Saltwater intrusion: Florida’s underground movement
Standing on a beach in southern Florida it is impossible to see that the saltwater gently rising and falling with the tide also is slowly creeping inland underground. But it is — at 100 feet per year under some beaches.
Saltwater intrusion is a problem that has crept up on South Florida water officials as quietly as the brackish groundwater has moved in from the coast and, like the water, it is leaving an unpleasant taste in their mouths. Continued development in the area has increased demand for water and the strain on the fresh groundwater supplies has opened the gates for saftwater intrusion.
For example, in Broward County, where the problem is particularly acute, the population grew by more than 200,000 people between 1980 and 1990 and is expected to increase by another half-million by 2010. According to the county’s water management division, residents use an average of 200 gallons per person per day; that means water use went up by more than 14.5 billion gallons per year last decade and will increase by more than twice that in the next 15 years.
“The impact of the additional people has caused there to be less area for water to percolate into to replenish the Biscayne aquifer,” says Fred Bloetscher, deputy utilities director for Hollywood, a Broward County coastal city, “and as a result it’s lowered the aquifer head, and that lowered head allows saltwater to creep farther inland. The increased demand only makes the problem worse during the dry season when there’s no real ability for replenishment to occur in most of the Everglades system.”
When people began moving to South Florida in large numbers early in the century, they settled along the eastern coastal ridge. Directly beneath this ridge is the Biscayne aquifer, one of the largest and highest quality concentrations of fresh groundwater in the world. Cities drilled their wells and everything was fine for the most part until about the 1960s, when water from the wells began to taste a little salty.
“Broward County had some of its earliest saltwater intrusion into wellfields in the city of Ft. Lauderdale,” says Roy Reynolds, director of the county’s water management division. “The city had one well out by the ocean, and it was moved further inland. Since then, especially in the last 15 or 20 years, we’ve had problems off and on with intrusion.”
HOW INTRUSION WORKS
Fresh groundwater floats on top of saltwater and the two exert pressure on each other; the point at which they meet is the saltwater interface or isochlor. Not a precisely defined barrier, the isochlor is more a measure of concentration of salts and chlorides.
Saltwater intrusion can happen in three different ways. The first method (depicted in a diagram on pg. 58) involves saline water from the coast slowly pushing back the fresh inland groundwater. Horizontal intrusion is caused by excessive freshwater pumpage, which relieves the pressure on the saltwater, thus allowing the isochlor to move inland.
“Drawing down” of freshwater also contributes to this form of intrusion. During drawdown, freshwater moves down toward the wellhead, allowing saltwater to rush in — the saltwater is, in effect, “pulled” in to take the place of the freshwater. Drawdown is essentially a local phenomenon around wells with a saltwater source in close proximity.
Pumpage can also pull saltwater up through an effect called “upconing.” As water moves toward the wellhead, saltwater in deeper aquifers rises in a process resembling an inverted funnel.
“If you’re real close to the coast, it doesn’t make sense to have a real strong pumpage demand,” says Mike Slayton, deputy executive director of the South Florida Water Management District (SFWMD), “because that can not only cause saltwater to move in laterally, it can actually pull the saltwater up to you.”
Secondly, canal-based intrusion occurs when drainage canals are dug with no salinity control structures. Because of the low, flat topography of South Florida and the high water table, land cleared for development must be drained via canals. If these canals have no control structures, such as dams or locks, water will flow unimpeded until the surrounding groundwater levels are lowered to those of the canals. Tidal saltwater then can move further inland through these canals and seep into the lowered groundwater.
“What you have is the ability during tides for saltwater to move quite a ways inland in these canals that are basically stagnant,” Bloetscher says. “If you look at some of the canal systems in Broward County, you can tell immediately which canals have no control structures, locks or dams. We have one — it’s a Y-shaped canal, and we have a Y-shaped isochlor.”
A third kind of intrusion involves deposits of remnant sea water, left over from when South Florida was submerged thousands of years ago. When the oceans receded, not all the water was flushed out of the surficial aquifer system. This source of contamination, also called “connate sea water,” is the least common and least studied form of saltwater intrusion.
Different factors contribute to how quickly the isochlor is able to move inland. The degree of pumpage in an area, the gradient along which the fresh groundwater naturally moves toward the coast and the permeability of the aquifer all determine the rate at which saltwater will migrate. The Biscayne is a highly permeable water source, with a maximum yield of 7,000 gallons per minute.
Yet another factor is seasonal rainfall variations. South Florida’s “dry season,” when monthly preciptation drops from 10 inches per month to about 2 inches, runs from November to May. Since the weather stays warm throughout the winter and evaporation continues, water levels can decline fairly rapidly.
Florida’s east coast counties south of Lake Okeechobee — Palm Beach, Broward and Dade, from north to south — are the most affected by intrusion. The Biscayne aquifer traces its way from the southern tip of the state northwest along a wavy line to south Palm Beach County. The aquifer’s cross-section resembles a wedge, becoming deeper as it gets closer to the eastern coast.
The west coast of Florida does not have as serious a problem with saltwater intrusion as the east for a few reasons. First, the shallow Tamiami aquifer, which feeds most of Collier County as well as parts of Monroe, Lee and Hendry counties, is not nearly as permeable as the Biscayne. Also, population density is lower, and property lots are larger on the west coast than on the east coast, which means less paved ground and more grass through which water can penetrate and recharge the aquifer. Certain local areas suffer from insufficient recharge and/or isolated groundwater intrusion but, as a whole, the west coast is not in trouble yet.
“There has been some documentatin of movement of saltwater, but it’s not as well-defined on the western side,” says Roy Sonenshein, a hydrologist with United States Geologic Surveys. “The mechanism isn’t understood. The source of the saltwater isn’t always known.”
REACTING TO INTRUSION
With so many different methods of intrusion, it is difficult not only to pinpoint exactly why saltwater has moved in different places, but also what to do about it. As in the case of Ft. Lauderdale in the ’40s, the easiest solution is simply to move wellfields inland where saltwater has not yet penetrated. In Broward County, Hollywood, Hallandale and Dania have abandoned certain wells close to the coast and have contracted with the county to tie into some of Broward’s inland wellfields.
Hallandale has discontinued use of six wells, capable of delivering 8.4 mgd, which were in danger of becoming saline. Two of the wells are retained for emergency use only. Hallandale still has two other functioning wells in the city and plans to supplement them with raw water from county lines in mid-March. The city now buys 35 to 40 percent of its water supply fron North Miami Beach at a dry season price of a little more than $1 per gallon; its consumption is roughly 5.75 mgd.
“With the 35 percent surcharge for the dry season, that’s a pretty big incentive for people to conserve water,” says Salvatore Schillaci, utilities operations manager for Hallandale.
Back in the ’60s, when intrusion first became an issue, communities retrofitted most of the faulty drainage canals with salinity control structures. This allayed the problem for a while, but continued development and increased demand cleared the way for more horizontal intrusion.
One of the most effective ways to minimize saltwater intrusion is to diversify water sources. Hollywood’s efforts in finding alternative sources earned the city a 1994 American City & County Award of Merit. Hollywood drilled deep wells through the Biscayne to the deeper Floridan aquifer, the brackish water from which is treated in the city’s new reverse osmosis plant.
Also, Hollywood laid an extensive pipeline network for water reuse. Finally, the city will take raw water from Broward County. These alternative sources ease the pressure Hollywood was putting on the Biscayne, which slows down the saltwater.
“I think utilities need to look at multiple water sources,” Bloetscher says. “We have three sources of water that we’ll be able to use to overcome the problem. By using saline sources, we don’t pull water off the Biscayne, so we don’t increase the problem.”
But reverse osmosis is an expensive technology and, for some communities, not economically feasible. According to Schillaci, Hallandale drilled some test wells to study the possibility of reverse osmosis and decided it would be cheaper to buy county water.
The SFWMD is looking into a project involving acquiring land for the purpose of “stacking” groundwater for storage. The land would be used as a “buffer” zone between the Everglades water conservation areas and the urban areas close to the coast. During heavy rainfall, pumps in the buffer zone would work to recharge the Biscayne at an accelerated rate in order to store water for the dry season.
“Right now when rain falls in the urban zone, it gets flushed out to sea and is no longer available for recharging the underground aquifer,” Slayton says. “We’ll use the land as additional surface reservoirs to hold rainfall.” According to a SFWMD report, the plan will involve purchasing over 50,000 acres, now privately owned, at a cost of close to $1 billion.
Water conservation, obviously, is a necessity in Florida these days. Almost all communities have some sort of restriction on watering lawns and other forms of aesthetic irrigation. Building codes contain requirements for water-efficient toilets and plumbing fixtures, and all water utilities send advisory notices and pamphlets on conservation in their monthly bills. The SFWMD even advertises on television to persuade customers to use less water.
CORRECTING THE PROBLEM
Though effective in terms of water supply, these measures do little to counteract what has already happened to the Biscayne. After years of continued drilling in eastern wellfields and with the yearly intrusion rate, the aquifer has been seriously damaged.
One idea is to inject water into the aquifer and artificially create pressure to push the saltwater back. “We need to undo what damage we’ve already done if we can, just to protect the current infrastructure,” Bloetscher says.
“We still aren’t good enough with our models to pinpoint exactly what it will take to get the system back into equilibrium,” Reynolds says. “One of the things that has been discussed is actually using reuse water to form hydraulic barriers over toward the coast to artificially raise water levels to hold the saltwater back.”
Reynolds is reluctant to say the Biscayne has been permanently affected by intrusion, but he does admit efforts to “fix” the aquifer have not been universally successful.
“In some cases, raising water levels to the point that we could force saltwater out may put us so close to flood elevations, or even above flood elevations, that it may be an impractical thing to do,” Reynolds says. “Is [the Biscayne] irreversibly damaged? No. Is it expensively damaged? Yes.”
“I don’t think technologically we’ve done a good job at identifying the solutions other than move wellfields inland,” Bloetscher says. “Broward County spent millions and millions of dollars to distribute water but, if their wellfields over time are going to degrade as well, what have we accomplished other than spend millions of dollars?”