A new study shows that approximately 20 percent of untreated water from across the country contain at least one contaminant, according to results from the U.S. Geological Survey (USGS).

A team of researchers collected samples from a total of 932 public wells, and compared their observations to treated water samples.

The results of the comparison showed that approximately 75 percent of the untreated water samples were concentrated with radon, arsenic and other natural contaminants, wchich can cause health problems in humans. The researchers also found levels of man-made chemicals, such as herbicides, insecticides, solvents, gasoline and nitrate in untreated water.

“Detections of contaminants do not necessarily indicate a concern for human health because USGS analytical methods can detect many contaminants at [different concentration levels],” said Patricia Toccalino, lead scientist of the study. She added that “assessing contaminants in these small amounts helps to track emerging issues in our water resources and to identify contaminants that may warrant inclusion in future monitoring.”

Unsafe water sources that have been contaminated with various chemicals cause more than 3 percent of all deaths in the world, according to the United Nations Environmental Program.