Barium as a serious water contaminant, although its occurence is not widespread.

Barium is a soft, silvery naturally occurring metal. It is found primarily in the midwestern section of the United States.

Traces of natural Barium can be found in surface or ground waters. It can also come from mining and oil and gas drilling operations, the wastes of coal-fired power plants and in automotive paints and jet fuels.

Barium is an EPA regulated water contaminant. The current MCL is 2.0 mg/l.

Barium is highly toxic when ingested.

Water treatment:
Barium is readily removed by cation exchange, sodium form (water softener), and by reverse osmosis. It can also be removed by electrodialysis.

Sources: Wikipedia and Enting Engineering Handbook.