HAB’s productivity remained steady and high. The crew took advantage of favorable lake conditions by working into the evening to apply 25,000 gallons of alum to a 65-acre section of the application zone. The application project is now about 70% complete and we remain ahead of schedule. Although the project is not near to Cedar Lake, the story of our alum application project at Pinto Lake is an interesting one to share. The application was conducted to remove toxic algae from the lake to, in part, reduce the deaths of a endangered sea otters in Monterey Bay.Pinto Lake is a 120-acre recreational lake in Watsonville, CA. The lake has developed massive algal blooms every late summer and fall and algal toxin levels typically exceeded the State health criteria. As a result, the lake was classified as “impaired” and was closed for contact recreation during the bloom periods. Of particular interest, the death of 31 endangered southern sea otters were traced to algal toxins which had accumulated in shellfish eaten by the otters. Pinto Lake was the likely source of the toxins in the shellfish and the cause of the otter deaths. Excessive amounts of the nutrient phosphorus was the main cause of the toxic algal blooms. Internal phosphorus loading (leaching from the lakebed sediments) and watershed runoff both contributed phosphorus to Pinto, but a study in 2011 showed that the majority (85%) was coming from the lakebed.
Core samples from the bottom of the lake confirmed that phosphorus was very high in the sediments and available to be released into the overlying water column. HAB Aquatic Solutions conducted a buffered alum application (80,000 gallons of aluminum sulfate and 40,000 gallons of sodium aluminate) over a ten-day period in early April, 2017. The application produced a “floc” that settled to the bottom of the lake. The floc had sites where phosphorus in the sediments became chemically bound as it leached from the bottom. The floc effectively intercepts and binds the phosphorus, which makes it unavailable for the algae to use for growth. The goals of the project were to dramatically reduce the internal loading of phosphorus from the sediments, lower the amount of phosphorus available to algae in the water, reduce the amount of algae and associated toxins and remove the recreational restrictions at the lake.
To date, the water quality monitoring results show a dramatic, 96% reduction in total phosphorus in the lake. Pre-application phosphorus concentrations were 450 ppb and dropped immediately after application to less than 20 ppb. The lake will continue to be monitored by the City of Watsonville to track the improvements.
“I was pleased to select HAB to design and perform alum application in-lake. Throughout the project, the team at HAB was responsive and professional. The team’s knowledge and experience working with harmful algal blooms were apparent in every interaction I had with them. What I appreciated as a project manager is that no question or request was too small for the team to call, email or facilitate a conversation with the public, Regional Board grant manager or other partner agencies. I would gladly work with them again to solve complex algal bloom and nutrient loading issues in the City’s lake and watershed.” ~ Jackie McCloud; Project Manager, City of Watsonville.