TRANSPORT OF FECAL INDICATOR BACTERIA FROM SAND TO SEA VIA SEAWATER-BEACH GROUNDWATER INTERACTIONS (Invited Presentation)

Marine beach sands harbor high concentrations of fecal indicator bacteria (FIB, e.g., Escherichia coli and enterococci), organisms strictly regulated in the ribbon of coastal waters adjacent to the shore. In this study, we conceptualize and test a model whereby FIB are transported from sand to sea via ‘through-beach transport’. In the conceptual model, FIB attached to unsaturated beach sands are mobilized by a wetting front of infiltrating seawater from wave uprush or rising tides. Once mobilized, FIB are transported through the beach vadose zone with the infiltrating seawater to the subterranean estuary where they are subsequently discharged to the coastal ocean via submarine groundwater discharge. Field, lab, and modeling experiments illustrate the first step in the process: the mobilization of sand-bound enterococci and their transport through the vadose zone. Beach sands naturally contaminated with enterococci where packed in a column and subjected to intermittent imbibing and draining using filtered seawater. A pan lysimeter was deployed at a field site with contaminated beach sands. Enterococci were readily mobilized and transported through the unsaturated zone during infiltration events in both the field and laboratory column experiments. Detachment mechanisms were investigated using a modified version of HYDRUS-1D to model column experiment results. Detachment kinetics that are first order with respect to the rate of change in the water content and attached surface bacterial concentrations were found to provide a best fit between predicted and observed data. This suggests that air–water interface scouring and thin film expansion are likely drivers of enterococci mobilization in the investigated system. The second step in the process, the discharge of FIB laden groundwater via submarine ground water discharge, was investigated at a beach with high levels of FIB in sands. Temporal intensive measurements of E. coli in the coastal ocean showed high concentrations at low tide. Subsequent fate and transport modeling confirmed a tidal modulated groundwater source could explain the field observations. Future work will couple infiltration and submarine groundwater discharge models to confirm the through-beach FIB transport pathway.