Most Montanans get their drinking water from groundwater–a source that mostly avoids high levels of manganese, according to a recently completed investigation by researchers at the Montana Bureau of Mines and Geology.

Manganese is a naturally occurring mineral and is present in many common foods. The mineral is necessary for proper nutrition, and too little of it may be harmful. But the Department of Environmental Quality also says that too much manganese ingested through drinking water may be harmful.

Distribution of Mn concentration in basin fill (west of yellow divide line) and alluvial fill aquifers. Blue dots indicate safe to drink; pink is poentially harmful to those under age 6; red exceeds recommended guidelines. (Montana Bureau of Mines and Geology)

Researchers with the MBMG’s Ground Water Assessment Program found manganese concentrations were low and safe for 85 percent of the investigation’s well samples. Only 7 percent of samples exceeded the recommended health standard limit for adults and children over 6 years.

The MBMG study examined 3,858 samples from across Montana. Lower manganese concentrations were found in samples from western Montana aquifers; higher levels were found in the east. Certain aquifers in areas near Flathead Lake, north and west of Great Falls, and alongside the Missouri River produced more samples with elevated levels.

Manganese is considered a secondary water contaminant under the Safe Drinking Water Act, along with 14 other contaminants, such as iron, pH, and sulfate. Water that meets secondary standards is safe to drink, but may have undesirable taste or odor or may damage water equipment.

Other research has found manganese may be harmful at certain levels–especially to children. Children and adults drinking water with high manganese levels over a long period of time may have problems with memory, attention, and motor skills.

The study comes at a time when Montana’s schools continue testing water fixtures for lead contamination. (Lead is a primary water contaminant, which public water supply systems must keep under a maximum contaminant level.) At last count, 380 schools have tested their water and are remediating contaminated fixtures.