A major oil pipeline owned by Sunoco Logistics Partners LP leaked thousands of gallons of crude oil into a nature preserve in southwest Ohio late on Monday.

Between 7,000 and 10,000 gallons (26,000-38,000 liters) of sweet crude leaked into the Oak Glen Nature Preserve about a quarter of a mile from the Great Miami River, according to early estimates from the Ohio Environmental Protection Agency.

The leak, which occurred on a line operated by Mid-Valley Pipeline Co, a division of Sunoco, was discovered at 8:20 p.m. EDT on Monday (0020 GMT Tuesday). The company shut the line, which helped reduce the pressure of the leaking oil, an EPA spokeswoman said, but it was unclear if oil was still spewing from the pipe.

Some oil reached a wetland a mile away and on Tuesday, clean-up crews were preparing to vacuum the wetland, located 20 miles north of Cincinnati.

The oil did not appear to have reached the Great Miami River, though tests were still being completed, the EPA said.

“The extent of impact to the resource is currently unknown,” said a statement from the Great Parks of Hamilton County, which oversees the Oak Glen preserve. “The EPA is assessing the situation to determine appropriate action.”

Sunoco was not immediately available for comment.

The pipeline is part of Sunoco’s mid-west system that runs about 1,000 miles from Longview, Texas to Samaria, Michigan, providing crude oil to a number of refineries, primarily in the U.S. Midwest.

Flows along the Mid-Valley pipeline decreased overnight to around 163,000 barrels per day from an estimated 229,000 bpd, according to data provider Genscape. The pipeline has a capacity of 280,000 bpd, Genscape said.

The loss of a key supply route from the south hit physical crude oil prices in Texas, already pressured by ample supplies. West Texas Intermediate crude for delivery at Midland, Texas, fell $2.50 a barrel on Tuesday to $15 below U.S. oil futures. West Texas Sour fell $4.