One complicating factor in the dispute is that water quality of Stream C was never tested before mining began, so no baseline exists.
'The fact of the matter is that it flows through an ore body,' the DNR's Fauble said. 'It might just have naturally higher levels of copper in it.'
That region of Wisconsin is rich in copper and other minerals that I wrote about back here concerning the Sudbury Basin. Copper Harbor, MI is not that far away as the glacier flowed. When the first European explorers came to this region, they reported finding nuggets and even small boulders of native copper. The fact that there's a mine there supports the natural occurrence.
Plants and trees sequester metals like copper and zinc; they also provide a chronological record via rings or even carbon-14. The answer to the question of whether the abnormally higher levels of copper came sooner or later may be found in the arboreal record.
Organic sequestration of copper link
Among the materials in the glacial drift of the Lake Superior Lowland are masses of native copper carried westward by the ice itself and in icebergs. One such mass found near Ontonagon, Michigan, weighs about 3000 pounds, and one from the Bayfield peninsula near La Pointe, Wisconsin, 800 pounds. One of the Jesuit fathers observed in 1669 that there were such bowlder of copper in the Apostle Islands. He relates that the squaws often found copper fragments of 20 to 30 pounds weight in digging holes in the sand to plant their corn, and suggests the transportation of this copper by floating ice, though he did not imply that this was glacial ice.Lawrence Martin, The Physical Geography of Wisconsin, 3rd Ed. ; University Of Wisconsin Press: Madison (1965), p. 436.