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South Carolina's beach waters are among the nation's least clean, according to the Natural Resources Defense Council's annual report on beach water quality.

Explore the Natural Resource defense Council's interactive guide to water quality at vacation beaches below:

- Beaches nationwide http://www.nrdc.org/water/oceans/ttw/default.asp

- South Carolina beaches: http://www.nrdc.org/water/oceans/ttw/sc.asp?loc=SC

The Palmetto State ranks 24th among the 30 coastal and Great Lakes states. Delaware has the cleanest beach water, according to the study, and Ohio comes in last place.

The advisory measure used to evaluate beaches' water quality is the Beach Action Value (BAV), which measures the number of enterococcus bacteria colony forming units per 100 mL of water in a sample. The maximum number per 100 mL is 60; if a sample exceeds that, officials may consider issuing a health warning to beachgoers. In total, 15 percent of samples taken from all South Carolina beaches in 2013 exceeded the maximum level of bacteria.

People can risk infection by swimming in waters with high levels of bacteria, especially children and the elderly. Those infected by bacterial beach water can contract infections as minor as pink eye or as serious as meningitis, according to Jon Devine, a senior attorney for the NRDC.

The BAV measure has been proposed, but not approved by the Environmental Protection Agency. Using this measure may mislead the public, according to Jim Beasley, a spokesman for the South Carolina Department of Health and Environmental Control.

"Use of a proposed EPA value, rather than the EPA-approved state standard that is in regulation, gives the wrong impression that our beaches got worse in 2013. This is not the case," Beasley said.

Many beach water monitoring stations along South Carolina's coast are placed at points where water quality is a concern. Sampling is also often conducted when water quality may be questionable, like after a heavy rain.

"They'll deliberately sample near outfalls and where water quality is expected to be poorest," Devine said. "It's a protective practice, and we encourage folks to do so. While that ultimate number may be higher as a result, they're protective of the public to do that."

This practice is not uniform from state to state, which may lead to results that are not comparable in the NRDC report, Beasley said.

The beaches with the most samples exceeding the BAV safety threshold in 2013 were all located in Horry County. Briarcliffe Acres Beach, just south of North Myrtle Beach, had the largest amount, with 45 percent of samples exceeding the federal safety threshold. For beaches between Little River and Georgetown, the average percentage of samples exceeding the threshold was 16.31 percent.

Between Isle of Palms and Hilton Head Island, the average was 3.8 percent. The beach with the highest percentage of samples exceeding the threshold was Sullivan's Island with 11 percent. Three beaches - Fripp Island, Hunting Island and Seabrook Island - had no samples exceed the safety threshold.

The difference between Grand Strand and Lowcountry beaches can be accounted for by the difference in development, according to Nancy Cave, director of the Coastal Conservation League's North Coast office. The major beachfront development in the Myrtle Beach area leads to increased stormwater runoff, which is the major source of beach water pollution, she said.

"When you think of the coastline of Charleston County, you're looking at an enormous length of coastline with little development. Sullivan's Island and Isle of Palms are certainly developed islands, but it's not a continuous strip of impervious surface as you have in Horry County," Cave said. "From Murrells Inlet to Little River is a continuous strip of pavement."

Reach Amanda Coyne at 937-5592 or on Twitter at @AmandaCCoyne.