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LESJWA investigates Canyon Lake water quality, fish mortality following fish die-off

Thousand of fish started washing ashore in Canyon Lake during an alum treatment in April. Photo by Jeff Taylor.

Following a fish die-off, the Lake Elsinore & San Jacinto Watersheds Authority (LESJWA) continues to work closely with its scientific team to investigate the recent water quality conditions in Canyon Lake, specifically the algal bloom and fish die-off that occurred during an alum treatment in April.

Following heavy winter storms, nutrient influx likely triggered an algal bloom, LESJWA reported. The April application of alum applied a lower dosage of alum than usual.

According to LESJWA, pre-alum monitoring on April 17 showed aluminum concentrations between 41 to 75 µg/L across the four TMDL sites. Although higher than routine findings, this was expected as samples were taken one-day post-application. Historical data shows much higher concentrations (up to 6300 µg/L) with no associated fish die-offs, typically after significant storm events.

LESJWA continues to investigate the recent water quality conditions in Canyon Lake after an algal bloom and fish die-off occurred during an alum treatment in April. Photo by Amiee Hunter.

The recent alum levels were below the acute criterion maximum concentration but slightly above the chronic threshold criteria in some areas, LESJWA said. Additionally, the algal bloom could have led to a drop in dissolved oxygen levels overnight, contributing to the fish mortality.

“Alum and algal blooms do not compromise drinking water quality or pose risks to human health and wildlife,” LESJWA said.

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