Re-using Aquifers

Water is becoming an ever more precious resource, and we need to develop new systems that help us make the most of what we have. Storing treated water in aquifers and then recovering it during dry seasons is one approach that is gaining ground around the world and has many economic as well as environmental advantages. In Canada, a municipality in Ontario recently built and tested a system which injects and extracts the treated water from an aquifer using the same well.

In a world where water is becoming a scarce and precious resource, the Aquifer Storage Recovery (ASR) approach holds a great deal of promise for supplying the water needs of a growing population. The term refers to the storage of treated drinking water by injecting it through a well into an existing natural aquifer. The water is stored underground during the times when it is plentiful, and then can be recovered later through the same well as needed when demand is high. In general, the recovered water does not need to be retreated other than for disinfection.

Aquifer storage recovery is a very efficient and cost-effective way to expand a water supply system because it optimizes the use of existing treatment and transmission facilities, matching water supply to water demand through underground storage. Typically, a municipality’s cost to develop additional peak supply capacity through aquifer storage recovery is less than half the cost of expanding a water treatment plant or distribution system. Capital costs to develop one megalitre per day (ML/d) of peaking capacity with aquifer storage recovery wells average about $150,000 within a typical range of $75,000. Operating costs average about $6,000 a year per ML/d recovery capacity, within a typical range of $3,800.

In addition to the economic benefits, aquifer storage recovery has several environmental benefits. Since the water stored underground is typically obtained during “wet” months when stream flows and surface water levels are higher, the environmental impacts of withdrawing surface water are reduced. Storing water underground may reduce the need for, and environmental impacts of, large surface storage reservoirs, and it will eliminate water losses due to evaporation. In areas where over-production from aquifers has caused declining groundwater levels, the aquifer storage recovery method can be used to replenish these aquifers and restore the groundwater levels. It could also be used to enhance or maintain stream flows and surface water levels in sensitive ecosystems.

Though there are other aquifer storage recovery systems in operation, particular technical and economic advantages exist in systems where the water can be injected and recovered through the same well. In the U.S. this method has been applied successfully at 27 operational sites and is in various stages of development at about 50 other sites. Systems are also operating or under development in England, Australia, Kuwait, and Israel.

The first aquifer storage recovery system began operating at Wildwood, New Jersey in 1969. Almost all other systems, however, have been developed after 1983. The rapid spread of the technology throughout the U.S. reflects its operational success. Several important technical and regulatory obstacles have been overcome, and the systems have shown their cost-effectiveness.

All the systems operating currently are storing treated drinking water, but many are being developed that draw from other sources such as reclaimed wastewater, untreated groundwater from other aquifers, and high-quality surface water following pretreatment.

Aquifer storage zones range in depth from 100 metres to 900 metres and use sand, sandstone, clayey sand, limestone, dolomite, basalt and glacial aquifers. Most storage zones are in confined aquifers although a few are in unconfined aquifers. Storage zones contain fresh water, brackish water and other adverse water quality constituents (such as high nitrate, iron, manganese, hydrogen sulphide and radionuclide concentrations) which make the water unsuitable for drinking unless it is treated. With aquifer storage recovery, the injected treated drinking water displaces the poorer quality water and as a result the stored water can be recovered without further treatment other than disinfection. Recovery capacities range from four to over 400 ML per day, and storage volumes range from 150 ML to 8,000 ML.

The well, wellhead, and wellfield design in aquifer storage recovery differ from conventional production or injection wells. The ASR wells and pipelines are commonly constructed of non-corrosive materials to prevent rust forming, for example, and the wellhead includes valves and piping that permit both injection and recovery of water using the same well. Wellfield design must consider the effects of both injection and recovery on the wells and aquifer.

Aquifer Storage Recovery at Mannheim, Ontario

Canada’s first aquifer storage recovery project that uses the same well for injection and recovery is a pilot testing project located near Mannheim, Ontario by the Regional Municipality of Waterloo. CH2M Gore and Storrie were the consulting engineers for the project and were responsible for the design, construction and testing of the system.

The project involved pumping raw Grand River water through a pipeline to the 73 ML/d Mannheim Water Treatment Plant. There it was treated to potable standards and injected through a well into a semi-confined, glacial moraine, sand and gravel aquifer beneath the plant. The water was later recovered from the same well and pumped to the distribution system after chlorination. The system could help to meet summer peak demands by adding 5 ML/d to the water system’s peaking capacity.

The objective of the project was to demonstrate the technology and confirm the capital costs. Phase I involved drilling test wells at three locations on the Mannheim water treatment plant site. We determined the hydrogeological and geochemical characteristics of the aquifer materials and analyzed the groundwater and treated water quality.

Field investigations identified two potential aquifer storage zones in which an ASR well could be constructed. The first is a semi-confined sand and gravel aquifer. The second is a deep bedrock (limestone) confined aquifer. The site geology and potential aquifer storage zones are illustrated schematically below.

We evaluated the aquifers on a scientific and technical basis to determine which aquifer had the greatest potential. We were very confident that an ASR well would operate successfully in the sand and gravel aquifer because it already supports high-yield municipal wells and it contains fresh water that is similar in chemical composition to the recharge water. The bedrock aquifer is also likely to be capable of supporting high-yield wells. There is, however, some uncertainty about its geochemistry and the potential for storage and recovery of potable water. We therefore decided to construct the ASR well in the sand and gravel aquifer.

The well was constructed of a 406 mm diameter stainless-steel well screen and a 442 mm diameter PVC well casing. A silica sand pack was installed around the well screen and the remainder of the borehole opening was sealed with cement grout.

A 100 hp submersible pump was installed below the well screen, and a Baski Flow Control Valve (FCV) connected to the surface with an epoxy-coated steel pipe (pump column). The FCV is a nitrogen-actuated valve that permits water to be injected and pumped out of the well using the same pump column, thereby preventing recharge water from cascading into the well. If not properly controlled, cascading water can plug ASR wells as a result of air-binding in the aquifer. The wellhead incorporated flow control valves, flow meters, level indicators, sampling ports, and alarms to monitor operations.

We performed aquifer pumping and injection tests to determine the initial hydraulic performance. The well was tested to simulate operational cycles (injection, storage and recovery) using potable water from the water treatment plant. Three cycle tests were co
mpleted with progressively increasing storage volumes of 19, 95, and 189 ML (5, 25, and 50 million gallons). Storage times for each cycle were 3, 15, and 42 days, respectively.

Our primary concerns during the testing was to collect data on the water quality and how well the system worked. We found that the only additional treatment required before the water is distributed is chlorination. We also found that the ASR well is capable of injecting, storing and recovering potable water from the aquifer at a rate of up to 63 L/sec.

The two-year pilot study was successfully completed last year. showing that aquifer storage recovery is a reliable and cost-effective means for storing seasonally available surface water to meet peak summer demands. It also makes more efficient use of existing facilities. The Waterloo Region is currently considering implementing an ASR system as one of three potential options for its long term water supply. The other supply options under consideration are developing new local groundwater supplies and constructing a pipeline to one of the Great Lakes. Of these alternatives, aquifer storage recovery has the lowest capital and operating cost.

Richard Wootton, M.A.Sc.is with CH2M Gore & Storrie Limited of Waterloo, Ontario.

References: Pyne, R. David, G. 1995. Groundwater Recharge and Wells A Guide to Aquifer Storage Recovery. Lewis Publishers, CRC Press Inc.