WOONSOCKET, R.I. — Hanora Spinning is one of the few textile mills still operating on the Blackstone River. Since 1911, the factory has literally been spinning yarns made of wool and synthetic fibers.

The production of yarn itself doesn’t pollute the river. But an underground oil tank has left a lasting impact.

In 1990, a 20,000-gallon tank holding No. 6 fuel oil leaked into the soil, the water table, a public storm drain and a narrow, open canal that helps control seasonal flooding.

Residue from the leak reached the Blackstone River, about a dozen yards away. Records detailing the extent of the 1990 spill and cleanup are absent from the Rhode Island Department of Environmental Management (DEM). Nevertheless, according to engineering reports filed with DEM, the leaking steel tank was replaced with a fiberglass-coated tank.

According to those reports, a second and much larger leak occurred in 2000, when the new tank’s underground fuel line punctured. The hole was less than a half-inch wide, but led to the release of about 2,450 gallons of oil into the storm drain, groundwater and a flood pumping station. The 200-foot-long canal, called a swale, was coated in an 8-inch layer of oil.

The cleanup required the removal of 380 tons of contaminated soil, 547,000 gallons of petroleum-contaminated water, and 28,000 pounds of oil-soaked waste.

Thirteen holes, called monitoring wells, were dug at the site of the leak for ongoing inspections, which are performed weekly. Containment booms, with large absorbent pads, float at the edge of the river to collect seepage of oil from the leak. Currently, oil residue drains into the river during severe storms or whenever the water table rises and falls significantly.

The cleanup is ongoing. Contaminated groundwater is periodically vacuumed from the containment wells. In November 2000, 12,413 gallons of polluted water was vacuumed from one of the wells. Since 2007, more than 11,000 gallons of contaminated water has been vacuumed from the wells.

Hanora Spinning didn’t respond to an ecoRI News inquiry, but the leak highlights the problem businesses and the environment face with underground storage tanks (USTs). Such tanks have been around for hundreds of years, according to DEM, and Rhode Island has about 2,000 them. About 85 percent of the existing steel tanks leak, with many along waterways such as the Blackstone River.

In the case of Hanora Spinning, the 1990 leak was caused by a deteriorating single-layer steel wall, according to reports.

Some property owners don’t even know they have buried storage tanks, according to DEM. Yet, underground storage tanks are still permitted, although most fuel tanks are now built above ground for better monitoring.

According the Environmental Protection Agency (EPA), some 581,000 USTs are in use nationwide for the storage of petroleum or hazardous substances.

Because they are so common, DEM has staff dedicated to managing them. It wasn’t until 1986 that the EPA established regulations for managing USTs.