Officials say the decision was based on Environmental Protection Agency standards for E. Coli bacteria.
They say acceptable levels of E. coli should not exceed 235 colony forming units (cfu) per 100 milliliter; 547.5 cfu per milliliter were found at the beach during the most recent test.
They're not sure if it can reopen in time for the busy 4th of July holiday weekend.
"We are trying to prevent people from getting sick, so we'll keep sampling and reopen the lake for swimming as soon as the levels return to normal," said Wake County Environmental Services Director Tommy Esqueda. "Two consecutive acceptable samples are required to reopen the lakes, which could be as soon as this weekend."
The lake is not closed to fishing and other swimming areas at Falls Lake will remain open.
