Lake Wheeler, a Raleigh park, is again closed to water skiing, tubing and swimming after tests showed elevated levels of bacteria. The recreational lake showed high counts of the bacterium enterococcus Sunday and in follow-up tests Tuesday, said Rob Richardson of Wake County Environmental Services. Elevated levels of the bacteria increase the risk of waterborne diseases, gastrointestinal problems and skin infections for swimmers. The lake has had restrictions at least five times this summer, more than any other recreational lake in Wake County, Richardson said. "This has been a consistent problem," he said. "We are in the process of studying what is going on." Among the possible culprits: remnants of a Cary sewage spill in June, algae blooms caused by warm weather, the presence of ducks and geese, and stormwater runoff. Richard Costello, who oversees lakes for the city parks department, said the restrictions have been in place since Monday. He said the city is not renting kayaks at Lake Wheeler but that boating and fishing are allowed. Costello said city officials are trying to determine why the lake is having a recurring problem with bacteria. "I wish I could give more definitive answers about what is causing it," Costello said. "I just do not know." The restrictions could be lifted Friday if follow-up tests on consecutive days show that bacteria levels have dropped below federal limits. Both Lake Wheeler and Lake Benson were closed to recreational activities after a five-day sewage spill in June sent an estimated 7.9 million gallons of raw sewage into Swift Creek upstream.