Environmental drivers of enterococci bacteria variability across water and sediment compartments in an urban coastal system.

Frequent exceedances of recreational water-quality standards for enterococci are a persistent problem at many urban beaches. In the state of VA, a small beach in a highly urban area has more swimming advisories than any other beach. To investigate environmental drivers of these exceedances, we analyzed six years (2018-2023) of department of health data from this beach-Hilton Beach- and three other nearby beaches. As a complement to the historical analysis, we conducted field sampling of water, sand, and sediment at Hilton Beach for most of a calendar year. Historical data analysis showed that rainfall and wave intensity were significantly and positively associated with enterococci concentrations across all four beaches, whereas tidal phase was not. During summer field trials at Hilton Beach, enterococci were consistently detected in water, the underlying intertidal sediment, and supratidal sand, with median densities in sediment and sand four to eight times greater than in the overlying water. Disturbing the sediment increased both turbidity and water-column enterococci, indicating resuspension of sediment-associated bacteria. Monthly sampling from late summer through early spring revealed a strong positive correlation between enterococci and water temperature, with concentrations declining under cooler conditions. Together, these findings demonstrate that sediments and supratidal sand can act as reservoirs for enterococci, likely deposited by rainfall flushing of stormwater pipes, contributing to recurring exceedances when resuspended by waves. The results highlight the need to integrate sediment processes and environmental drivers into predictive beach- monitoring frameworks and emphasize the importance of data-driven models and the difficulties in improving water quality at urban recreational beaches.