SCCWRP presents HF183 stormwater study: markers widespread, modeled threshold roughly 100 gene copies/100 mL

Researchers from the Southern California Coastal Water Research Project on the work-group call presented regional wet-weather sampling and a quantitative microbial risk assessment (QMRA) to explore whether HF183 (a human-associated marker) can be tied to risk-based management thresholds in stormwater.

Joshua Steele, a microbiologist with SCCWRP, summarized the study design and key findings. The study sampled 31 sites across Southern California (Ventura to San Diego and inland to Riverside and San Bernardino) over two wet seasons, collecting 70 samples within 24 hours of rain events. "We found HF183 in about 90% of the sites we looked at, and around 80% of the samples," Steele said, adding that norovirus and other pathogens were also detected frequently though with greater variability.

SCCWRP reported a median HF183 concentration of 557 gene copies per 100 milliliters across the stormwater samples with concentrations ranging up to 10^4 copies per 100 mL. Using dose–response relationships and exposure assumptions for body-contact recreation, the QMRA produced a combined median risk of about 190 illnesses per 1,000 swimmers when swimming in undiluted stormwater; norovirus and Campylobacter were the principal contributors to modeled illness risk.

To link marker concentrations to an EPA-style benchmark (32 illnesses per 1,000 swimmers), the team modeled dilution of stormwater into cleaner waters. Steele said a roughly 5.5-fold dilution — producing a median HF183 concentration near 100 gene copies per 100 mL — would correspond to that benchmark in the study's median case. "When we model dilution, that gets us to about 100 HF183 gene copies per 100 mL," he said. The team also calculated a single-sample statistical threshold of about 1,875 gene copies per 100 mL that may be more operational for stormwater managers who collect individual event samples.

Steele cautioned these numbers are preliminary and region-specific: "There are no water quality criteria for HF183 — EPA doesn't have one, the state doesn't have one — so this is something stormwater managers are looking to use in the absence of a regulatory threshold." He described a management committee with representatives from stormwater and public-health agencies, water boards, EPA Region 9, wastewater agencies and NGOs (including Heal the Bay) that is drafting recommendations for how to apply HF183 in monitoring and management.

Attendees asked about measuring HF183 directly in ocean water and about sample volumes; SCCWRP said ocean measurements require larger volumes because HF183 is typically more dilute and that stormwater grab samples were taken in 1–2 L bottles but filtration can be difficult in turbid samples. Steele said outstanding questions include appropriate sampling locations, site-specific dilution, and what actions to trigger when thresholds are exceeded. The management committee is working on recommendations expected next year.