The French Gulch site, which includes the former Wellington-Oro mine, is located about two miles upstream or east of the confluence of French Creek with the Blue River near the town of Breckenridge, Colorado. Extensive underground mining occurred in the valley from the late 1850s to the 1960s. Lode mining recovered lead-zinc-silver sulfide and gold ores from an extensive network of tunnels and adits originating on the steep valley sides. Large floating dredge boats were used to placer-mine the valley floor for gold. The placer dredging disrupted French Creek and its associated alluvial valley material. This resulted in large dredge piles covering the French Gulch valley floor and extending upstream approximately one mile east of the former Wellington-Oro mine.

Ore veins targeted by the underground mines are commonly associated with faults and fractures that serve as a conduit for groundwater flow. In 1934 the mine workings were reported to consist of more than 12 miles of tunnels. In the 1940s, the present owner, B&B Mines, acquired the Wellington-Oro properties.

Sporadic mining and milling operations occurred at the mine from the late 1940s to the early 1970s. Since this area contains a large quantity of sulfide-bearing minerals, these conditions promote the formation of acid mine drainage. This is caused by the oxidation of sulfur in the presence of water, forming sulfuric acid in the mine pool water which makes the water acidic. Metals such as cadmium and zinc are soluble in acidic water. As a result, acid mine water flowing through the mine workings becomes highly contaminated with dissolved metals, exits the mine in the form of seeps, and enters French Creek.

Elevated zinc concentrations in the seep water are primarily responsible for the absence of fish populations in the downstream portion of French Creek and a segment of the Blue River. EPA investigations in the late 1980s determined that the Wellington-Oro Mine pool was the major contributor of zinc and cadmium load from French Creek into the Blue River.

In 1998 mine wastes including roaster fines, tailings, and waste rock were removed from the mine site to an area with reduced potential for human contact. The materials were capped with impermeable clay and clean gravel. Drainage ditches were installed to reduce infiltration of rain and snow melt into the mining wastes. This material provides little or no contribution to the contamination of French Creek and the Blue River.