If the Foster City lagoon seems too blue to be true, that's because it is.
Every year from April to October, when aquatic plants are growing, the Department of Public Works dumps 100-150 gallons of blue dye in the lagoon each month to keep algae and aquatic weeds in check. The public works crew loads the dye onto a boat and introduces it into the lagoon through spray heads placed under the water's surface.
The annual cost of this sapphire blue treatment: $35,000. That money comes from the city's general fund, said Mike McElligott, streets and lagoon superintendent, the Department of Public Works.
Although Cygnet says the dye is non-toxic to marine and other wildlife, the company does caution that contact with the product in its concentrated form before dilution may cause slight eye irritation and redness, skin irritation, nausea if inhaled and "gastric disturbances" if ingested.
"The dye colors the water and reduces depth penetration of ultraviolet light," said McElligott, explaining that ultraviolet light allows a plant to produce chlorophyll. If the ultraviolet light can't penetrate to the depth of aquatic weeds growing near the bottom they can't produce chlorophyll, an essential ingredient for keeping them alive and growing.
McElligott said without the dye weeds, particularly widgeon grass, would choke the waterways and potentially damage sailboat engines or hinder other recreational activities.
Horne confirmed the dye is non-toxic, describing it as very similar to that used for dying clothes. Even if some dye got into San Francisco Bay when the city releases lagoon water during the rainy season, Horne said it would not be harmful at all. The dye degrades in sunlight and eventually breaks down into a colorless compound.
Another justification for weed control is municipal liability.
"You need them down because if you got little sailboats, canoes and people paddling or even swimming, you don't want [them] to get tangled up." said Horne. "From a city's point of view, one of the things where it would really be in trouble is if somebody drowned - some child got wrapped up in weeds and drowned."
William Kennaugh, a Foster City resident since 1977 with a home on Plum Island, said he was surprised to learn the lagoon's deep blue color is attributed to dye, but he wasn't too concerned about either the dye or the cost.
"If it's not harmful to other things, I'm okay with that," Kennaugh said. "The algae used to be so bad. It was unsightly 7 to 10 years ago."
McElligott has used the dye for the six years he's been an employee and estimates it's been in use for at least a decade, if not more.
San Mateo resident Sharon Darling said she's always suspected that the lagoon color wasn't natural. "I know they're doing something to make it that way," said Darling, who works at Foster City Eye Care, which is seated along the lagoon at the Edgewater Place shopping center. "I'm not opposed to it, but if people want to swim in the water, that's their prerogative. I rather use my pool."