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If the ground filters water, is groundwater always clean?

Drinking from an  artesian well in Buenos Aires, Argentina. Sign says to drink at your own risk.

Drinking from an artesian well in Buenos Aires, Argentina. Sign says to drink at your own risk.
Credit: TravelBlog - LizzZ

Water being drawn from a well was once precipitation that fell onto the Earth's surface. It seeped into the ground and, over time, occupied the porous cracks, spaces, and pores of the underground rock. People drill wells into rock formations that are permeable and porous enough to hold significant amounts of water (an aquifer) and pump water to the surface for use.

Naturally, big particles that can be found in streams, such as leaf chunks, bugs, and bubble-gum wrappers, will not be seen in groundwater. So, yes, big particles are filtered out. But groundwater can contain other items that you can't see. Some are naturally occurring and some are human-made substances. Groundwater can contain hydrogen sulfide (makes the water smell like rotten eggs) or other naturally occurring chemicals. Groundwater also may 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 also can contaminate groundwater. A septic tank can introduce bacteria to the water, and 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 contaminates.

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