Transport pathways of microbial contamination in coastal waters from a seasonally forced, anthropogenically influenced stream in the Eastern Mediterranean

Microbial water quality (MWQ) along the Israeli Mediterranean coastline is influenced by coastal stream inputs, yet the seasonal discharge variability and associated transport remain insufficiently resolved. This study integrates field observations and hydrodynamic modeling to quantify the dispersion of fecal coliforms (FC) originating from the Alexander Stream. FC concentrations were measured at the stream outlet and along the adjacent coastline over the period 1/1/2021–6/15/2023 and analyzed together with daily stream discharge that varied in the range of ∼0.1–0.2 m³ s⁻¹ and ∼10 m³ s⁻¹ during summer and winter, respectively. The mean FC concentration at the outlet during the study was (1.05 ± 0.05)·10⁵ CFU·100 mL⁻¹, with a maximum of 10⁶ CFU·100 mL⁻¹. Peak values were associated with high-flow conditions and declined to below detection limits during summer low-flow periods. In contrast, coastal sediments and beach sands sampled during the dry season (September 2023) retained FC concentrations (∼10¹–10⁴ MPN·100 mL⁻¹), indicating persistent storage of fecal contamination in the nearshore environment. A passive tracer was implemented in the hydrodynamic model to simulate contaminant transport and evaluated using an exceedance threshold of 0.01% of the source concentration. During the winter months, elevated discharge generates a coastal plume extending ∼2–3 km offshore and alongshore, with exceedance frequencies exceeding 50% in the nearshore zone. Surface transport is predominantly southwesterly, whereas near-bottom transport is directed northward along the coastline. During April-June, the plume becomes more confined (∼1-2 km from the outlet). In July-September, tracer exceedance is negligible (<1%) despite FC discharge observed in the stream outlet and bathing waters. In October-December, exceedance reappears but remains spatially limited. These results demonstrate that FC inputs from Alexander stream dominate nearby coastal contamination during high-flow periods, while summer conditions reflect minimal transport and a decoupling between discharge and observed MWQ indicating the sedimentary reservoir as a possible source.