LAKE KETCHUM — Snohomish County is two treatments in to a long-term water quality project here, and officials estimate that phosphorus levels — the key to controlling toxic algae blooms — are about a tenth of what they once were.
Yet the lake remains one of the most polluted in the county. It’s moved from the No. 1 spot to fourth out of 40 lakes monitored by the county’s Surface Water Management program.
Even with treatments, the lake will not be as clear as others around the county because there is too much pollution. The county had been studying Lake Ketchum for about two decades before they started treating the water in 2014. Contractors release aluminum sulfate, or alum, into the water to bond with the phosphorus, trapping the element before it can nourish the algae.
Lake Ketchum is a quiet, 25-acre lake just north of Stanwood. It’s surrounded by houses, with a small parking area and boat dock for public access.
“It has been the most polluted lake in Snohomish County, and mainly polluted by phosphorus, which feeds algae,” said Gene Williams, a senior planner with the program. “In recent years, the algae blooms in the late summer and fall on Lake Ketchum have been toxic to the point that people couldn’t use it.”
Though most algae show up as harmless but unappealing clusters in the water, some varieties are poisonous to animals.
In the summer, when feeder creeks run low, the weather is calm and the water is still, the bottom of the lake has very little oxygen. Phosphorus gets released from sediment there and algae thrive. When those algae die, they sink to the bottom of the lake and pile up until the phosphorus is recycled back into the system.
It’s a cycle that upped phosphorus levels in Lake Ketchum until they were about 13 times greater than regional water quality standards recommend, according to the county.
A phosphorus concentration of 25 micrograms per liter of water is high enough to feed unwanted algae growth. In 2014, after 15 years of regular testing, the county placed the summertime average for Lake Ketchum at 277 micrograms per liter near the lake’s surface.
In the depths of the lake, it was more than 1,700 micrograms per liter.
After a partial treatment last year, the levels are down to about 34 micrograms per liter near the surface and 186 in the depths.
The county attempted its first alum treatment in May 2014. Aluminum sulfate is not normally hazardous, but the amount needed to treat Lake Ketchum is enough to change the water’s acidity, Williams said. Another chemical — sodium aluminate — is added as a buffer.
The first treatment didn’t go as planned. Though phosphorus levels dropped nearly 90 percent, the acidity changed enough that about 40 trout and several hundred yellow perch died. The treatment was stopped about two-thirds of the way through, as soon as contractors noticed the dead fish, Williams said.
They tried again the first week of March 2015. By doing it earlier in the season, mixing the chemicals differently and having wildlife experts on hand, contractors were able to complete the process without killing fish, Williams said.
Moving forward, they plan to do smaller yearly treatments to keep phosphorus levels down.
About 73 percent of the phosphorus has been coming from the bottom of the lake, according to a county study conducted from 2010 to 2012. Another 23 percent comes from a creek that runs past a former dairy farm. Phosphorus is found naturally in soil and most living things but tends to be more concentrated in agricultural areas because it’s used in fertilizers and found in animal waste.
About 4 percent of the phosphorus in Lake Ketchum has been coming from rain, groundwater, septic systems, pet waste and fertilizers.
This year’s treatment required more than 13,000 gallons of aluminum sulfate and about 8,100 gallons of the sodium aluminate buffer. Water samples will be collected over the next few months to determine how effective it was.
“We expect to have much better water quality this year compared to last year,” Williams said. “That doesn’t mean there won’t be some algae blooms, but there won’t be as many and they shouldn’t be as severe.”
The project cost about $128,000 last year and $120,000 this year. Future treatments will be smaller and are expected to cost about $40,000 each year.
The project is paid for by a mix of state grants, county dollars and increased surface water management fees approved by property owners around the lake.
Kari Bray: 425-339-3439; firstname.lastname@example.org