Applying underground hydro-logic

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ASR offers a low-cost alternative for strapped water providers to extend their resources, according to Gord Cope. It is gaining traction in water-scarce areas around the world.

Precipitation is the most important source of fresh water in the world, but it is also blithely indifferent to human needs. When it falls as snow during winter months, spring runoff can be preserved through above-ground reservoir and canal systems, but when it falls as rain, all too often it drains unbidden into the sea. Part of the reason is due to timing; many regions get the majority of their rain in a short time period, often when it is not needed.

The other reason is cost; most river valleys amenable to storage have already been dammed, and above-ground storage is prohibitively expensive.

Aquifer storage and recovery (ASR), an application with almost three decades of large-scale use, can provide a realistic alternative, and the concept is gaining traction globally. The ASR sector measures activity in terms of the installed pump capacity at each wellhead to inject and withdraw water on a daily basis. “Globally, there is about one billion gallons per day [3.79 million m3/d] of ASR capacity right now,” says David Pyne, a hydrogeological consultant based in Florida. “By 2020, it wouldn’t surprise me to see 5 BGD [18.9 million m3/d] of capacity.”

ASR involves the injection of water under pressure into a well connected to a natural underground reservoir during times when water is available, and the recovery of water through the same well during times of need. In many cases, the storage zones are aquifers that have experienced longterm declines in water levels due to heavy pumping to meet increasing water needs. Groundwater levels can then be restored if adequate water is recharged.

ASR’s commercial application began in the early 1980s. “From a cold start in 1983 with the first system in Manatee, Florida, ASR grew to about 25 operating systems in 1995,” says Pyne. “By 2005 there were over 70, and now there are over 95 in the US alone.” Slightly more than half of all commercial systems are located in the US, with the remainder in a broad range of countries, including the UK, Canada, Australia, South Africa, Israel and the Middle East. Thanks to several major projects, capacity has tripled over the last decade, from about about 1 BGD (3.79 million m3/d) today.

The ASR service industry is broadly divided into engineering services, such as those offered by Schlumberger Water Services (SWS), CH2M Hill and CDM, and a widely dispersed array of water well contractors that install the physical infrastructure. Capital costs for an ASR system average about $1.00 per gallon per day ($264/m3/d). “A 10 MGD [37,850m3/d] system would cost $10 million,” says Pyne. “That includes designing, which is a relatively small portion, as well as drilling wells and equipping them, which is most of the cost. Water well contractors are very supportive of ASR, because in addition to injection and recovery wells, you need monitor wells and pumps and other equipment.” If, as predicted, global capacity expands to 5 BGD (18.9 million m3/d) over the next decade, the annual average spend on new services and equipment would be in the $400 million region.

Underground vault
There are several compelling arguments to adopt ASR. The first, and most popular, is to use the technology to reduce capital investment in potable water treatment plants needed to meet peak demand. A medium-sized city in the US might need an average of 100 MGD (378,500m3/d) of potable-grade water. But usage can seasonally fluctuate between 50 and 150 MGD (189,250-567,750m3/d). A system is thus generally built to meet peak daily demand. “Since the capital cost of water treatment is about $5.00/gal/d [$1,321/ m3/d], meeting peak demand can add several hundred millions to capital costs,” says Pyne. “You can build a 100 MGD [378,500m3/d] plant and put in 50 MGD [189,250m3/d] ASR, often directly underneath the plant, for a fraction of the capital cost. Generally, you see savings of at least 50% on capital costs, and it can be up to 90% if you are eliminating expensive items such as pipelines or a treatment plant.”

ASR also offers much greater storage capacity, allowing operators to increase reserve supplies. “Above-ground distribution systems usually have only one or two days of storage,” says Tom Missimer, global segment leader for new water at Schlumberger Water Services. Some of the most recent ASR projects in the Middle East now offer potable water plants a year’s storage. “A one-year storage above ground for a 100 MGD [378,500m3/d] facility would require an area 10 metres deep by one kilometer wide by 12km long,” says Missimer. “That’s a lake.”

In addition, ASR offers the opportunity to treat potable water when supplies are plentiful, such as during the rainy season, and store them until the a period of high use ensues during the dry season. “In Denver, where summer demand exceeds winter demand by eight times, ASR works very well,” says Pyne. “The largest system is in Las Vegas, with 157 MGD [594,245m3/d] capacity. San Antonio has in excess of 50 MGD [189,250m3/d]. There are 18 operations in Florida alone.”

The second argument involves security of supply. “SWS has four large projects devoted to strategic storage in the Middle East,” says Missimer. “In the case of a [treatment] system being destroyed, you can pump water out of the aquifer.”

More commonly, a treatment system can be disrupted by earthquake or flooding. In 1993, Des Moines, Iowa, lost its 100 MGD (378,500m3/d) potable water facility for 17 days due to flooding. Afterwards, the city planned for an emergency backup in the event of subsequent catastrophe. “It designed an ASR system that could supply 30 MGD [113,550m3/d],” says Pyne. “The second objective was to smooth out seasonal variations and make their system more cost-effective.” The third objective was to reduce operating costs. “During spring run-off, fertilizer that had dissolved in snow would cause a spike in nitrogen content. The de-nitrogenating equipment was expensive to build and operate. The ASR system meant they didn’t have to input the high-nitrogen flow.”

Sorting the snags
ASR has several drawbacks. Firstly, there may not be adequate aquifer reservoirs nearby, which drives up costs. “Longdistance transmission piping can run $5 million per mile,” says Pyne. “San Antonio has a large ASR system in which they need to pump the water 30 miles to the aquifer. The pipeline is the biggest cost.”

Some aquifers can contain harmful trace elements. “There are small amounts of arseno-pyrites in some sediments, and if you create an oxidizing environment, it leaches out a tiny amount of arsenic,” says Missimer. “There were examples in Florida where this happened. You can flush it out with several cycles, but then there are regulatory concerns because what do you do with it when you flush it out?”

An ASR project also requires sophisticated planning by experts. “While many engineering firms offer ASR services, there are only about 50 hydrogeologists with ASR experience in the world,” says Missimer. “There have been a few projects that didn’t work. It gave the technology a bad name. There were situations where the reservoir was too small, or there were steep flow gradients or karst geology, and the water went away.” To help train professionals, Missimer has authored a textbook on ASR, to be published later this year. “I’m a strong proponent of doing ASR carefully, in phases.”

The key to a successful project is to plan upfront. “You need to decide upon your source, aquifer and users,” says Pyne. “There are about 26 objectives, and you go through the checklist and delete the ones that don’t apply. You then prioritize the ones that do. The next step is to look at variation of supply, demand and quality. You define your volume stream and rate of recovery. You look at what aquifers are good for storage. You also look at the proximity of the aquifer to supply and usage, and geochemical issues. There are also aboveground issues, such as legal, ecological, political.”

A comprehensive ASR study involves high-resolution geophysical logging, regional hydrogeological interpretations, geologic modelling, geochemical analysis and flow simulation. Storage zones can include a wide range of sediments, although clean, homogenous sands are highly rated. The storage zone can range in depth from as shallow as 75 metres to as deep as 900 metres. Natural water quality in the storage zone ranges from fresh – which is suitable for drinking without treatment – to water with total dissolved solids concentrations of up to 5000 mg/l. Capacities range from 10 million gallons (37,850m3) to over 2 billion gallons (7.57 million m3).

Once the aquifer has been quantified, engineers create a well network pattern that will optimize injection and recovery. “You can recover up to 100%,” says Pyne. “Even storing in very saline water aquifers, you can recover up to 75%.” During the operation of an ASR system, injected potable water displaces aquifer water laterally around a well, creating a pocket of pure water surrounded by a buffer zone (see diagram opposite). The water is then withdrawn at a calculated rate in order to avoid coning, in which aquifer water is drawn up to the well. “When you draw the water out, typically, you don’t have to re-treat, just add some residual chlorine.”

All’s well for the future
Over the next decade, sector participants expect ASR to expand on many fronts. Some fresh water aquifers that have been over-used for many years, for instance, are now under threat of saltwater intrusion. “In New Jersey, you have salt water intrusion into existing aquifers, so you have to shove the salt water out or it could ruin the aquifer,” says Missimer.

New uses other than potable water storage will proliferate. “There are over two dozen different applications, including storing reclaimed water, maintaining system pressures and heat storage value,” says Pyne. “Computer chip manufacturers use ASR to store cold water in the winter in order to modulate heat in the summer. Several years ago, farmers in eastern Oregon were being forced out of business after depleting local reservoirs. Now, ASR systems are pumping several million gpd back into the aquifer during wet winter season run-off, and up to 50 ASR fields are under development. In Washington State and California, ASR systems are being developed in order to protect salmon runs.”

A very large ASR programme is being planned to protect the environment of the Everglades in south Florida. “The state receives about 60 inches of rain a year, mostly during June to September,” says Pyne. “During that time, the Everglades fill to capacity and spill to tide water. But during the dry months, the ecology is threatened with low water.” When the programme is completed, it is expected to have over 300 ASR wells storing and recovering water at combined rates of up to 2 BGD (7.57 million m3/d).

ASR will also become more common in sectors which consume a lot of water, such as the energy industry. “We have given six presentations on ASR in Calgary,” says Missimer. “The oilsands and drilling fluids people are interested.”

Ironically, federal statutes designed to protect drinking water can act as a deterrent to ASR, which is generally seen as much more environmentally benevolent than alternative water storage systems, such as dams. “Regulations are struggling to accommodate ASR because it wasn’t contemplated when the Safe Water Act was passed in 1977,” says Pyne. “We are treated by the EPA as through we’re storing and recovering hazardous waste. EPA is looking at ways of improving the regulatory framework. There are lots of options to follow.” On the other hand, ASR projects may qualify for carbon credits under “cap and trade” legislation. “We’re seeing TOC drop in ASR, so some of the carbon is staying in the reservoir,” says Pyne. “This is one potential way of generating a carbon credit.” Pressure on an increasingly scarce resource is expected to be the prime motivator for the expansion of ASR. “The economics are positive, and the need isn’t going to go away,” says Pyne.

The global ASR market at a glance

* Aquifer Storage and Recovery (ASR) activity is generally measured in terms of the installed pump and well capacity able to inject and withdraw water on a daily basis. There is approximately one billion gallons per day (3.79 million m3/d) of ASR capacity installed globally.

* Over half of all ASR capacity is in North America, with the rest spread throughout the world, including the UK, Canada, Australia, South Africa, Israel and the Middle East. The largest system is in Las Vegas, with a capacity of 157 MGD (594,245m3/d).

* Prominent ASR engineering service providers include Schlumberger Water Services (SWS), CH2M Hill and CDM.

* Capital costs for an ASR system average about $1 per gallon per day ($264/m3/d). ASR capacity is estimated to grow to 5 BGD (18.9 million m3/d) by 2020. The average annual spend over the next decade on engineering, wells and associated equipment is projected to be in the $400 million region.

* The most common use of ASR is to inject potable water into an aquifer during times of higher supply (such as the rainy season), then withdraw it during times of high demand (such as the dry, irrigation season).

* Potable water plant capital costs run at about $5.00/gal/d ($1,321/m3/d), meaning that utilities can save several hundreds of millions of dollars in capital costs through ‘peak shaving’ (using ASR to replace peak load capacity).

* ASR systems must be carefully engineered to avoid problems such as arsenic contamination and water loss through high hydrostatic gradients.

* Aquifer storage capacities range from 10 million gallons (37,850m3) to over 2 billion gallons (7.57 million m3). Recovery rates of stored water can range from 75% to 100%.