This link is to the Maine State Drinking Water Program private wells page. It has additional information on well testing, arsenic testing and treatment
A general overview on arsenic in drinking water is available at the World Health Organization web site.
This USGS website discusses the distribution of arsenic in the United States
This excellent website from a Harvard University researcher has a lot of information about arsenic, affected areas and other resources. There is also a comprehensive list of links.
The EPA document here provides detailed information about community water treatment technologies for arsenic and their costs.