Factors such as water temperature, algae, the presence of birds and the number of patrons at Evanston’s beaches can be determinants of when water will be so polluted as to cause beach closings, Carl Caneva of the City’s Health Department told members of the Human Services Committee on May 7.

A partnership among the United States Geological Survey, the United States Environmental Protection Agency and the City of Evanston are developing a model to predict water quality along the beaches. The City would like to switch from a “reaction” model to one of prevention, Mr. Caneva said. Under the reaction model – the City’s present one – the City tests the lake water at each of the beaches on a daily basis. When the E. coli count reaches a certain level, the beaches are closed.

The main problem with that method, said Mr. Caneva, is that it takes 18 hours for the E. coli to grow and so the test results are not available until the following day. Thus the notification that the water is polluted and the consequent closing of a beach come a day after the fact. People may unknowingly swim in polluted water one day and be banned from swimming in what is likely safe water the next.

Mr. Caneva said that to switch from reaction to prevention, “we have to alter what we’re looking at. Using an E.P.A. beach survey, we will look at what causes a beach to close: air, rainfall, water quality [temperature, odor and turgidity], bather load [how many people are using the lake], pollution [“floatables” from other parts of the lake, since liquid pollution does not seem to be a problem here], birds and algae [which can trap bacteria]. We will assign weights to those variables and add them up to give a predictive value.” As an example, he said, high water temperature and the presence of many gulls would be two parts of the predictability.

Representatives of all three agencies – USGS, EPA and the City – will study these factors, he said, to see how they predict when a beach must be closed – which this summer will still be based on the water tests. If the predictions come close to the actuality, he said, the groups will be fairly certain they have a model for predicting beach closings. “The [models] will be tested over the summer, and the results will be available in the fall and the program implemented next summer, he said.

OhioNow and Nowcast in Lake County are examples of this type of forecasting beach closures, Mr. Caneva said.

“This will provide better service for Evanston residents,” Mr. Caneva said, noting that there were 125,428 visitors to Evanston’s beaches last year.