Influence of surface water - hyporheic zone interactions on pathogen persistence in an urban Mediterranean stream

Rivers transport pathogenic microorganisms (including fecal indicator bacteria and human enteric viruses) long distances from non-point and point sources, posing a direct risk for human health. Yet, pathogens can be immobilized and accumulated in streambed sediments via vertical hydrologic exchange between surface water and the hyporheic zone, which delays downstream transmission. Persistence of pathogens may be exacerbated in intermittent streams under extremely low flow conditions. Direct measurements of pathogen accumulation are rare, and further it is unknown if this is a local phenomenon constrained near to the point source or if the continuous deposition and resuspension of pathogens in streams results in the transmission of active pathogens further downstream. In this study, we sampled streambed sediments along a 1km reach of an intermittent Mediterranean stream receiving inputs from a WWTP, during the extreme scenario of a drought when the effluent constituted 100% of the stream flow. We measured abundance of total bacteria, Escherichia coli (as a fecal indicator bacteria), and presence of enteric rotavirus (RoV) and norovirus (NoV). These analyses were supported by measurements of environmental characteristics (temperature, oxygen, total benthic particulate matter, % of organic matter). We found high abundance of E. coli ( 4 ng/μL based on qPCR detection of E. coli) in the sediments within the first 100 m downstream of the WWTP effluent. Furthermore, E. coli concentrations tended to increase with temperature, oxygen, and nutrients. We also found that NoV transported to 300 m, RoV to 700 m and E. coli to 900 m downstream of the WWTP input. These results indicate that at a very low flow condition, inputs of pathogens are transported long distances, but that the extent of longitudinal transport varies between pathogens. We also show that hyporheic sediments act as hot spots for transient storage in the stream, that then could be resuspended and transported downstream during subsequent higher flow events.