Volunteer ‘First Flush’ monitoring finds widespread fecal‑indicator exceedances; some metal and phosphate hotspots flagged
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Summary
The Resource Conservation District presented 2025 First Flush results showing enterococci exceedances across sampled sites, isolated sediment and metal spikes at several outfalls, and orthophosphate exceedances at five sites; RCD emphasized this is the program’s typical 'worst‑case' annual snapshot and recommended continued monitoring and targeted follow‑up at persistent hotspots.
The Resource Conservation District (RCD) presented the 2025 First Flush volunteer water‑quality results to the San Mateo County Harbor District on April 15.
Clifton Herman, who introduced himself as representing the RCD, said First Flush samples the first major rain of the wet season to capture runoff‑related contaminants that have accumulated over the dry months. Herman described the program as a long‑running community‑science effort with data back to 2008 and said the 2025 sampling event took place on Oct. 13 after an intense storm.
Key findings Herman highlighted: • Indicator bacteria: E. coli and enterococci were above human‑health thresholds at most sites in 2025; enterococci exceeded recommended health limits at all sampled sites this year. Herman noted this program is designed to capture a ‘worst‑case’ event for pathogen indicators. • Total suspended solids: One site (Surfer South outfall) exceeded recommended ecological thresholds for sediment; RCD said this spike was collected upstream of sand‑placement activities and is unrelated to the Surfers Beach replenishment project. • Metals: Copper exceedances occurred at four sites (including West Point Ditch and Vassar outfall); zinc exceeded thresholds at two samples. West Point Ditch showed a history of repeated metal inputs and was identified as a persistent hotspot. • Nutrients: Nitrate did not exceed ecological thresholds; orthophosphate exceeded the program threshold at five sites in 2025 (Herman noted laboratories’ detection limits can make low phosphate readings appear as exceedances).
Herman emphasized the program’s limitations: First Flush identifies where contaminants are entering the ocean, not specific sources, and it is best used to prioritize follow‑up sampling and remediation. He invited volunteers to participate in future events and said the full dataset and technical report are in the board package.
Commissioners asked whether microplastics might be monitored in the future and whether the data indicate any linkage to the Surfers Beach sand placement. Herman said microplastics sampling would require different methods, but he expected microplastics likely increase during first flush events; he reiterated that several exceedances (including sediment spikes) were sampled upstream of the beach‑placement area and therefore are unlikely caused by the dredge project.
Commissioners thanked the RCD for the long‑running partnership and noted the data will inform public education and targeted interventions at persistent outfalls.
