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Environmental groups are demanding that the U.S. Environmental Protection Agency investigate a noxious smell emanating from the Dominguez Channel in Carson.

LOS ANGELES – A rotten egg odor continues to hang in the air along the Dominguez Channel in Carson, but measurements from air-quality monitors show the levels of hydrogen sulfide gas have diminished since the noxious stench suddenly emerged last month.

Officials with the South Coast Air Quality Management District said they are continuing to investigate potential sources of the stink along the channel.

“Since the beginning, we have been evaluating petroleum refineries, wastewater treatment facilities, nearby landfills and other types of industrial facilities,” said Terrence Mann, AQMD’s deputy executive officer for compliance and enforcement. He said they are also investigating whether the stinky gas erupted naturally from the channel itself, or whether it might have occurred in combination with other factors.

“We are continuing to work every day to identify and potentially rule out potential sources,” Mann said.

The air agency has been working with officials from Los Angeles County and other agencies, including the federal Environmental Protection Agency.

Some residents have grown concerned, however, insisting that the investigation has failed to get to the bottom of the environmental mystery.

Members of the Wilmington-based Coalition For A Safe Environment held a news conference in Carson on Wednesday calling for federal officials to step up their involvement to pinpoint the origin of the hydrogen sulfide.

“We are all here because there is a crisis going on in the city of Carson,” said Jesse Marquez, the group’s executive director. Marquez said his organization was compelled to act because citizens in Carson and surrounding South Bay cities have suffered adverse health effects.

“Our concern now is the fact that it’s been four to five weeks,” Marquez said.

He and other members of the group spoke between two homes by the Dominguez Channel, saying it’s time the EPA takes over the investigation, which is currently headed by county agencies.

They held the news conference exactly one month after residents complained to the South Coast Air Quality Management District of noxious smells that have led to headaches, nausea and other symptoms.

Marquez said he’s not convinced by the theory proposed by county officials that decaying organic material in the channel may be to blame.

“The problem with that is that it takes tons and tons of decaying organic matter and it must also be in a confined space,” Marquez said.

Marquez said three key things need to be investigated: the origin of the hydrogen sulfide in the Dominguez Channel, the possible effects of oil refineries and whether a September earthquake played a role in the smell.

Some steps in the investigation, laid out by Marquez, should include reviewing nearby refineries, checking for corrosion and leaks of toxic chemicals and inspecting abandoned wells.

In the last few weeks, county officials have said the colorless gas would not likely cause long-term or severe health effects, but acknowledged short-term issues like watery eyes and nausea.

Jill Johnston, University of Southern California assistant professor of population and public health sciences, said more health issues are possible.

“Hydrogen sulfide is known to have adverse effects as well as chronic effects” with even short-term exposure, she said.

Johnston said that along with eye, ear and throat irritation, other reactions can include wheezing, asthmatic and allergic reactions and elevated blood pressure.

Steven Leonido-John, director of the EPA field office in Los Angeles, said his agency has been involved in investigating the smell since about Oct. 18, following requests by Rep. Nanette Barragán (D-San Pedro) and the Carson city government. He said he has participated in daily calls with officials from other agencies and has provided input on the investigation and how to address the odor.

“It’s very, very typical in a situation like this that the local authority, be it the county or a city or the state, would take the lead,” Leonido-John said. “And that EPA would be there in an advisory role, to provide expertise, to bring in technical experts as necessary.”

“On a daily basis, we are providing input to the interagency group that is looking at this,” he added.

The levels of hydrogen sulfide measured by the flood-control channel last month were unprecedented, said Jason Low, head of AQMD’s air monitoring division.

“Once we found out where the largest area of hydrogen sulfide emissions was for this incident, we had placed a hydrogen sulfide monitor nearby that location.” On Oct. 14, Low said that location was at 213th and Chico streets in Carson.

The highest hourly average measured was about 7,000 parts per billion overnight on Oct. 16 and 17, Low said.

“For context, that’s about 230 times the nuisance standard established for California,” Low said.

It’s also far above the highest levels that have been measured in recent years at the Salton Sea – about 250 parts per billion. Hydrogen sulfide sometimes fills the air near the shrinking lake as decaying algae and other matter on the lake bottom are disturbed by winds.

One such event in 2012 sent the odor of rotten eggs wafting across much of Southern California.

The highest level measured during that event, which was dubbed the “big stink,” was about 150 parts per billion, Low said. The levels in Carson were much higher, albeit in a smaller area.

The levels tend to rise at night and decrease in the day, Low said. Since mid-October, the concentrations have been declining. On Wednesday evening, the hourly level by the Dominguez Channel had dropped to below 60 parts per billion.

The AQMD has other air-quality monitors in surrounding communities, and nearby oil refineries also have monitors at their fences.

”What we’ve seen is that the levels, as you go away from that central area in Carson at the Dominguez Channel, really decrease a lot,” Low said. “And that’s both within the nearby community monitors as well as some of the fence-line monitors that are at the refineries.”

The agency’s officials have also measured the air for volatile organic compounds and other toxic pollutants associated with oil-related air pollution. They said their findings thus far don’t indicate elevated levels of those pollutants.

“We haven’t 100% ruled out any of the nearby sources,” Mann said. “We are continuing to look under every rock and consider every possible potential other source, or if not a source, contributing factor to the incident.”

County public works officials have said the hydrogen sulfide gas likely was created by organic material clogging the channel. They have been spraying the channel with a biodegradable neutralizer.

The decreasing levels of hydrogen sulfide indicate that these efforts are having a positive impact, said Leonido-John of the EPA. Local officials have also been receiving fewer odor complaints.

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