MARYSVILLE — A study of odors in the Snohomish River Delta — including the target of many complaints in recent years, Cedar Grove composting — has yielded mixed results.

The Puget Sound Clean Air Agency’s $375,000 study was conducted over 13 months through November using electronic odor monitors and the observations of 11 volunteers.

The volunteers and the monitors differ in their impressions.

The volunteers, who live in Marysville and north Everett, recorded their olfactory impressions during that time and reported 122 instances of odors that smelled like compost. The volunteers reported 43 times in which they smelled fresh yard-and-food waste of the type used to make the compost at the Cedar Grove plant on Smith Island, said Joanne Todd, a spokeswoman for the Seattle-based clean air agency.

Volunteers reported only three instances they said seemed to be from other sources — two from biogas, one from sewage treatment.

The results from the electronic odor sensors, or “e-noses,” were less conclusive, officials said.

They showed that the strongest single-incident odors came from, in order, the Everett sewage treatment plant, the Marysville sewage plant and Cedar Grove, said Brian Renninger, an engineer for the clean air agency.

The most persistent odors, however, were recorded as having come from Cedar Grove, the Everett plant and the Marysville plant, in that order, he said.

Mini weather stations were located at Cedar Grove and in downtown Marysville and were used in the data collection.

Cedar Grove vice president Susan Thoman said in a written statement that the results show that Cedar Grove is not the major source of odor in the area.

“The science can’t be denied,” she said. “There are multiple sources making odor contributions to (Marysville), and the city’s own wastewater treatment plant has reported odors as much as six times more intense than Cedar Grove.”

Marysville Mayor Jon Nehring on Friday said the city will not comment until it has time to review the study carefully. The mayor said he will meet with the clean-air agency on Monday.

The measurements were based on computer modeling before the study — including odor characteristics and prevailing weather conditions — and information from e-noses at two of the three sites.

A monitor was not placed at the Marysville sewage plant because city officials opposed the study and did not give permission. Data for that location was extrapolated from the model, including weather information, Renninger said.

The weather data made the e-nose results harder to interpret because the models are based partly on prevailing wind, and during the study year the air was calmer than normal, he said.

Three other e-noses were placed in surrounding communities. One was installed just east of downtown Marysville, one in the Sunnyside neighborhood and one in north Everett — the areas from which most of the complaints have come.

“They did not provide a lot of meaningful information,” Renninger said.

The e-noses were purchased from Odotech of Montreal, Canada.

At this point, the information won’t result in any enforcement action against Cedar Grove or any other potential source, Todd said. The clean air agency will continue to comb through the results, she said.

“That information could very well help us understand how to go forward,” Todd said.

She said it’s unclear whether the agency will need to gather more data or will be able to extract more insight from that already collected.

Twelve other potential odor sources, including Cemex in north Everett and the Lake Stevens sewage treatment plant, were ruled out as persistent nuisances by e-nose information and pre-study testing, officials said.

Inspectors for the clean air agency several times have traced odor complaints to Cedar Grove. The company receives yard and food waste from Snohomish and King counties and grinds, cures and sells it for use in gardens.

The firm has been fined for odor violations at both its Everett location and its other plant in Maple Valley in King County. The company is also the target of four lawsuits filed recently on behalf of nearby residents in both areas.

Cedar Grove Composting is paying $200,000 toward the study. Fines recently paid by the company for odor violations, totaling $119,000, were applied toward that amount.

The city of Seattle and King County, which send yard and food waste to Cedar Grove, are pitching in $100,000 and $50,000, respectively. The Clean Air Agency is spending $25,000.

Bill Sheets: 425-339-3439; bsheets@heraldnet.com.

Discuss the results

A meeting to discuss the results of the Puget Sound Clean Air Agency study of odors in the Snohomish River Delta is scheduled for 7 p.m. Thursday at the Snohomish County PUD auditorium, 2320 California St., Everett.

The study is available online at tinyurl.com/mmd3yon.