drinking-water-feature

Are we going back to the future for our drinking water?

Top photo provided by Southwest Florida Water Management District

Aquifer storage and recovery — or ASR — was a promising technology for storing water in the early 1980s. Water injected into the aquifer during rainy season stayed in place so it could be recovered during dry season and droughts. While it does require specific geological features, ASR needs far less land than a reservoir and eliminates evaporative losses that can be quite significant.

That was before a happenstance test at the City of Tampa’s brand-new ASR facility in 1997 revealed higher-than-expected levels of arsenic. “The Florida Geological Survey had been testing for elevated radionuclides and they decided to test for the other metals typically found together,” recalls Don Ellison, senior hydrogeologist at the Southwest Florida Water Management District and its point person for ASR. “After the Tampa tests came back, we started testing other facilities and they were all positive for arsenic too.”

He stresses that arsenic levels in the water were treated prior to distribution to the public and never exceeded federal limits for drinking water — even though the limits dropped from 50 parts per billion to just 10 ppb. Ongoing tests also showed that the arsenic-laden water never traveled far — very rarely 300 feet from an ASR well. Distance decreased and concentrations declined as water was injected and withdrawn over the years. Still, federal regulations prohibit any activity that adversely impacts groundwater, and permits for ASR operation were effectively halted.

It was only a matter of days to identify what caused the problem, but it took nearly 20 years to prove that it could be eliminated.

“Jon Arthur, the state’s chief geologist, theorized that the dissolved oxygen in the injected water was oxidizing arsenic-bearing iron pyrite in the limestone,” Ellison said. “Basically, we were creating a situation where the iron could rust and the naturally occurring trace levels of arsenic in the pyrite was released into the water. The ‘rusting’ process also created an iron oxide compound that arsenic attached to, so it didn’t travel very far.”

The issue with arsenic isn’t limited to Florida aquifers. The same chemical reaction allows arsenic to leach into surface waters when mine tailings are exposed. And technology to eliminate oxygen from water existed. Nuclear power plants must remove oxygen from water because it corrodes pipes, and beer and beverage makers minimize oxygen to allow room for carbonation.

The technology, however, wasn’t easily adaptable to Florida’s aquifers and no one had ever field-tested Arthur’s theory. The large water utilities chose to re-treat water from their ASR facilities to remove arsenic rather than experiment with unproven technologies.

Only the city of Bradenton, which had recently completed an ASR well, was willing to test the theory. “Their ASR site was eight miles from the water treatment plant, so piping it back and forth to re-treat wasn’t really an option either,” Ellison said.

Six years, three major research initiatives, an on-site pilot program and nearly $1 million later, the dissolved oxygen theory was proved. Once the theory was verified, federal and state regulators approved an approach that would allow the limited release of arsenic around an ASR well to occur under certain conditions in facilities operated by public drinking water systems.

BrackishGroundw Resources Bay Soundings updated

Across the SWFWMD, there are 34 active potable ASR wells operated by four separate water utilities. Some are full ASR in which potable water is injected into the aquifer and then recovered for use as drinking water at a later date. They include the facilities in Tampa and Bradenton which work much like an underground reservoir.

Others are more focused on recharging the aquifer to halt saltwater intrusion with no immediate plans for recovery. Close to home, the city of Clearwater is moving ahead with a new initiative to recharge the aquifer with reclaimed water purified using state-of-the-art technology and the same oxygen removal process as Bradenton. While reclaimed water is fully utilized during the dry season, it’s typically discharged into Tampa Bay during rainy months when irrigation is not necessary. That discharge can be a significant source of nitrogen in the bay.

The plans call for using up to 3 mgd of reclaimed water purified to meet or surpass all drinking water standards. The recharge wells are all located more than a mile away from water supply wells; ongoing tests indicate that water from the recharge well will not travel more than one-quarter mile in 10 years. Test wells were completed last year, with design and construction scheduled to begin this year.

In south Hillsborough County, an ambitious pilot program designed specifically to halt or minimize saltwater intrusion into the region’s aquifer is being tested. It’s near what water managers call the “Most Impacted Area” of the Floridan Aquifer where long-term withdrawals of underground water have lowered aquifer levels by up to 50 feet. It’s also near the county’s reclaimed water facility in Ruskin, where reclaimed water from across the southern part of the county is treated and already has multiple monitoring wells nearby so the movement of water in the aquifer can be tracked.

A joint effort between Hillsborough County and SWFWMD, the South Hillsborough Aquifer Recharge Program — or SHARP — will inject two mgd per day of highly treated reclaimed water into a deep non-potable zone of the aquifer during summer months when irrigation is not needed.

“There are still a lot of technical issues about what happens once the water is in the ground, and how it moves,” notes Mark Barcelo, SWFWMD’s project manager. “It’s going into a portion of the aquifer that’s already so salty it could not be used for potable water.”

There also are significant questions about whether the same results would be seen in another area with different geology, Barcelo said. “This project alone won’t stop saltwater intrusion, the goal is to reduce the rate of landward movement.”

If the pilot program is successful, then the county may expand the facility to inject up to 10 mdg during rainy season, reducing nitrogen loadings to Tampa Bay by up to 200 tons per year.

Following two pilot tests in South Florida, ASR is also likely to be a key component in the Everglades restoration, said Bob Verrastro, lead hydrogeologist at South Florida Water Management District. Two ASR wells – north of Lake Okeechobee and west of Boca Raton – were injected with minimally treated surface waters from the lake and the canals in west Palm Beach County.

Although results are still being compiled, highly treated water seems to be “chemically hungry” while surface waters are much less active. “The surface water is chemically stable because it’s saturated with typical nutrients and elements found in nature,” he said.

Original plans for the Everglades restoration called for more than 300 ASR wells that would receive surface waters during rainy season to be released during dry season and droughts, but Verrastro said that only about half could actually be built. “Arsenic wasn’t the issue but that many wells could have a regional impact on nearby users and affect the groundwater flow patterns many miles away.”

The final locations and numbers of wells will be determined by additional local and sub-regional analyses and most likely the wells will be integrated with reservoirs or other storage features constructed as Everglades restoration continues.

Sea level rise is making headlines across the state, but a more insidious movement is taking place beneath our feet.

Even with significant reductions in groundwater withdrawals over the past 20 years, saltwater is sneaking into the Floridan Aquifer, one of the most productive sources of potable water in the world. In 1992, the Southwest Florida Water Management District identified a Southern Water Use Caution Area – or SWUCA – to address the declines in aquifer levels that were increasing the rate of saltwater intrusion.

Over the past 20 years, multiple strategies have been implemented to slow the rate of saltwater intrusion – including 50 reclaimed water projects expected to replace nearly 30 million gallons per day (mgd) of groundwater withdrawals. Still, saltwater continues to move into the aquifer. “It’s a complicated scenario that’s difficult to track because it’s happening underground,” says David DeWitt, chief professional geologist with SWFWMD’s water quality monitoring program.

“From a theoretical perspective, it makes sense to say that sea level rise is making the problem worse, but we don’t have the science available to make that determination,” he said. “We don’t quite know yet how it will impact groundwater quality over the next 20 to 60 years.”

Saltwater has already invaded most of the aquifer under Pinellas County – forcing St. Petersburg to purchase land in Pasco County in the 1970s to ensure a long-term water supply. The city of Clearwater pumps potable groundwater for public supply use, but it is drawn from the upper portion of the aquifer because the deeper sections already are saline. The city uses this brackish groundwater as potable water only after it is treated with reverse osmosis.

To some degree, saltwater intrusion can occur even without groundwater withdrawal and sea level rise. It has a higher mineral content which makes it denser, so it can push inland underneath the freshwater by convection and tidal forces. And the problem isn’t limited to Florida’s west coast either — groundwater stored in aquifers is retreating in south Florida as well as Louisiana and up the Atlantic Coast to New Jersey.

“I don’t think we’ll be able to stop saltwater intrusion and once the saltwater is there, it’s really hard to push it back,” DeWitt said. “The goal is to slow down the rate of intrusion by managing groundwater resources until new technologies are proven to work.”

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