https://water.usgs.gov/edu/gw-well-contamination.html https://water.usgs.gov/edu/gw-well-contamination.html https://water.usgs.gov/edu/gw-well-contamination.html 43 million people in the US https://water.usgs.gov/edu/gw-well-contamination.html millions Recent study millions millions https://water.usgs.gov/edu/gw-well-contamination.html https://water.usgs.gov/edu/gw-well-contamination.html