The problem with relying on groundwater is that it comes from the ground.

The image of drawing water for drinking, bathing and other household uses from a “pure underground aquifer filtered by Mother Nature” is an appealing one. But the problem is that Mother Nature doesn’t always do a good-enough job.

That’s why Maine homeowners who have private wells are wise to have their water tested for contaminants on a regular basis.

Among the things that can show up on such tests are potentially hazardous levels of radon, a radioactive gas, and heavy metals such as magnesium and even uranium.

And arsenic.

While public water supplies are regularly examined and filtered for such substances, about half the state’s population gets its water from private wells, and homeowners are essentially on their own. State officials encourage every Mainer with a private well to have the water professionally tested.

Arsenic is in the news because a report by the U.S. Geological Survey says that tests of 11,000 wells in 530 communities from 2005 to 2009 showed contamination above federal safety standards in a number of communities across the state.

The state says its figures show that about 10 percent of the wells tested in recent years had arsenic levels above the federal maximum of 10 micrograms per liter. Since arsenic can cause serious health problems, including cancer, heart disease and diabetes, that’s something to be seriously concerned about.

There are remediation methods, some expensive and some not, but before they can be applied, well owners have to know the facts about what’s in their water.