Swimming is not recommended at Brushy Creek State Recreation Area due to high bacteria levels, according to the Iowa Department of Natural Resources.
Park Manager Amber O'Neill said this is the first time this year the lake has exceeded the limits, and she doesn't expect the problem to last.
"They did band 80 geese down there, that week," O'Neill said. "I wonder if that's what spiked it, you know, fecal material from the geese.
"It doesn't happen very often here, maybe once a year, if even that. We don't usually don't have E. coli."
She said the DNR will retest the lake in one week.
DNR swimming recommendations are based on the geometric mean of the last five samples taken within a 30-day period. Warning signs are posted when the mean E. coli levels reach 126 colony forming units of bacteria per 100 milliliters of water.
Jason McCurdy, DNR beach monitoring coordinator, said a level of 1,700 E. coli per 100 milliliters of water was measured at the lake last week. When they went back to sample again later that week, it had risen to 3,100 per 100 milliliters of water. Those two measurements are what elevated the geometric mean, he said.
McCurdy said the lake was measured at 52 E. coli per 100 milliliters of water this week. Up-to-date information can be found at http://is.gd/IAbeaches.