The waters of Blackman Lake as seen on a cloudy day in July.
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
