USGS NAWQA Transport of Anthropogenic and Natural Contaminants to Supply Wells Study

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Why Some Public-Supply Wells Are More Vulnerable To Contamination Than Others
More than 100 million people in the United States - about 35 percent of the population - receive their drinking water from public groundwater systems, which can be vulnerable to naturally occurring contaminants such as radon, uranium and arsenic, as well as commonly used manmade compounds, including fertilizers, septic-tank leachate, solvents and gasoline hydrocarbons. Public-supply well vulnerability to contamination starts with groundwater vulnerability to contamination. Even wells within a single aquifer, however, may not be equally vulnerable to contaminants in the aquifer because individual wells produce unique mixtures of the groundwater.

This USGS video podcast uses examples from four locations with very different aquifer-well combinations to illustrate why some public-supply wells are more vulnerable to contaminants in aquifers than others.

For more information on how the science behind public-supply well vulnerability to contamination can be used to protect drinking water sources, please refer to the accompanying USGS fact sheets below.


  • unconsolidated sediment (sand, gravel, silt, & clay)
  • oxic conditions (defined as > 0.5 mg/L dissolved oxygen)
  • long well screen (hundreds of feet)
  • range of young to old water in well (10 to 1,000s of years)

  • unconsolidated sediment (sand)
  • mostly oxic conditions
  • short well screen (tens of feet)
  • young water in well (mostly < 10 years)

  • layered unconsolidated sediment (sand)
  • anoxic conditions in confined aquifer(defined < 0.5 mg/L dissolved oxygen)
  • well screen beneath clay confining unit
  • mix of young and old water in well (10s AND 1,000s of years)

  • carbonate rocks
  • oxic and anoxic conditions
  • open hole well
  • mix of very young and generally young water in well (< 1 year and 10s of years)

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