Environmental Factors Impacting Coastal Beach Health

Abstract

The objectives of this research were to 1) determine the variability of fecal-indicator bacteria in Gulf of Mexico marine waters, 2) determine the effect of environmental variables on enterococci concentrations, 3) determine if beach sand at Holly Beach, LA was capable of acting as a reservoir for fecal-indicator bacteria, and 4) determine if a model could be developed to reduce the time it takes to issue a beach swim advisory. Fecal-indicator bacteria concentrations varied from year to year in historical data collected from a Gulf Coast beach. Afternoon fecal-indicator bacteria concentrations were consistently lower than morning concentrations, demonstrating daily variability. Environmental variables recorded from morning sampling events also varied from year to year and presented no clear connection to yearly fecal-indicator bacteria concentration variations. Beach sands contained enterococci (geometric mean of 43 ± 848 MPN per 100 g wet sand) and also provided enterococci the ability to persist and even regrow (geometric mean of 57 ± 1,541 MPN per 100 g wet sand). Laboratory experiments confirmed the ability of solar radiation to inactivate large concentrations of enterococci (>24,196 MPN per 100 mL) across a range of salinities (0, 5, 15, 20, 25 ppt) in as little as four hours, which explained the decrease in enterococci concentrations over the course of the day. Increased turbidity (70 and 140 NTU) prevented complete inactivation of enterococci, which partially explained why enterococci, although lower than morning samples, were still found in afternoon samples at coastal beaches. Experiments demonstrated that after solar inactivation, no appreciable reactivation of enterococci occurred overnight. An exponential decay model and a regression model were applied to predict enterococci concentrations along a Gulf Coast beach, although their use alone was not effective in issuing beach swim advisories. The decay coefficient (k) generated for the exponential decay model was 0.120 hr-1. The regression model found that enterococci were influenced by the variables year, salinity, water temperature, wind speed, total rainfall within 48 hours prior to sampling, sunny, weather, and mean tide level. However, the regression model only accounted for a small portion of variability (R2 = 0.20) in predicting enterococci concentrations.