The waters of Blackman Lake as seen on a cloudy day in July 2021.
Photo by Michael Whitney.
#SNOHOMISH — To reduce toxic algae blooms in Blackman Lake, the city is recommending to use treatments at first, while keeping an eye on modern underwater oxygen pumps as a future non-chemical option to implement, city senior utilities engineer Cory Morton told the City Council last week.
#Either option can help address why cyanobacteria has thriving conditions in the lake. Also known as blue-green algae, when cyanobacteria blooms, it has organic toxics that can poison people and pets that come into contact with it.
#Cyanobacteria feeds off phosphorous. Blackman Lake happens to have abundant phosphorous and nitrates right now.
#The lakebed is releasing phosphorous from its deep sediment.
#Scientists identified the lakebed, not outside pollutants, as the lake’s top phosphorus source from a year of collecting and analyzing periodic lake water samples.
#A draft of the lake management plan came out July 12. A final version is expected in August.
#Rob Zisette, the primary consultant from the firm Herrera Environmental Consultants, is recommending Snohomish installs an oxygen saturation system only.
#Aluminum sulfate (alum) treatments help control phosphorous from releasing. They lock in phosphorous to the bottom of the lake. An alternative to alum is lanthanum, a treatment made from clay. It’s sold as EutroSorb. This stuff binds to the phosphorous, taking away its usefulness as food for cyanobacteria.
#Oxygenation is a comparatively new technology. It’s been used in about one dozen lakes nationally so far, Zisette said.
#Its method is to continually pump oxygen into the lake. The constant movement breaks the cycle of stratification, where during warm months the lake splits into a top layer of oxygen and bottom layer rich in phosphorous and nitrogen that feed cyanobacteria. Oxygenation would let other, non-toxic algae thrive, plus benefit other species. Lake fish would get deeper levels of water to breathe in as well.
#Costs
#The two routes have differing costs, from figures Zisette’s team provided.
#Using alum would cost $350,000 up front to set it up, and then cost $50,000 or $125,000 a year depending if you do two treatments a year or quarterly treatments annually, in latest figures. Over 20 years, two treatments a year plus the initial treatment would cost $1.35 million.
#Installing an oxygenation system would cost about $600,000 up front to set it up, and then cost $12,000 a year in maintenance, in latest figures. Over 20 years, it would cost $840,000.
#Herrera proposes a package that includes also doing lakeshed monitoring and other lake work.
#It also gave policy recommendations for the city: These are to require adding phosphorus reduction treatments at new developments and redevelopments, and have the city require stormwater treatment retrofitting on developments. Stormwater retrofitting can include requiring gravel-topped underground stormwater basins (instead of retention ponds) or using rain gardens. All of these steps are to bring water slowly into the watershed.
#One visible result of oxygenation is it doesn’t produce a crystal-clear lake, but an organically colored lake, Zisette said post-meeting.
#Problem-solving for Blackman Lake is Zisette’s last major project before retiring, and expressed its been a joy working on it with the city. He’s not going away, though: He bought a house around Storm Lake in the Three Lakes area.
#The draft plan is posted to the city’s Blackman Lake Cyanobacteria Management Plan web page, www.snohomishwa.gov/808/Blackmans-Lake-Management
#A shortened link to the 314-page draft plan is www.tinyurl.com/blackmanlakedraftplan
