Refined ambient water quality thresholds for human-associated fecal indicator HF183 for recreational waters with and without co-occurring gull fecal contamination

A number of studies have used quantitative microbial risk assessment (QMRA) to derive risk-based water quality thresholds (RBTs) for a diverse set of novel water quality indicators in recreational waters. Over time, the QMRA approach has been refined with respect to model inputs and hazard characterization. Recent work considered differential decay of pathogens and indicators, and mixtures of contamination of diverse ages. In the present study, we present a refined ambient water quality RBT for the human-associated marker HF183. The new estimate updates previous work as it specifically considers contamination aging through the inclusion of temperature-specific organism decay, and the presence of mixtures of human sewage contamination of diverse ages. Based on these analyses we derived a RBT of 525 HF183 copies/100 mL as representative of conditions consistent with those described in the 2012 Recreational Water Quality Criteria (32 illnesses /1000). In recreational waters, microbial contamination due to gulls is also common. To account for this, we consider the case where human contamination from sewage co-occurs with contamination from gull feces. The resultant proposed RBTs for HF183 range from 1 to 525 copies/100 ml and are a function of the amount of gull fecal contamination that is present in the water. The proposed RBTs can be considered for use in the evaluation of recreational water quality.