‘Cat urine and burning plastic.’ The foul odor in Kuna is back — and so is the mystery.

On certain nights for the last few weeks, Russ and Tara Evans have woken up to a smell they describe as “cat urine and burning plastic.”

“It’s really heavy,” Russ Evans said. “It’s almost like you can taste it.”

The smell rolls in around midnight, when the Evanses are sleeping with their windows open to cool down the house. The Idaho Statemsan reports petroleum odor might just last for 10 minutes, but it sometimes lingers for over an hour.

The smell has become familiar to the family. It first appeared last year in July. Kuna and Department of Environmental Quality officials later traced it back to a wastewater and soil treatment pond owned by the L&R Group, south of Boise on Thompson Road. DEQ called the smell “out of the ordinary.”

Businesses bring their industrial and septic non-hazardous waste to the L&R plant, which treats it in large ponds until it can be disposed of in a landfill or recycled and made into bio-fuels.

Ada County Planning and Zoning Administrator Mark Perfect said the county granted a conditional use permit to L&R in 2004 to remediate contaminated soil and water. In 2016 it granted the company another permit that allowed for the treatment of septic waste. Waste-treatment plants like these are only allowed in rural districts, near the outer edge of the county, he added.

The company stopped accepting septic material in 2017 but was still treating a pond that contained the waste as of last year.

Last summer, L&R paused treatments on its septic pond as it completed a regular test for DEQ to show it wasn’t leaking a significant amount of waste into the groundwater.

During that time, the summer heat spawned bacteria blooms within the septic pond, releasing an odor DEQ said was the source of residents’ complaints.

L&R contested DEQ’s conclusion, arguing it isn’t the only source of odor in the area. Still, the company took steps to clean its ponds.

Regan Hardy, vice president at L&R, told the Statesman in a phone interview that his company still doesn’t believe it is the source of the smell, and that tests from an independent engineer it hired last year did not prove the L&R pond was the source of the odor.

Still, the company has spent more than $500,000 to chemically treat the waste and drain the pond, which contains about 12 million gallons of wastewater.

“We took action because we wanted to be good stewards of the community,” Hardy said.

Just a few inches of solid waste remain in the affected pond.

“The pond that supposedly caused the smell is gone,” Hardy said. “If the smell continues, then we know it’s not us.”

Dave Luft, air quality manager for the Boise Regional office of DEQ, told the Statesman that his office has received some odor complaints in Kuna in the last few weeks when the wind comes from the east overnight.

“As things heat up in the day, the winds come out of the west and northwest,” he said. “As it cools down overnight around midnight, you’ll see a change where the winds come from the east.” Odors also tend to linger more at night in the cool air.

Luft’s office has not conducted any tests yet this year to determine the source of the smell.

Kevin Ryan, an engineer at DEQ who has worked with L&R, said there are other properties in the area that can sometimes smell.

Those properties include wastewater treatment centers other than L&R; the Darling Ingredients rendering plant that recycles livestock carcasses into industrial fats, oils and fertilizers; the state prison, which has its own septic treatment system; and several livestock farms.

Additionally, L&R manages six other wastewater ponds at its treatment center beyond the one it drained.

The county doesn’t require L&R to keep its odors below a certain level, but it can revoke the permit that allows the company to operate there if there are a significant number of complaints.

Hardy said that L&R has tried to comply with Ada County throughout the process, but he feels like L&R has become a scapegoat for other companies contributing to the smell.

“We’ve spent a lot of money cleaning up to address the concerns of the county, DEQ and the public,” he said. “While any wastewater pond does give off smell, we’re not the only ones. We just want the proper people to be held accountable.”