To determine the concentration of HF183 in runoff that corresponds to an elevated illness risk for swimmers, scientists ideally need to begin by measuring levels of both HF183 and pathogens in the field. These data points can then be plugged into health risk modeling tools to estimate the health risks of swimming in water with a given level of HF183.

But this process is expensive and time-consuming. Also, it’s only been over the past decade that scientists have had access to the technology necessary to reliably make these measurements in complex environmental water matrices like stormwater.

Consequently, efforts to answer the question of how much HF183 is too much have been incremental, with scientists needing to overcome multiple limitations:

• Scientists initially relied on data that were available in published literature, including concentration distribution data for raw, fresh sewage during dry weather – an imperfect proxy for fecal contamination that may come from different human sources and be of potentially variable ages, and that may be mixed with wet-weather runoff.
• Scientists opted not to use well-established bacteria culturing methods to quantify fecal contamination levels in a sample, as stormwater can be so contaminated with other types of debris that these methods do not perform reliably.
• The technology that the SMC study used – droplet digital PCR (polymerase chain reaction) – was not vetted for routine fecal contamination monitoring until the mid-2010s. Thus, it has been only recently that managers have gained confidence about relying on ddPCR to quantify HF183 levels in runoff.

The newly completed SMC study has overcome all of these limitations. For this three-year study, researchers invested in field data collection at 31 sites across coastal southern California during two wet-weather years, measured both HF183 and pathogens in the field samples, and took advantage of the latest updates to health risk modeling tools.