Thurmond Lake -- one of Georgia's largest and busiest swimming holes -- is also one of the cleanest.

"Believe it or not, it's almost too clean, if there is such a thing," said Billy Clayton, director of water utilities for Columbia County, which uses the 70,000-acre lake as a drinking water source.

Because the county taps the reservoir for residential use, the raw water undergoes a battery of tests for constituents including E. coli and cryptosporidium, a spore-forming protozoan known as "crypto" that causes gastrointestinal distress.

"We do a whole slew of tests, and basically, we don't find anything that causes any kinds of concerns for us," he said. "Actually, there are times we wish it was more turbid. Sometimes it can be too clear, and water is easier to treat when small particles are there that can be settled out."

From a water engineer's perspective, Thurmond and its sister lakes upstream -- Russell and Hartwell -- are like mammoth settling basins, where particles and solid material sink to the bottom and the cleaner water flows from the surface to the next lake.

Thurmond, Mr. Clayton said, is also a water source with almost no major industrial sources discharging into it, and very few agricultural operations or sewage discharges that could create issues with fecal coliform -- the intestinal tract bacteria from warm-blooded creatures, including humans.

The Army Corps of Engineers conducts fecal coliform tests at the lake's dozens of swimming beaches, but no one at the project could recall any instances where levels were elevated enough to require closures or advisories, corps spokesman Billy Birdwell said.

The corps protocol requires monthly samplings from May to September, but also requires beaches to have at least 3 feet of water, as they currently have for the first time in many months.

Samples were collected this week and are being analyzed in anticipation of July Fourth weekend crowds, Mr. Birdwell said.

"In the event that samples exceed water quality criteria for fecal coliform, we begin repeated sampling, followed by public notices when warranted."

Thurmond is also a "no discharge" lake, where both Georgia and South Carolina have banned the discharge of marine toilets, Mr. Birdwell said. All boats with toilets must be configured so they cannot discharge directly into the lake, and such toilets must be pumped out by an approved facility.

Mr. Clayton said fecal coliform contamination typically originates from sources like leaking septic tanks, livestock operations and sewage spills.

In a 2007 study by the U.S. Environmental Protection Agency, portions of two Columbia County streams -- Jones and Euchee creeks -- were found to have mildly elevated levels of fecal matter, possibly due to whitetail deer droppings.

"Crypto is probably the new animal of concern on the block and is hard to test for," he said. "Back in 2007, we had to set up traps to identify them (in Thurmond Lake). We did it monthly for a whole year, and we did not find a single one."

Aiken County tests Langley Pond twice monthly, as required by the South Carolina Department of Health and Environmental Control. Samples are analyzed for fecal coliform and if there's a high enough level the swimming area must be closed until an acceptable limit is recorded.

Currently, Aiken County Assistant Administrator Todd Glover said the pond is safe for swimming and boating, but there is a fish advisory warning against eating any largemouth bass or channel fish from the pond.

Although Thurmond Lake is mostly free of unhealthful bacteria, Mr. Clayton said the reservoir's recovery from a long drought is likely to bring some aesthetic issues this fall.

"There was a lot of vegetation that grew up during the drought, and now much of it is underwater and will decay," he said. "When that zone of water turns over in the fall, it will form manganese, which will mix with the water and rise to the surface.

"It can turn the water in some areas a very dingy color, and it can stain porcelain -- or clothing," Mr. Clayton said. "It has no health concerns, though."