Diving to find fresh water and sinkholes in Florida

It ranks among the most hazardous ways to gather data, but a group of cave divers wants to use their adventurous sport for scientific research - research that could provide Florida with new supplies of fresh water, protect known water supplies from pollution, and find potential sinkholes.

''One little sinkhole that's no bigger than my desk can connect to thousands and thousands of feet of tunnels and caverns,'' said Dr. William Fehring. ''A lot of people dive into caves for the thrill of exploring the unknown, but there's a place for someone with the interest and the training to search out the scientific unknowns, too.''

Fehring spends a great deal of his free time from his job as environmental affairs director of the Tampa Port Authority diving into caves and squeezing through crevices to explore the caverns under Florida.

He was recently appointed chairman of the cave-diving section of the National Speleological Society, and he is now trying to encourage interest in the scientific value of his sport.

A homeowner sitting next to his pool in a west Florida subdivision was startled recently to hear a voice come out of a small nearby pond, shouting, ''Where are we?'' Two divers were splashing around, and as mysteriously as they appeared, they disappeared back underwater.

That was Fehring and a fellow diver studying the interconnecting cave system under the subdivision. He said he did not think that the homes were in danger of dropping into sinkholes. But he warned developers to be careful what they dump into the area's ponds, because the water in them is linked directly to a public water supply well.

Fehring is quick to emphasize the hazards of cave diving, and to warn people that unless they are highly trained and have the right equipment, they are taking a great risk.

''Sixteen people have died in caves during the past year,'' he said. ''There's a very limited number of people who are willing to make the total commitment to safety that this requires.''

Much of Florida north of Tampa and west of the state's central ridge is permeated with tunnels and caverns that have formed in limestone deposits over millions of years.

Layers of limestone, which is largely compressed coral, were created each time the sea level rose to wash over the land. Those layers make up Florida's aquifer, the freshwater-laden rock that supplies most of the state's drinking water.

As the sea level receded, the limestone was exposed to rainwater, which is slightly acidic from carbon dioxide and soil acids. The acidity dissolved the limestone along its cracks and fissures, which created sinkholes dropping scores of feet, tunnels hundreds of feet long, and caverns as large as a football stadium.

The caverns became interconnected by the tunnels to form underground rivers that eventually pop out as springs. Some of those springs are along the marshland that borders the Gulf of Mexico, and the fresh water pouring out of them has become an integral part of the coastline ecology.

Until the mid-1960s, researchers working with drills had only a vague idea that these underground caves and rivers existed. But with the widespread use of the aqualung, people began to dive down into springs and sinkholes and discover the tunnels.

''We can actually see the geology laid out down there just like the Grand Canyon,'' Fehring said in explaining why diving into the caverns produces more information than drilling rock samples. ''For the price of one fill-up of air, we can gather much more information than a number of core borings, and we can do it a lot faster,'' he said. ''A core boring can only look at a vertical sample of the rock. A diver can look at the formations laterally.''William Sinclair, a researcher for the US Geological Survey, agreed that cave divers could provide important information that would be difficult to get any other way.''We have computer models of the aquifer, but they are only generalities of what is underground,'' he said. ''The divers are looking at the anomalies. An underground river is a unique thing.''This information could be valuable in providing Florida with a more reliable water supply, he said. Now, most of the state's water comes from well fields where wells are drilled into the aquifer and water is sucked out from the crevices in the porous limestone.The City of Tampa experimented by pumping water from a sinkhole near its well field and from the well field itself, Sinclair said. The results showed that pumping from the sinkhole lowered the ground-water level less than pumping from the well field.Finding these underground systems could save Floridians untold amounts of money and controversy in supplying water-poor southwest Florida in the future, he said.''There may already be natural pipelines,'' he said. ''We have to find where they are.'' And while the divers are exploring, he said, they may be able to spot potential sinkholes.''If they look up and see a dome that's lined with clay rather than limestone, and it's got roots coming out of it,'' Sinclair said , ''they will have found a spot that sooner or later is going to cave in.''Fehring said water conservation and pollution control are as much a part of his group's research as water supply.Some subdivisions drain their storm water runoff into ponds that are actually sinkholes, he said. That means all the oils, metals, and chemicals that wash down suburban streets are dropping into these interconnecting rivers of fresh water, which may stretch for miles.Some of these tunnels open under the Gulf of Mexico, he said, so that the water running through the system ebbs and flows with the tides. That means the pollutants dumped into sinkholes could be pushed farther inland with incoming tides until they are under water-supply wells.If a garbage landfill is built over a cavern system, he said, toxic pollutants could seep down into the underground river system and be carried farther than anyone realizes.While Sinclair is interested in using the caverns as a freshwater supply, Fehring said he is concerned about pumping a lot of fresh water out of this underground system. The tunnels that connect to the Gulf, he said, may act like straws, sucking salt water into the aquifer as fresh water is drawn out.During their explorations, the divers have swum through the barrier between fresh water and salt water, Fehring said, and it is a lot farther inland than many researchers know.But doing organized, scientific research in water-filled caverns under the earth may be impossible because of potential hazards, Fehring said. Few government agencies or private companies can afford the liability insurance that would be required to support cave-diving studies.Until they can be supported, Fehring and his fellow divers will have to continue their research as a very expensive and risky hobby.''I think you can cut the risk in cave diving down to a level acceptable for research,'' he said. ''But it's like working in space. You're sending a man into an environment that is every bit as foreign as outer space.''