Risk Assessment: Exposure to Pathogens in Sewage-Contaminated Beach Sand

In mid-winter in a temperate area, untreated municipal sewage overflowed onto ocean beaches. To assess the infection risk posed to beachgoers by sewage-borne pathogens, we developed a framework to predict the concentration of pathogens in beach sand, and coupled the environmental concentration to sand-exposure parameters and dose-response relationships for representative sewage-borne pathogens. Our risk assessment focused on risk from the ingestion of contaminated sand, but other possible routes of pathogen exposure include ingestion of ocean water, inhalation of ocean spray, inhalation from wind-driven dispersal, consumption of shellfish, and penetration through dermal abrasions. In our case scenario, raw sewage overflowed onto a public beach and approximately 238,000 L entered the ocean before a sand berm was constructed to impound the remainder. Approximately 3,230,000 L were recovered from the impound area and returned to the sewage treatment plant, while an estimated 2,270,000 L infiltrated an 8770 m area of the beach. Sand samples were collected by the municipality and area public health organizations and analyzed for fecal coliform bacteria. After several weeks, the beach was eventually decontaminated with a dilute chlorine solution. In our study, we model, evaluate, and compare the risk of pathogen exposure to the beach sand when (1) left untreated and (2) treated with chlorine. To validate the level of beach contamination predicted by our model, we compare the predicted fecal coliform bacteria concentration to the results of the municipality’s sample analysis. We used a traditional approach to risk assessment: hazard identification, exposure assessment, dose-response assessment, and risk characterization.