Saturday, August 17, 2013

Thirsty, and Surrounded by Water

Rolling Stone looks at challenges facing South Florida:
In South Florida, the drinking-water supply comes from a big lake just below the surface known as the Biscayne aquifer. Engineers examined the situation and determined that the combination of draining the swamps and pumping out the aquifer had changed hydrostatic pressure underground and allowed salt water to move into the aquifer. To stop this, the Army Corps of Engineers and the South Florida Water Management District built dozens of these salinity-­control structures at key points on the canals. When they were closed, salty water wasn't able to flow into the canals. But if there was a big storm and intense flooding, the gates could be opened to allow drainage.
That worked pretty well for a time. The gates were engineered so that, when they were closed, the fresh water was about a foot and a half higher than the salt water. This freshwater "head" (as engineers called it) helped keep pressure in the aquifer and kept the salt water at bay.
But in the 50 years since the structures were built, much has changed. For one thing, nearly 80 percent of the fresh water flowing into the Everglades has been diverted, some of it into industrial-­agriculture operations. At the same time, consumption has skyrocketed: The 5.5 million or so people who now live in South Florida consume more than 3 billion gallons of water every day (including industry and agriculture). Almost all of that is pumped out of the aquifer, drawing it down and allowing more and more salt water to move in. At the same time, the sea level is rising (about nine inches since the canals were first dredged), which also helps push more salt water into the aquifer.
"Here, you can see the problem," Obey says, pointing to the saltwater side of the gate. "The water is only 10 inches lower on this side than on the canal. When this structure was built in 1960, it was a foot and a half. We are reaching equilibrium."
Obey explains that when there is a torrential rain (a frequent occurrence) and inland Florida floods, there is nowhere for the water to go. Cities on the western edge of Miami-Dade County, such as Hialeah and Sweetwater, are now at risk of massive flooding with every big storm. To solve this, the South Florida Water District is installing pumps on the freshwater side of the control structures on the canals. The pumps, which cost about $70 million each, can take the runoff water from storms and pump it into the ocean to alleviate flooding.

But stopping saltwater incursion is more difficult. The town of Hallandale Beach, just a few miles north of Miami, had to close six of its eight wells due to saltwater intrusion. The town now buys half its water from a well field in Broward County and is working on a deal to drill six new wells of its own, at a cost of about $10 million. Fort Lauderdale has also faced saltwater intrusion, as has Lake Worth, a community just south of Palm Beach. "In the long run, the whole area is likely to have problems," Obey says.
The conventional solution to this was simple: Drill new drinking wells farther west, away from the salty water. The trouble is, engineers have done that already and can't move any farther west without running into the Everglades. Instead, engineers are now turning to more radical solutions, such as trying to capture storm water and store it underground, or reuse water from sewage-­treatment plants. This will help, but ultimately South Florida is likely to rely more and more on desalination, a complex industrial-­scale process that eliminates the salt from the sea water. Right now, South Florida has 35 desalination plants operating, with seven more under construction. They have the capacity to produce 245 million gallons of potable water per day. But desalinization is expensive and requires huge amounts of energy. In 2008, the city of Tampa opened a new $158 million desalination plant, one of the largest in the nation, which produces up to 25 million gallons of fresh water a day – about 10 percent of the region's water needs. Construction costs alone will run about $6 billion to desalinate just one-third of the water used for southern Florida.
For many cities in South Florida, securing a reliable supply of drinking water is going to be a heavy financial burden. "South Florida is not going to run out of drinking water," says Fred Bloetscher, an associate professor of civil engineering at Florida Atlantic University. "But it will be an expensive fix." Bloetscher estimates it will cost upward of $20 billion to $30 billion to re­plumb South Florida and armor it with pumps and a stormwater-recapturing system to deal with a three-foot sea-level rise. And when the waters keep rising? "Well, you just have to believe that we will come up with some kind of a solution," Bloetscher says.
There's a ton of interesting civil engineering detail in the story.  I think some of the concerns are misplaced, but it is very interesting.

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