GEOLOGIC MAPS AS A BASIS FOR UNDERSTANDING GROUND WATER QUALITY IN CENTRAL MAINE

Nearly 40% of Maine residents derive their domestic water from private wells drilled into bedrock. The water quality in these wells is dependent in large part on the nature of the bedrock and the fracture system that conveys meteoric water to the wells. Our central Maine study area (Augusta, Winthrop, and Purgatory 7.5-minute quadrangles), is primarily underlain by metasedimentary rocks: several units of thickly bedded quartz-feldspar-biotite granofels and schist, a unit of thinly bedded quartz-mica schist, several thinly bedded marbles, and several thin sulfide-rich units. These units are variably calcareous, have been regionally metamorphosed to as high as sillimanite grade and intruded by granite and syenite.

We analyzed water from 790 bedrock wells for a full suite of analytes, but focus here on arsenic and uranium. Arsenic concentrations (range < 0.07 – 325 ug/l) exceed the EPA maximum contaminant limit (MCL) of 10 ug/l in 31% of the sampled wells. Wells with the highest concentrations are not clearly related to a single bedrock unit. Detailed geologic maps demonstrate that As concentrations are not clearly correlated with individual calcareous units, as some regional analyses based on generalized maps have suggested. Concentrations of U (median 1 ug/l, range < 0.0007 – 484 ug/l) are highest in and near the two-mica granites and mostly less than 10 ug/l in the metasedimentary rocks. Less than 4% of the sampled wells have U concentrations in excess of the MCL (30ug/l).

The detailed maps provide some guidance for public outreach to well owners concerning the need for water testing and subsequent remedial action, but without a thorough understanding of the origins of elevated arsenic and uranium concentrations, the Maine Bureau of Health recommends that all wells should be tested periodically. Mapping by the Maine Geological Survey is part of a regional multi-year effort, through the National Cooperative Geologic Mapping Program, to create detailed bedrock geologic maps for the area from Augusta to Rockland.