of the 45 case studies, 71% use "natural" source waters (rivers, lakes and groundwater), and the remainder use treated sewage effluent (20%) or urban stormwater runoff (9%) (Figure 1). Highly treated sewage effluent is commonly used in the United States of America. This yields a very consistent, high quality (but expensive) water at a relatively uniform flow rate, making this attractive as a source of water for ASR (National Research Council, 1994). Retention in an aquifer provides the necessary contact with the natural environment to make recovery for potable reuse palatable to consumers. The sustained injection of urban stormwater was found in only four documented cases, three of which were in South Australia.

The question of whether the quality of stormwater and treated effluent is adequate for ASR has been addressed and a survey of the characteristics of these classes of water, and the effects of passive treatment in wetlands reported (Pavelic and Dillon, 1995). Stormwater quantity and quality are determined by rainfall, catchment processes and human activities, which cause its flow and composition to vary in space and time. Municipal treated effluent on the other hand, is much more consistent in flow and composition, and water quality is determined by source water, the nature of industries connected to sewer, and their proportion of sewer discharge, and the effluent treatment processes. For the set of Australian samples considered stormwater had higher suspended solids, heavy metals and bacterial numbers, and lower dissolved solids, nutrients and oxygen demand than secondary treated sewage effluent. Guidelines on the quality of water for injection have been developed subsequently, as explained later in this paper.

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Much has already been written about the benefits of ASR with stormwater (eg Dillon et al 1997, Gerges and Howles 1998). These have demonstrated that saline and brackish aquifers can be freshened for use as irrigation water supplies. ASR takes two un-utilised water resources, and adds value to them both by blending them at times of excess supply, storing until times of peak demand, and then recovering the water for the highest valued uses to which it can be applied. The economic viability of ASR with stormwater may depend on the establishment of urban stormwater detention ponds for flood mitigation or other purposes.
The viability of ASR with post-secondary treated effluent is currently being evaluated on the Northern Adelaide Plains. This will make use of installed effluent treatment capacity for direct reuse of reclaimed effluent for irrigation of food crops. Effluent supply is relatively constant throughout the year, but irrigation demand varies, and effluent supply in winter exceeds irrigation demand. Storage of surplus treated effluent in depleted aquifers over winter will enable it to be recovered in summer to meet peak demand, and thereby allow expansion of the irrigated area. A distributed network of ASR wells will enable this expansion without increasing the capacity of trunk pipelines in the irrigation distribution system (Gerges, 1996).

In USA, UK, Netherlands and Israel, ASR with mains water or its equivalent is practised. This is used to serve peak water demand where this exceeds water treatment capacity, and storage within the distribution system is small. Surplus treatment capacity in the off-peak season is used to treat water for injection into an aquifer, for subsequent recovery and return to the distribution system at times of peak demand, usually with minimal post-treatment (eg chlorination). In one system in UK, ASR is used for mains pressure compensation even on a daily basis.

An unexploited aquifer underlying or near a city is latent water resources infrastructure, which has a capacity to store, treat, and distribute water. A good aquifer, can be considered therefore as both a dam, treatment plant, and reticulation network, for which the capital cost is a comparatively trivial access and restoration charge, and the operating costs involve pre- and post- treatment, pumping, and monitoring costs. The access charge is simply the cost associated with drilling a well. Restoration costs are negligible where the groundwater is of suitable quality for the intended use of the recovered water. However in arid and semi-arid areas aquifers are commonly brackish or saline, and some injection of water is required to establish a water quality buffer zone.

As a principle, improving the environmental value of the groundwater resource should be an aim of ASR. As drinking water has a higher resale value than lower classes of water such as irrigation, it has been suggested that the utility of an aquifer would be improved most by insisting that injected water needs to be of potable standard. In reality, treatment of reclaimed waters to potable standards is rarely economically viable. Furthermore, this is not precluded as a future option so long as the principle of improving the environmental value of the resource is adopted. In addition, the needs of existing environmental values of the groundwater system need to be taken into account, including existing beneficial extractive uses of the groundwater and ecosystem support values. That is, the potential for negative aspects of ASR on groundwater quality and pressures should not be overlooked.

This paper advocates ASR not disposal of unwanted water into aquifers without thought of reuse. The latter may have significant and undesirable impacts on groundwater quality and pressures. The recovery element is important for maintaining a longer term hydrologic equilibrium in the aquifer and ensuring that there is an ongoing vested interest in the quality of the injectant.

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