Appropriate Indicators for Recreational Waters: Lake Pontchartrain Basin Case Study

The Pontchartrain Basin is one of the largest and most productive estuaries in the continental United States. Due to the actions of man, the Basin's ecosystem has changed significantly over the past half-century. Recreational activities including swimming have been banned in areas of the lake.A current Tulane University project sponsored by the Lake Pontchartrain Basin Foundation through USEPA is designed to characterize the movement and ultimate fate of indicator microbes in urban runoff from drainage canals to Lake Pontchartrain. The ultimate goal is to determine the conditions under which swimming and other recreational activities in specified areas of the lake would be permissible by Louisiana Department of Health and Hospitals. E. coli, Enterococci and Fecal coliform were employed as indicator organisms following storm events and during dry periods (background conditions). In addition to plume sampling of pumped drainage water, samples were collected and analyzed for E coli, Enterococci, and fecal coliform at sites along and adjacent to Lincoln Beach in New Orleans which is considered to be the prime candidate for initial opening of Lake Pontchartrain for swimming. Pertinent physicochemical data and hydrological were also taken as inputs for model correlations.As the indicator organisms E. coli, Enterococci and fecal coliform exit from the outfall, a rapid decrease in numbers is observed. Due to organisms die off, sedimentation and dilution by the receiving water of Lake Pontchartrain indicators are not detected a short distance from the outfall following two to three days after pumping. Spearman correlation coefficients were analyzed to statistically compare indicator titers for pairs of these three indicator organisms (E. coli, Enterococci and fecal coliform). All indicators correlated relatively closely for fresh water conditions with less variability observed for Enterococci. Spearman correlation coefficients for Lincoln Beach water samples, however, showed a significant decrease as compared to that of drainage canal water. This demonstrates the importance of water environment characteristics on indicator organism fate and applicability. Variability of all indicator counts increased in lake water. Box plots of E. coli, Enterococci and fecal coliform indicate that Enterococci may be a more sensitive indicator than the other two groups for brackish water conditions. Logistic regression with Lincoln Beach water quality data supports that E. coli criteria may be better suitable for freshwater conditions; whereas, Enterococci seems to be the indicator organism most appropriate for brackish water with counts well correlated to physicochemical and pumping activity parameters. Both descriptive and regression studies support Enterococci as the most appropriate indicator under the conditions of the study.