The first confirmed documentation of the cyanotoxin, microcystin, in a recycled water pond occurred in 2018 at a municipality in California, where recycled water is provided for residential, agricultural, commercial, and urban landscape irrigation. Microcystin, which is produced by the cyanobacteria genera Microcystis, was detected in the recycled water pond on numerous occasions throughout the 2018–2019 sampling season. RBI conducted a human health risk assessment (HHRA) according to U.S. Environmental Protection Agency guidance to identify and characterize potential risks from microcystin to users exposed to the recycled water. Microcystin exposure routes were identified and assessed for municipal and agricultural workers, children who might play in sprinklers, people consuming food grown in gardens or crops irrigated with recycled water, and for domestic pets (i.e., dogs). Conservative human health and domestic dog benchmarks, developed by State and Federal agencies to be protective of recreational exposures in waterbodies containing microcystin, were used for the HHRA. Although no potential for human health or domestic animal risks from microcystins were identified in 2018–2019, the HHRA identified the potential for future risks if microcystin concentrations were to increase.
Results from the HHRA were used by the municipality to inform a cyanobacteria harmful algae bloom mitigation and prevention strategy, refine routine monitoring, and to design a communications plan for public outreach. These efforts will help minimize future potential health risks associated with microcystin exposure from this recycled water system.