Enumeration, variability, and transport of Escherichia coli in the foreshore reservoir and surface water at freshwater beaches

Beaches are of immense recreational, societal and economic value. This value, however, is considerably diminished by poor water quality. Fecal indicator bacteria (FIB) are measured at recreational beaches worldwide to assess the water quality. A beach closure or advisory is issued if FIB concentrations in surface water exceed recreational water quality standards. Due to the lengthy time required to enumerate FIB (24 – 96 hours), statistical and mechanistic models have been developed to predict water quality exceedances a priori and to better understand why and under what conditions water quality exceedances occur. These models as well as beach water quality management strategies are often based on limited mechanistic understanding of the fate and transport of FIB in the beach environment. For instance, FIB are known to accumulate at very high concentrations in foreshore sand and porewater at beaches (herein referred to as the foreshore reservoir). The dynamics of FIB accumulation in the foreshore reservoir and its subsequent release, including the impact on surface water quality exceedances, is unknown. It is also unclear how to best quantify the abundance of FIB in the reservoir including its partitioning between the sand and pore water. An increased understanding of the behavior of FIB at beaches is needed to improve the accuracy of predictive water quality models, develop effective measures to reduce water quality exceedances, improve water quality monitoring strategies, and ultimately to better protect human health at recreational beaches. This thesis focuses on addressing key knowledge gaps regarding the behavior and quantification of FIB in the foreshore reservoir. In the first study, seasonal and daily variabilities in FIB concentrations in the foreshore reservoir and surface water are evaluated including determining the influence of environmental factors, such as temperature, waves, and rainfall. In this study, seasonal variability in FIB concentrations in the surface water and foreshore reservoir were found to depend on environmental factors, with some beaches showing a gradual increasing trend through the summer, then decreasing towards the beginning of fall. However, daily variation showed that FIB variability is much more complex and FIB may not simply accumulate over the summer months as previously thought. Further, this study showed for the first time that FIB may be able to replicate in unseeded natural foreshore beach sand not subjected to external stimuli. The second study uses experimental and field data to evaluate the behavior of FIB in the beach environment during intensified wave conditions including the transfer of FIB from the foreshore reservoir to the surface water. This study showed that as wave height increased foreshore sand erosion resulted in elevated E. coli concentrations in surface water, as well as depletion of E. coli from the foreshore sand and pore water. E. coli initially attached to foreshore sand rather than initially residing in the pore water was found to be the main contributor to elevated surface water concentrations. Surface water E. coli concentrations were a function of not only wave height (and associated sand erosion) but also the time elapsed since a preceding period of high wave intensity. This finding is important for statistical regression models used to predict beach advisories. While calculations suggested that foreshore sand erosion may be the dominant mechanism for releasing E. coli to surface water during intensified wave conditions at a fine sand beach, comparative characterization of the E. coli distribution at a coarse sand-cobble beach suggested that interstitial pore water flow and discharge may be more important for coarser sand beaches. The third study compared the partitioning of FIB in the foreshore reservoir between the sand and pore water and evaluated different sampling methods for quantifying FIB in the foreshore reservoir at beaches with varying grain sizes. This study showed that the collection of the top 1 cm of unsaturated sand resulted in higher and more variable concentrations than the top 5 cm of sand. There were no statistical differences in E. coli concentrations when using different methods to sample the saturated sand. Overall, the unsaturated sand had the highest amount of E. coli when compared to saturated sand and pore water (considered on a bulk volumetric basis). Pore water sampled with a shovel resulted in the highest observed E. coli concentrations (only statistically significant at fine sand beaches) and lowest variability compared to other sampling methods. These findings presented will help determine the appropriate sampling strategy for characterizing FIB abundance in the foreshore reservoir as a means of predicting its potential impact on nearshore surface water quality and public health risk. Overall, this thesis presents valuable information to health departments, beach managers, and scientists interested in improving water quality and water quality predictions at recreational beaches. Findings from this thesis increase understanding of FIB behavior, especially in the foreshore reservoir, and can be used to improve predictive water quality models, develop strategies to reduce FIB levels at beaches, and identify where and when a foreshore reservoir may be an important source of FIB to the surface water at a beach