Microcystins at the tap: A closer look at unregulated drinking water contaminants

This case study compared microcystins in tap water from 18 public water systems (PWSs) and 46 households with private intakes or nearshore wells (called self-supplied water systems [SSWS]) from Clear Lake, California during a major cyanobacterial bloom in 2021. Microcystin concentrations were below the US EPA health advisory (HA; 0.3 μg/L) for PWS with Clear Lake intakes and for SSWS drawing from wells. However, for SSWS with Clear Lake intakes (intake SSWS), 24 of 79 microcystin samples collected were at or above the HA (maximum 3.9 μg/L). Further, microscopy showed that 20 of 28 tap samples (post-treatment) from intake SSWS contained either cyanobacteria or algal cells. This demonstrates that robust multi-barrier treatment systems at PWS reliably removed microcystins, while the less sophisticated treatment systems at intake SSWS were compromised during a major cyanobacterial bloom. Intake SSWS should consider alternative sources of drinking water, including physically connecting to an adjacent PWS or drilling a well.