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.

However, groundwater can contain many different elements that are not filtered out by the ground. Contamination can come from human activities above the ground, the geology below the ground, or from the water system itself.

Elements in soil and bedrock can dissolve and contaminate groundwater over time. Some contaminants are widespread and come from geologic materials such as arsenic and manganese. Long-term exposure to detectable arsenic in drinking water can cause cancer and other health problems. Consuming elevated levels of manganese over time can cause negative neurological health effects, especially in young children and infants.

Some contaminants are more localized and come from human-made substances such as nitrate and other chemicals. Nitrate contamination comes from fertilizer, septic systems, or animal waste. Pesticides and fertilizers that seep into farmed soil can eventually end up in water drawn from a well. Water seeping downward can carry nitrate and other chemicals into aquifers, contaminating the groundwater. Nitrate in drinking water can be especially harmful to bottle-fed infants.

Private well users should also be aware of lead, which can leach into water from pipes and other components of the water system. Even low levels of lead exposure can be harmful to children’s development. For adults, lead exposure can harm the heart, kidneys, and reproductive systems.

In any case, it is wise to have your well water tested for common contaminants such as arsenic, manganese, nitrate, and lead.