Water is generally classified into two groups: surface water and groundwater.
In general: Groundwater is located underground in large aquifers and must be pumped out of the ground after drilling a deep well. Surface water is found in lakes, rivers and streams and is drawn into the public water supply by an intake.
Surface water is just what the name implies; it is water found in a river, lake or other surface cavity. This water is usually not very high in mineral content, and is often called “soft water” even though it is probably not. Surface water is exposed to many different contaminants, such as animal wastes, pesticides, insecticides, industrial wastes, algae and many other organic materials. Even surface water found in what seems like pristine mountain streams can be contaminated by wild animal waste, dead animals upstream or other decay.
Groundwater is water contained in or by a subsurface layer of soil or rock. There are many sources recharging the supply of groundwater, including rain that soaks into the ground, rivers that disappear underground and melting snow. Because of the many sources of recharge, groundwater may contain any or all of the contaminants found in surface water as well as the dissolved minerals it picks up underground.
However, groundwater commonly contains less contamination than surface water because the rock tends to act as a filter to remove some contaminants. Imagine that rain falls and the rainwater soaks into the ground. The plants use as many nutrients as they can and then the water continues to filter down through clay, sand and porous rock filtering the water much like a charcoal filter might clean your drinking water at home. Eventually this groundwater finds a home in an aquifer or trapped between levels of rock creating a water table. This is the water you most often drink from your well. Due to the minerals picked up while filtering through the rocks, groundwater is typically considered to be “hard” water.