Manthey said there are also other potential sources of molybdenum and added that it can occur naturally in rock.

But those natural levels are much lower than the levels found in the wells examined by Clean Wisconsin. The report said a survey of 2,700 wells in northern Wisconsin by the Department of Health Services found that 98 percent of samples had concentrations below 20 parts per billion.

The 1,000 wells examined in the Clean Wisconsin study, however, had an average of nearly 50 parts per billion of molybdenum, higher than the enforcement standard of 40 ppb set for drinking water by the DNR. That is the limit beyond which the agency can require action to correct the problem.

One in five of the wells exceeded the health standard of 90 ppb set by the Department of Health Services. That is the point where DHS recommends “that you not use your water for drinking or in foods where water is a main ingredient and that you find a different source of safe water to drink.”

The state health standard is more than double the EPA advisory level and, according to Clean Wisconsin, allows more molybdenum in drinking than the federal agency recommends children be exposed to for even a single day, 80 ppb.