NASHUA – It’s an instantly recognizable smell: unattended garbage.

Heaps of it are piled high and circled by ravenous seagulls at the Four Hills Landfill, referred to by most local residents as “the dump,” a spot that has handled Nashua’s trash for more than 40 years.

Every day, the landfill is visited by an average of 800-1,000 cars that unload a total of 300 tons of material. That’s a lot of smelly trash.

But the odor, pungent with rotting food and garbage under the hot summer sun, is surprisingly minimal because of technology and the continual efforts of staff at the Solid Waste Department.

“We are designed to be as environmentally benign as possible,” said Kerry Converse, an environmental engineer who has worked at the dump for 10 years.

His responsibility is to monitor and report the environmental conditions of the dump in compliance with government regulations on the state and federal levels. Among waste management, air perimeters and storm water, one of these conditions includes odor emissions.

Converse’s arsenal of smell-combating devices includes machines to monitor methane, which is released by decaying material and is a major component of a dump’s stinkiness. Converse scans the surface of the ground with a wand to gather the smell and register levels of methane.

Marc Morgan, superintendent of solid waste for the city, said the site has had few complaints in the past few years. Complaints are taken as evidence of a breach in their smell-proofing security and trigger concern, he said.

“If we do detect odors,” Morgan said, “then we investigate to try to figure out what the source is. But I don’t think it’s overwhelming.”

The staff employs more than 40 wells that use hundreds of yards of piping to vacuum methane to a power plant, where it’s burned to generate electricity.

The conditions of the landfill change constantly, making the fight against smells harder.

The weather and wind direction, for example, will affect odors. Incoming storms yield a dramatic drop in atmospheric pressure, which pulls odors from underground into the air.

“There is never a typical day here,” Morgan said. “We always have to plan a different strategy.”

To minimize the smell, the staff maintains a small operating face, which is a designated area to which workers refer as the active site for the day where trash is dumped and then covered under layers of double-lined plastic and clay composite material.

The landfill uses 300 acres, with four landfill mounds – which look like manmade hills – where trash is layered.

Workers have several trucks and tractors at their disposal, including excavators, bulldozers and trash compactors that transport, level and bury layers of discarded junk in a specific fill sequence.

The Nashua landfill is one of four municipal landfills still in operation in the state, Morgan said.

There is a lot of pressure to use the space to the maximum because it would be so expensive and difficult to expand it or create a second site.

This is why they use the meticulous process of flattening and jam-packing trash nearly 80 feet deep into the landfill mounds between layers of plastic liner and composite material. It’s also why the city is pushing recycling efforts so strongly – recycled material doesn’t end up here.

“The landfill is not a static thing,” Converse said. “It’s constantly changing.”

Alexandra Churchill can be reached at 594-6411 or achurchill@nashuatelegraph.com.