Those two-headed guys can’t be too healthy.
The culprit is apparently elevated levels of selenium, a toxic nonmetal. The drinking water standard for selenium is 0.05 mg/L, or 50 ppb, and human health effects include hair and fingernail loss, numbness in fingers or toes, and circulatory problems. But as is the case with many contaminants, its effects on aquatic organisms can be much worse. Eggs laid in contaminated water are very susceptible to mutations. In the 1980s, elevated selenium in agricultural runoff in California produced fatal deformities in waterfowl. The U.S. Environmental Protection Agency has set a 5 ppb chronic standard for selenium in surface waters. There is some controversy about this standard, with some agencies recommending a lower standard.
Of the almost 20,000 samples in our groundwater quality database, fewer than ten had selenium levels about the drinking water standard, and almost all of those can probably be explained as transcription or analytical errors. There may be areas in Illinois where selenium contamination of shallow groundwater by human activities (the main sources being petroleum and metal refineries and discharge from mines), I haven’t seen any. Even in the highly industrialized and heavily polluted area around Lake Calumet in south Chicago, with elevated levels of many toxic metals, no selenium contamination was found. Plenty of other pollutants, but not selenium, as you can see from the map below:
Map from Kelly, W.R., and G.S. Roadcap, 1994. Shallow ground-water chemistry in the Lake Calumet area, Chicago, Illinois. In Proceedings of the National Symposium on Water Quality, American Water Resources Association, pp. 253-262.