Analysis of data on arsenic concentration in groundwater from several sets of random and pseudo-random bedrock wells indicates that 1- to 3-percent of wells in Maine have arsenic concentrations above the present maximum contaminant level (MCL) of 0.050 mg/l and 12- to 13-percent above the proposed standard of 0.010 mg/l. These are comparable to levels found in New Hampshire (Peters and others, 1999) and New Brunswick (Brinsmead, personal communication, 2000), but significantly lower than values reported by Marvinney and others (1994) based on a self-selected sample of private wells in the Buxton-Hollis, Maine, area.

The distribution of wells with elevated arsenic concentrations is not random, however. Statewide, the random and pseudo-random wells show a much higher occurrence of elevated concentrations in zones of biotite grade or higher metamorphism or adjacent to igneous intrusions.

The Buxton-Hollis data support clustering of elevated arsenic on a scale of 10s of kilometers. The 1994 census data indicates that there were 3,793 households in Buxton and Hollis, most single family and all on private water supplies. Of this, we analyzed 1,111 self-selected wells and found 153 above the MCL. Applying a rate of 3-percent to the total possible wells in the study, only 114 wells in the two towns should have been above the MCL. This is confirmed by the random and pseudo-random wells, where several zones of elevated arsenic concentrations can be observed along an axis from northern York County (Buxton and Hollis) through central Kennebec County and in eastern coastal Maine.

Finally, locally high concentrations of arsenic in groundwater on the scale of kilometers (for example, Northport and Surry) appear to be associated with igneous activity within a belt of sulfidic pelites.

Marvinney and others, 1994, NGWA Focus Conference on Eastern Regional Groundwater Issues, Burlington, VT, p. 701-714

Peters and others, 1999, Environmental Science and Technology, 33:1328-1333