If the ground filters water, is groundwater always clean?

Water drawn from a well was once precipitation that fell onto Earth's surface. It seeped into the ground and, over time, occupied the porous space in some subsurface material. Big particles that are in surface streams, such as leaf chunks, will not be seen in groundwater. So, yes, big particles are filtered out by the ground, along with some minerals and chemicals that are too small to see.

But groundwater can contain many different elements that are NOT filtered out by the ground and that we don't want in our drinking water. Some are naturally-occurring (like hydrogen sulfide) and some are human-made substances. Groundwater can contain petroleum, organic compounds, or other chemicals introduced by human activities.

Contaminated groundwater can occur if the well is located near land that is used for farming where certain kinds of chemicals are applied to crops, or near a gas station that has a leaking storage tank. Leakage from septic tanks and/or waste-disposal sites can also contaminate groundwater. A septic tank can introduce bacteria to the water. Pesticides and fertilizers that seep into farmed soil can eventually end up in water drawn from a well. Or, a well might have been placed in land that was once used for something like a garbage or chemical dump site. In any case, it is wise to have your well water tested for contaminants.

Learn more: Quality of Ground Water

Related Content

Filter Total Items: 15
man under pop-up tent leaning over pumping equipment on table

Collecting Groundwater Samples - Long Island, New York in July 2017

Collecting Groundwater Samples from a Well on Long Island: The image above shows USGS scientists pumping a deep Lloyd aquifer observation well in Long Beach, NY.  To ensure that water-quality samples represent the formation water, the well is pumped for an extended period of time or until at least three casing volumes are removed and measured field

Diagram of the water cycle, aimed at schools.
February 20, 2017

The Water Cycle for Schools and Kids (PDF)

The Water Cycle for Schools

The U.S. Geological Survey (USGS) and the Food and Agriculture Organization of the United Nations (FAO) have teamed up to create a water-cycle diagram for schools.

We also offer three age-related versions of an interactive versions

Image: Acidifying a Drinking Water Quality Sample for MTBE Analysis

Acidifying a Drinking Water Quality Sample for MTBE Analysis

Sample collected from Madison Springs Hut in the White Mountains (~ 5,000 ft above sea level).