Water at Mammoth Cave has shown spikes in fecal coli form and E. coli bacteria.
Spokeswoman Vickie Carson said no one has become ill, including park workers who are warned to wash their hands every time they leave the cave.
Park workers have installed a cover at the cave’s entrance where dripping water naturally finds its way downhill and onto visitors to the world’s longest cave.
Part of the historic section of the cave was closed in October 2006 after a Western Kentucky University researcher found the presence of the contaminants at the cave’s waterfall. The historic section was reopened in mid-November after the shelter was installed.
National Park Service employees and WKU researchers are trying to find out why water at the tourist attraction has shown the contamination.