Doug Campbell of Snohomish uses a measuring tool to take a sample of water from near the center of Snohomish's Blackman Lake on Wednesday, Dec. 14, 2022. The work is part of a monthly water test volunteers such as Campbell are doing for the city to create a years-long catalog of water quality information. This information will be used for strategies to help the lake.
Doug Ramsay photo
SNOHOMISH — Alleviating Blackman Lake’s algae problems will take time, but scientists have pinned down the source.
The lakebed is releasing phosphorous from its deep sediment. It’s contributing to an overabundance of nutrients in the water.
The nutrient-rich imbalance creates lake conditions in which toxic blue-green algae thrives, and oxygen for fish gets suppressed and depleted.
The solution, broadly, will be to control or prevent these releases from the deep. One way is by putting treatments in the lake.
The nutrients were deposited decades ago, possibly centuries. A professional study answered whether outside pollutants are behind the nutrient spikes.
Instead, it’s “to the extent that in the summer, 63% of phosphorus input to the lake is from the lake bed (sediment) and 6% is from stormwater,” aquatic scientist Rob Zisette said. His team at Herrera Environmental has now analyzed a year’s worth of water samples to strategize the next steps.
Zisette said Herrera plans to deliver a draft plan to the city in the coming weeks.
Its presentation to council is tentatively scheduled for May 21, City Council President Tom Merrill said.
A pact of lakeside homeowners and stakeholders, Friends of Blackman Lake, plans to host a public presentation, too.
Herrera’s final plan might be released around the end of June, Zisette said.
From there, it is the City Council’s call. The city has about $400,000 set aside to do something.
About algae and how we (probably) got here
Good algae and bad algae both live in Blackman Lake’s ecosystem.
The bad algae is cyanobacteria, which resemble mats of blue-green paint on the water when it blooms. It’s bad because it has organic toxins that can make people and animals sick. It’s serious enough that warning signs to stay out of the lake are posted when blooms are spotted.
When summertime hits, the warmth causes the lake to stratify into layers. Oxygenated water rises to a layer up top, while a layer of oxygenless water stagnates below. Oxgyenless conditions allow phosphorous releases.
In the fall, when the lake
de-stratifies and the layers rejoin, the now-nutrient-rich bottom layer spreads nutrients through the lake. Cyanobacteria love it, and bloom their blooms.
The lake’s history as a logging area plausibly contributed to the lakebed becoming rich with nutrients.
Forests prevent soil erosion, which prevents nutrient-rich soils from washing into the watershed and settling into the sediment.
Logs slowly decaying also deposit nutrients.
Friends of Blackman Lake co-founder Kay Ditzenberger said sunken logs can be seen underwater from her dock.
The lake is named for the three Blackman brothers, who began logging around the lake in the late 1870s, local historian Warner Blake wrote. Soon after, they built a big sawmill on the Snohomish River’s bank roughly where the Snohomish Public Works shop is today on west First Street.
Zisette declined to point any accusative finger. His team is not sure how old the sediment is. The scientists didn’t ‘date’ the sediment core, which could gauge when the sediment’s composition became richer with phosphorous.
A caveat is the geese.
Geese are causing about one-fourth of the problem with Blackman Lake’s enrichment. The city can work to dissuade geese along the shore, but can’t easily prevent them from depositing nutrient-rich contaminants while on the lake surface.
In the fall, hundreds if not thousands of geese visit. The good news here, though, is they do not reside on the lake.
What are the treatments, and at what price tag?
Treatments have been done in many lakes in Western Washington to get algae cleared up. Lake Ketchum by Stanwood is a good example.
It’s what would be looked at for Blackman Lake.
Two choices are aluminum sulfate treatments or lanthanum treatments.
Lanthanum is mixed with Bentonite Clay that absorbs water to create a way of sealing off phosphate releases by binding to the phosphate.
Lake treatments using alum sulfate or lanthanum binds the phosphate ions in the water and sediments with metal ions to make them unusable by algae as a food source.
One heavy treatment or a few light treatments of either choice would cost several hundred thousand dollars.
Zisette said effectively treating Blackman Lake could pencil out to $300,000 to $400,000 for a series of treatments lasting up to 10 years.
“The good news about Blackman Lake is not a lot is coming in from the watershed” so it means “those treatments last much longer.”
The city has some money set aside for remediation.
There is approximately $417,000 left budgeted toward implementing a plan for the lake, the city said.
Mayor Linda Redmon’s 2023-2024 budget allocated $470,000 overall for Herrera’s work product and to act on the lake management plan.
“We will provide resources in the budget, while seeking any additional funding sources available, to care for the health of the lake,” Redmon said when asked if her 2025-2026 city biennial budget will have money for the lake.
An alternate, but more expensive, concept is whole-lake aeration, which mechanically reduces the conditions that let bad algae thrive without harming good algae. An expert in oxygenation technology designed a modern system that injects oxygen into the lowest water layer and uses a water pump to mix it up vigorously.
Cold-water fish also benefit because they can live in deeper waters in the lake all year as there’s oxygen there.
Blackman Lake is shaped like a bowl, and gradually slopes at the deep end toward Hill Park. The fishing docks, by the way, are put at the deepest areas.
