Public water supplies fed by wells are widely contaminated, often at pollutant concentrations that potentially pose a human health risk, according to a US Geological Survey report released May 21, 2010. About 105 million US residents (more than one third of the population) drink water from such systems, of which there are about 140,000.
- Quality of Water from Public-Supply Wells in the United States  (includes a map of well locations, providing some context for water systems of interest to your audience).
The agency's findings, which were presented to Congress the same day the study was released, are based on sampling done on 932 wells in 41 states. The wells were chosen because they tend to be representative for the country
About 20% of the wells tested had at least one natural or synthetic contaminant at a concentration above a government standard. More than 80% of the wells had multiple contaminants at concentrations near or above a standard; one of the glaring weaknesses of current science is that little is known about interactions of multiple contaminants, and standards are set almost entirely based on limited knowledge of health effects for just one contaminant at a time.
Water from the majority of wells was tested prior to treatment. However, testing in more than a quarter of the wells found that many synthetic contaminants, such as pesticides, solvents, and nitrates, were little diminished after treatment. More than 80% of the contaminants identified are not regulated. Other known contaminants, such as pharmaceuticals and hormones, were not tested for.