Canyon Lake Alum Application – Response to Floating Alum Floc

May 17, 2023

Rick Whetsel
SAWPA Senior Watershed Manager


The following statement has been prepared in response to the recent photos from a Canyon Lake resident concerning alum flocs in areas of the lake.

“The cause of the floating alum floc shown is mostly likely due to the significantly high concentrations of algae present in the water column. Due to the delayed start on the alum application, a  combination of the warming weather, and the abundance of nutrients in the water column from the numerous winter storms, conditions are more conducive for the algae to feed on. The alum floc shown in the photos will eventually settle out of the water column in this area as an inert (harmless, non-reactive) compound, but it is uncertain how long this time-period will be. The settling could take as little as a day or as long as several days.

However, a benefit of the extended time that the alum remains suspended in the water column, is that there is greater opportunity for the alum to bind with any remaining phosphorus in the water column and provide for the maximum amount of phosphorus removal, which is used to feed algae.

On day 2 of the alum application, Aquatechnex switched their application technology from a surface spray application to sub-surface injection to reduce the amount of alum floating on the lake surface. Aquatechnex plans to complete the remainder of the alum application (Days 2 through 4) using sub-surface injection. This should be most beneficial when applying alum to the shallower Easy Bay and North Ski Area above the causeway.

It is not recommended that additional water spraying be done to break up the alum that has concentrated in the narrow coves, as this could result in the floating alum floc spraying onto any nearby boats or docks.”

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