The smell of gasoline at the top of Spring Hill Road in Harrison led to a potentially dangerous discovery by state officials.

Ten underground fuel storage tanks, nearly 100 years old, were revealed by ground-penetrating radar beneath the parking lot at the Stop’n Shop convenience store at Spring Hill and Freeport roads.

The tanks date to 1929 and are structurally compromised, officials from the state Department of Environmental Protection said.

At least some of them appear to be leaking after samples collected from the area revealed groundwater contaminated with gasoline.

Laina Aquiline, DEP regional communications manager, said remediation is underway “to address an imminent and substantial threat to public health and welfare.”

Contaminated materials are being sampled to ensure proper handling, transportation and disposal, Aquiline said. The project is expected to last at least a month.

Excavation is being conducted by DEP and Warrendale-based contractor EnviroTrac. Crews have unearthed 10 steel tanks so far, she said.

Because the tanks are so old, they were never registered with the state. The Storage Tank and Spill Prevention Act, which requires most underground fuel storage tanks to be registered and tested for safety and environmental protection, didn’t come along until 1989.

The number of unregistered tanks in Pennsylvania is not known, according to DEP online data, but there are more than 21,000 registered underground tanks in the state.

Aquiline said the work is being done under the state’s Hazardous Sites Cleanup Act fund, which provides resources for DEP to carry out investigation, cleanup and monitoring related to the release of hazardous substances or nonhazardous contaminants that represent a danger to public health, safety or the environment.

“In many cases, it allows DEP to address legacy contamination resulting from past industrial activity where there is no longer a viable responsible party to fund cleanup activities,” she said.

Information about the property goes back only to 2002 on the Allegheny County real estate website. It housed a Sinclair gas station in the 1960s and, since at least the 1980s, has been home to multiple convenience stores, including 7-Eleven and Natrona Heights Food Mart.