A few years ago, Fred Pinkney stood in Hershey Run, a tributary of White Clay Creek, collecting mummichogs for a fish sampling project.

Like the fish, the water, which eventually flows to the Christina River and on to the Delaware, looks OK.

Until you look closer.

“We were standing in very highly oily sediments,” Pinkney, a U.S. Fish & Wildlife Service biologist, recalled. “As you stood there, you could see the sheen at the surface.”

The fish looked healthy enough. But back at the lab, a high percentage had liver tumors, Pinkney said.

The likely source of contamination, a closed-down wood processing company, is now a federal Superfund site.

Elsewhere in the Christina River Basin, a 565-square-mile area that includes the Brandywine, Red Clay, White Clay and Christina River watersheds, fish are loaded with PCBs, and in some areas, chemicals like dioxin, chlorinated pesticides and other pollutants. There are fish consumption advisories throughout the Delaware reaches of the basin.