Rainfall Effects on E. Coli Concentrations at Chicago Beaches
Hepler, Jordan T.
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Chicago has 24 beaches used for recreational swimming from May to September. Beaches are monitored daily for E. coli using the Colilert IDEXX culture method. The 24-hour interval between sampling and obtaining results for the Colilert method presents a significant limitation in the utility of the monitoring for protecting the public’s health. Precipitation has been shown to elevate microbial concentrations in recreational swimming waters. The goal of this research is to predict the need for beach notification (swim bans or beach advisories) using prior-day Escherichia coli (E. coli) concentration (measured by culture) and precipitation information. Beach monitoring data have been provided by the Chicago Park District (CPD) and precipitation data obtained from the National Climatic Data Center. Eight logistic regression models were used to measure the association between precipitation and beach notifications. Precipitation variables consisted of cumulative rain over 12- and 24-hour periods, and the presence of wet conditions defined by greater than or equal to 5 mm of precipitation over 12- and 24-hour periods. The prior day’s culture results were also considered with the presence of wet or dry conditions to predict a swim advisory or closure. During the time interval of 2003 to 2011 there were a total of 12,806 monitoring observations used to guide decision making. Presence of wet conditions was associated with elevated concentrations at 11 of 21 locations. After considering the previous day’s culture results with the presence of rain, this association grew stronger. Not all Chicago beaches are affected uniformly by precipitation. Prior day culture results and precipitation information can be used to issue a beach notification without additional testing.