BYRNES MILL, Mo. -- The Environmental Protection Agency plans to test 256 Jefferson County residential properties amid concerns that lead in the Big River is contaminating soil.

The St. Louis Post-Dispatch reported that testing is expected to be completed by midsummer. Recent studies and samples indicated widespread lead contamination in the flood plain that extends from Leadwood in St. Francois County to the confluence with the Meramec River near Eureka.

While testing will be done at 256 properties, nearly 2,000 residentially zoned properties sit in the flood plain in Jefferson County.

The Southwest Jefferson County Mining Superfund site covers Jefferson County but excludes Herculaneum, home to the Doe Run lead smelter, which has its own Superfund site.

The contamination is linked to long-gone lead mining operations that date to the early 1800s. Records show more than 3 million pounds of lead were shipped out of the county per year by the mid-1800s, making it one of the largest lead-producing areas in the nation.

"Lead is naturally found in the county, and it's going to be in the environment," said Dennis Diehl, director of the Jefferson County Health Department. "Unfortunately, in years past, the way it was mined and handled, it caused some -- if not most -- of the contamination we see these days."

Lead contamination in children can interrupt normal brain development and has been linked to behavioral problems. Nearly 2,400 Jefferson County children ages 1 to 6 were tested last year for elevated lead levels, but only 19 had levels considered very dangerous. Still, health officials say no level of lead is safe.

Lead concerns in the Big River also prompted the Missouri Department of Health and Human Services to issue a fish advisory, urging people not to eat carp, suckers, long-ear sunfish, green sunfish, bluegill, warmouth sunfish or rock bass caught in the Big River in St. Francois or Jefferson counties.

Gail Beyer has lived in the Big River Hills subdivision in Byrnes Mill for 20 years, about a quarter-mile from the river. Last month, she bought a dump truck load of dirt taken from that flood plain to level her sloping backyard.

"I'm worried about the dirt," Beyer said. "I guess I'll have to wait and see if they find a lot of lead in it."

The EPA is trying to determine how many people are hauling and selling dirt from the flood plain in an effort to see how widely contamination may have been spread.