June 28, 2007
A solution to our dwindling water supply lies below
By PHIL BROWN
Limited water resources combined with competing demands and the potential for extended drought or climate changes require innovative approaches to water resource management. Managing surface water and groundwater resources together is crucial to maintain adequate streamflows for the environment and provide sufficient water for our economies, particularly during low-flow periods.
With the adoption of conjunctive groundwater and surface water management policies in the past decade, both supply sources are now viewed as a continuum. Consequently, increasing groundwater production to supplement surface water supplies or to support additional growth may not be possible.
Continued declines in streamflow, snowpack and groundwater levels have increased the anxiety regarding how much longer current groundwater pumping and diversion practices can sustain growth. Agencies, water utilities and agricultural communities all recognize that increasing water storage is an important component of future water management. However, the environmental and economic costs of new storage are of concern when considering what actions can be taken to develop new sources.
Aquifer storage and recovery (ASR) is a technique that has demonstrated itself as a practical, economic and environmentally favorable method to satisfy increasing water demands, while sustainably managing surface and groundwater systems.
Many utilities have implemented subsurface storage projects to assist in meeting their water supply needs. These projects typically involve storing large volumes of water underground and recovering it through wells for later use. The recharge periods correspond with winter and spring when surface water is available for appropriation when streamflows exceed minimum instream requirements and irrigation/municipal demand is low. The system usually involves a municipality or water utility with surface water rights, a water treatment plant, and existing municipal backup or supplemental supply wells.
ASR projects are under consideration where:
• A utility will install wells to store water to meet peak demands without expanding a water treatment plant.
• A utility will use existing wells to store water purchased at off-peak rates to reduce operating costs.
• A utility will install ASR wells to optimize the size of a new water treatment facility, avoiding a larger plant to meet peak demands with unused winter capacity.
• Long-term solutions are expensive and utilities install ASR systems to extend the supply, buying time for planning and to generate revenue for larger projects.
ASR projects are typically less expensive than aboveground storage facilities and have a smaller environmental footprint. A rule-of-thumb estimate for ASR feasibility studies, permitting, design, construction and a year of monitoring is $500,000 per existing municipal well converted to ASR use (assuming existing surface water rights can be used and a water treatment plant delivers high-quality water to the well through existing infrastructure).
These costs are largely independent of the volume stored and recovered. The aquifer characteristics are the primary control of the storage volumes and recovery rates, so the dollar-to-gallon ratio of an ASR system would vary depending on whether the aquifer could support a 3 million-gallon-per-day (MGD) well or a 0.25 MGD well.
Conversely, aboveground concrete or steel storage tank costs are roughly $1 per gallon and increase in a linear fashion with target storage volume. A simple ASR system using a moderately productive well recharging water at 500 gpm over five months would store about 108 million gallons. That’s 20 times as much water for less than one-fifth the cost of an aboveground storage tank or reservoir.
One limitation of ASR relative to tanks or reservoirs is that water can only be returned to the supply system at the pumping rate of the well. If it is necessary to add water to the supply system at rates greater than the well’s capacity, either a multiple-well recovery system or combined well-plus-tank system could be developed.
ASR buys time
The configuration and operation of an ASR system is site- and situation-specific. Most Pacific Northwest ASR systems developed in the past 15 years have used existing infrastructure to treat, convey and store water in aquifer systems previously targeted for extractive use. By optimizing and extending the life of existing infrastructures, ASR systems can delay investment in costly upgrades.
Golder recently assisted the city of Dallas, Ore., which is not only growing in population but is supplied by a surface reservoir that is losing capacity to silt, resulting in a forecasted supply shortfall by 2010. Dallas’ unique ASR system uses a subsurface storage zone not historically considered an ideal target: No high-capacity supply wells are present in the project area because the bedrock has little permeability and the groundwater is saline. Although the ASR well yield is limited to about 300 gpm, the 50 million gallons that each well can store annually will extend the date of the expected shortfall by five to seven years. This gives the city the opportunity to develop the resources to fund a much larger reservoir project while minimizing the effect on ratepayers.
The city of Walla Walla operates seven supply wells, all completed in a basalt aquifer and sealed through a gravel aquifer and clay aquitard that overlie the basalt. Two wells have been converted to ASR wells. The first has a recharge capacity of 1,300 gallons per minute, while the second, converted in 2003, has a recharge capacity of about 1,600 gpm. In six years, about 3.8 billion gallons of water have been recharged to the basalt aquifer system.
The concept of subsurface water storage with an ASR system can be applied to a wide range of water resource issues, such as maintaining cooler water temperature in heat-stressed streams. One promising addition is river bank filtration (RBF) as a means to treat raw surface water to standards suitable for direct or indirect recharge.
The Environmental Protection Agency has recognized bank filtration as an acceptable means of meeting its filtration requirements. Where cities currently rely on groundwater systems, the cost of a treatment facility to treat water available only in winter months can be daunting. However, installing wells adjacent to streams to capture surface water and treating it by filtration through natural media could lead to a more cost-effective approach to meeting water-quality requirements prior to recharge.
Where groundwater-dependent utilities are experiencing declining water levels and must replace or deepen existing wells, a combined RBF-ASR system would provide a reasonably priced alternative for a new supply source.
Predictable regulations developed for ASR systems in Washington and Oregon, combined with the use of existing intakes, treatment facilities, conveyance and supply wells, will keep ASR economically competitive with other storage options. As the need for additional storage increases, ASR will become more common because it will frequently be lower cost and less controversial than new surface impoundments. Overall, it will be a favored solution because it is cost-effective, environmentally friendly and able to increase the availability of finite water resources when demand is greatest.
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