From bloom to tap: Connecting harmful algal bloom indicators in source water to cyanotoxin presence in treated drinking water.

Cyanotoxins produced by harmful algal blooms (HABs) can negatively impact human health through ingestion of drinking water. Under most circumstances, public water systems (PWSs) have demonstrated that they can successfully manage the removal of low-to-moderate levels of cyanotoxins with conventional treatment. Intense or frequent blooms put some PWSs at higher risk for treatment breakthrough, meaning cyanotoxins can pass through the drinking water treatment process, and there are still some instances of reported cyanotoxin levels in drinking water. This study links cyanotoxin drinking water occurrence data from PWSs sampled under the Fourth Unregulated Contaminant Monitoring Rule (UCMR 4) with qualitative source water HAB information to understand the relationship between source water indicators and drinking water occurrence. During the monitoring period (2018 to 2020), 153 of approximately 110,000 total cyanotoxin analytical results from 67 PWSs were observed at or above the minimum reporting level (MRL), resulting in a 0.14 percent rate of occurrence in drinking water. The associated qualitative data indicated that 23.9 percent of systems with a cyanotoxin reported value ≥ MRL in drinking water also reported the presence of a HAB indicator while 1.7 percent of systems with a reported value ≥ MRL in finished water did not report any HAB indicators. Odds ratio testing revealed that a system is more than twice as likely to have a cyanotoxin detection in drinking water if a HAB indicator is observed. This study demonstrates the importance of monitoring source water changes and observed treatment changes for vulnerable systems, as these can facilitate timely adjustments to treatment and mitigate the risks of cyanotoxin breakthrough in finished water.