Minnesota cities launch system to treat water underground
Just northwest of the Twin Cities, in the bedroom communities of St. Michael, Albertville and Hanover, something unusual is happening. A pump is taking water from the jointly run treatment plant and rather than sending it to people's homes and faucets, it's injecting it into the ground at a rate of 300 gallons a minute. The pumping won't stop until 100 million gallons of treated drinking water have been stowed in an aquifer beneath the cities.
The process is called aquifer storage and recovery, and it involves capturing water during times of plenty, storing it underground and pulling it out later when it's needed. It's a strategy used a lot in the western and southeastern parts of the country, where drought and water shortages are common. There are hundreds of these storage wells operating in the United States; the project in St. Michael is the first in Minnesota.
"This is the first year where the project has been fully up and running," said Kelly Daleiden, project manager for Veolia Water, which operates the well and other joint water efforts for the three cities. The water will be pumped out of the aquifer this summer, when residents want it for lawn watering and drinking. "It's interesting," she said, "the water comes out just like it went in for the most part."
People across the country are managing groundwater differently as drought, contamination and other factors cause supplies to dwindle. Even in Minnesota, where aquifers were once thought to be limitless in their bounty, cities and regulators are bumping up against limitations and, by necessity, developing new systems.
In St. Michael, Albertville and Hanover, the problem wasn't a strained aquifer, but rather a strained water-treatment plant. In the winter, water demand is just over a million gallons a day. But during the summer lawn-watering season, demand spikes to between six and eight million gallons, near the limit of what the plant can produce. City leaders looked at their rapidly growing communities--the population in St. Michael rose 80 percent between 2000 and 2010 to 16,000--and considered building an additional plant.
But then they talked with a consulting company with offices in Iowa, a state with several underground storage systems, and realized that instead of spending $5 million or more on a new treatment plant and related wells and tower, the cities could spend just around $2 million to build a system that would store treated water in a bubble underground.
"When we were looking at the best bang for our buck, building another treatment plant to sprinkle lawns didn't seem like the logical thing to do," said St. Michael city administrator Steve Bot. "That's what's great about (underground storage). We already have a plant that can produce well above our winter usage. We have all this extra capacity in the plant you are only using during the summertime. Now, we are able to use that capacity."
Here is how the process works:
Water is filtered and treated, with manganese and iron removed and fluoride and a type of bleach added. Then the water is injected 500 feet underground, via a stainless-steel tube, into the Mt. Simon aquifer, where it displaces existing water and remains pretty much in one place until needed. The sandstone aquifer is akin to a bucket of sand and rocks that have been cemented by time. Water there moves slowly, perhaps only several feet per year. When it's pulled back out, all it needs is a little added bleach and it's ready for consumption.
Given that aquifer storage is new to Minnesota, the three cities had to clear a lot of hurdles with the state before they were given the go-ahead to start pumping. The testing and monitoring process took nearly six years and cost around $200,000. "It's good to be on the cutting edge of things," said Bot. "But we were on the bleeding edge of this one, given all the stuff the Department of Health required us to do."
The Minnesota Department of Health had concerns, and in general the state doesn't like people injecting things into the ground. "Once you screw up an aquifer, it's really hard to fix it," said Ed Schneider, a hydrologist in the department's well management section. "Anytime somebody is suggesting doing something to change the characteristics of an aquifer, we get very nervous."
Mainly, Schneider, who reviewed the St. Michael project, wanted to make sure water that had been treated and oxidized up top wouldn't pick up contaminants once it was back underground. Arsenic was a worry and so were radioactive materials, like uranium and radium, which are present in the Mt. Simon. "We were concerned that injecting water into the aquifer could mobilize more of the radioactive materials," Schneider said. "But that doesn't seem to be the case."
The arduous approval process involved a series of pilot studies, in which ever-greater amounts of water were pumped into the aquifer, along with a one-year, full-scale test. The aquifer was monitored to measure the chemistry and location of the injected water and also to keep track of pressure. The project wasn't fully approved until 2012.
So far, the water is fine. "It's hard to know over time if things might change," Schneider said. "But after a full-scale operation of the system, we haven't seen any changes in the water quality... In this case, (aquifer storage) seems to be a good choice and saves consumers some money."
It's common practice in the West to treat and store water from rivers and lakes underground, but that seems unlikely to pass muster in Minnesota. Here, officials were less concerned about the introduction of pathogens into the aquifer at St. Michael because the system removes and then replaces the same potable water.
But, Schneider said, "We have a lot of concerns about taking surface water and putting it into our precious aquifers." "It has such a different chemistry, we don't know what is going to happen. We would have to look long and hard before accepting any such proposal."
Schneider doesn't expect to see a lot of underground storage wells in Minnesota. "It has a limited application," he said, noting that aquifers in many parts of the state aren't suitable. "You couldn't do it in a limestone aquifer with big caverns and caves," he said. "The water would flow out the door and you would never see it again."
But Bot is pleased with the system and said other communities have come to have a look. "Our ultimate plan is to have three wells out there," he said, once housing demand warrants an expansion. "From the standpoint of our happiness with the results and savings, things have gone smoothly.
"People are interested, even more so now that we have broken ground in Minnesota for it to be a possibility. It takes quite a few years to pull off with the monitoring wells and so on, to get the full-scale go ahead. But it wouldn't surprise me, as the economy and development come back, if other communities will be looking at this. The choice might be building a new treatment plant or trying something like this."