Oso Reservoir Stench Subsides

By the time board members of the Santa Margarita Water District met last week, the rotten-egg smell in north Mission Viejo was tapering off. Officials explained that the three-week-long “odor event” had been caused by an algae bloom in Upper Oso Reservoir. Although residents near the reservoir complained that the smell made them sick, board members indicated they avoided calling a special meeting to save money.

On Nov. 11, the Air Quality Management District said SMWD had caused a public nuisance. Residents living near the reservoir complained of nausea, coughing and other ailments stemming from the foul-smelling air.

SMWD officials said the algae bloom was set off by a change in weather. According to their report, the weather turned cold in October, chilling the top level of water. As the cooled water sank to the bottom, the odor was caused by oxygen-depleted air rising to the top. Winds stirred and dispersed the air, and the foul smell spread well beyond the lake.

A Mission Viejo resident who lives near the reservoir said, “The water district’s explanation might sound good until you look at how many times we’ve experienced cold snaps, and the reservoir never had this problem before. I’ve heard about a couple other factors that haven’t been discussed by SMWD. One is that pumps in the reservoir were either turned off or they were the wrong kind of pumps, so the water wasn’t being aerated at all or the pumps were inadequate. Upper Oso is a deep-water reservoir, and the pumps they were using are intended for smaller, shallow-water ponds. It could be that an employee was supposed to monitor water quality and didn’t. The reservoir is filled with treated wastewater, and SMWD stopped adding fresh water in 2008.”

According to information published in the Nov. 21 OC Register, the water district spent $148,142.27 to address the problem with chemicals, renting ozone-pumping equipment, purchasing equipment and paying for tests and consultants.

The greatest cost to SMWD officials could be the loss of confidence in the board of directors for not calling a special meeting to face the public or resolve the problem quickly.