Fecal indicator bacteria on sand and wrack at marine beaches [electronic resource] : impacts on microbial water quality

Fecal indicator bacteria (FIB), such as enterococci (ENT) and E. coli (EC), are used as a proxy for pathogens in recreational waters around the world. When concentrations of FIB exceed standards in recreational waters, the beaches are closed or posted with a health advisory. FIB standards are based on epidemiology studies and are designed to be protective of human health. These epidemiology studies typically have been conducted at beaches where the sources of pollution were treated wastewater or urban runoff, presumably contaminated with raw sewage. However, non-fecal reservoirs of FIB have been identified that have no clear connection to human fecal contamination, such as river sediments, sands and decaying aquatic plants (wrack). This dissertation examines two of these non-fecal reservoirs of FIB, beach sands and wrack, and their impacts on coastal water quality. Understanding these impacts has direct implications on current and future beach management decisions. The first research chapter in this dissertation investigates a transport pathway for ENT on beach sands to serve as a source of ENT to the coastal ocean. Although beach sands have been identified as a potential reservoir for FIB and pathogens, little is understood about the mechanisms by which these microbes can move between sands and the ocean. Two transport pathways have been suggested, 'over-beach transport' and 'through-beach transport'. In through-beach transport, microbes are eluted from sands by infiltrating water, transported down through the vadose zone and out to the ocean through submarine groundwater discharge. This study focuses on the first component of through-beach transport utilizing a combination of field and laboratory experiments. The results show that ENT are readily eluted from beach sands during infiltration events in the vadose zone. ENT detachment kinetics and potential mechanisms are revealed through use of a computer model utilizing the results of the laboratory column experiments. Detachment kinetics that are proportional to the rate of change in water content are found to best describe the observed behavior, suggesting that detachment may be caused by thin film expansion and/or air-water interface scouring. The second research chapter in this dissertation presents a two year microbial source tracking (MST) study aimed at identifying the sources of the high FIB concentrations observed at Cowell Beach in Santa Cruz, CA. Cowell Beach consistently has the worst summertime microbial water quality of any beach in California. Local agencies had been unable to identify the source of this pollution but believed that a non-fecal source, namely wrack, was responsible. Potential sources investigated included a river, a storm drain, a wharf, a harbor, sand, wrack and contaminated groundwater. The microbial pollution was identified as originating from a shoreline source, ruling out the river, storm drain, wharf and harbor as relevant sources. Based on a 24 h study and near-shore modelling results, two separate sources were identified as being dominant, sand for ENT and contaminated groundwater for EC. Wrack was found to be only a minor source, contributing less than 2% of the FIB compared to the dominant sources. The final research chapter presented in this dissertation assesses the impact beach grooming for wrack removal has on FIB concentrations at Cowell Beach. Beach grooming of wrack is considered a potential remediation strategy for beaches with high concentrations of FIB in the water and large amounts of wrack on the beach. No study prior to this work had investigated these impacts. The impacts were studied on both seasonal and short-term time scales. Grooming was generally found to have negligible impacts on concentrations of FIB in the ocean at Cowell Beach. Grooming did however increase nutrient concentrations (phosphate, silicate and dissolved inorganic nitrogen) and turbidity in the ocean. Grooming was found to be an ineffective remediation strategy at Cowell Beach. The research presented in this dissertation documents how FIB from beach sands and wrack can impact coastal microbial water quality. This dissertation provides evidence for FIB from sands being transported to the ocean via through-beach transport. This dissertation also provides evidence that wrack is a minor source of FIB to the ocean and that beach grooming for wrack removal does not provide relevant improvements to coastal water quality.

Advisor: Boehm Alexandria primary advisor. Thesis advisor
Holden Patricia advisor. Thesis advisor
Luthy Richard G. advisor. Thesis advisor