An in Situ Determination of the Disappearance of Coliforms in Lake Michigan

Six Fecal coliform sampling surveys in the near-shore waters of Lake Michigan off the Milwaukee metropolitan area showed that the counts dropped rapidly with distance from the shore. Outside the breakwater structure the counts dropped to insignificant levels. The reasons for this decline are natural die-off and environmental dilution. An in-situ test was conducted at the Milwaukee South Shore wastewater treatment plant to determine the rate of total coliform die-off excluding dilution effects. This was accomplished by tagging a slug or primary effluent with fluorescent dye and monitoring it in the lake. The first order reaction die-off coefficient for this test was found to be 8.72/day, a value higher than most reported in the literature for fresh water. Most previous studies were, however, conducted in rivers or in the laboratory.