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Bedrock Ground Water Characterization

Types of Aquifers

Subsurface water that fills the spaces between particles of rock and soil, or in fractures in solid rock, is called ground water. An aquifer is a subsurface water supply which yields useful quantities of ground water to wells and springs. There are two types of aquifers in Maine: surficial materials and fractured bedrock.

Maine's bedrock has been metamorphosed and recrystallized, and the original porosity between mineral grains has been eliminated. In a bedrock aquifer, ground water is stored in fractures in the rock, and areas with a large number of fractures may contain significant amounts of water. Fractures are sufficiently abundant to provide enough water for a single-family home most everywhere in Maine, and most domestic water supplies are wells drilled in bedrock.

Uses of Aquifer Information

When deciding where to locate a domestic or municipal well, aquifer information is very important. Information such as high yield zones in bedrock and the location of sand and gravel aquifers can influence where a well driller locates a well, and what type of well is constructed. Knowledge of aquifers and their recharge areas is also critical when siting possible sources of pollution such as landfills, salt piles, and hazardous wastes. Aquifer maps and texts from the Maine Geological Survey contain information on aquifer favorability and vulnerability and are widely used by local and state officials in making environmentally sound siting decisions, and by well drillers, developers, and geological consultants as a base for detailed hydrogeological studies.

Bedrock Ground Water Characterization

Characterization of the bedrock ground water resource is complicated by the nature of ground water flow through crystalline bedrock. This flow is controlled by the distribution and characteristics of brittle fractures in the rock. Brittle fracture systems cannot be mapped as easily as coarse sand and gravel deposits at or near the land surface, and estimating the hydraulic properties of these fracture systems is difficult.

At the present time, the bedrock ground water resource program consists of collecting, analyzing, and publishing basic information on bedrock wells drilled by commercial well drillers in Maine. These data are published on a series of 1:125,000 to 1:150,000 scale maps presenting information on well yield, well depth, and estimated thickness of surficial materials (Bedrock Ground Water Resources Basic Data Maps). A database containing over 43,500 located wells is available for download.

A portion of the bedrock well database consists of data collected from well drillers in the mid-1970's which includes information in their records up to that time. This data was used to prepare an earlier series of county ground water resource maps published at a scale of 1:250,000. The remainder of the database consists of information collected on wells drilled since 1985. In 1987, passage of the Water Well Information Law by the State Legislature created a mandatory reporting program for water supply wells drilled in the State. Since that time, approximately 100 commercial well drillers have been supplying MGS with ownership, location, yield, depth, and related information on wells drilled in Maine. Contact the Maine Geological Survey for more information on this database.

MGS is beginning a program to develop a better understanding of the ambient water quality of ground water in the bedrock aquifer system. This program is in part a result of contamination of ground water by MTBE, a gasoline additive, and the occurrence of arsenic in wells in Maine. The program will be a cooperative with the U.S. Geological Survey and is partially funded by the Maine Bureau of Parks and Lands. The program will consist of two primary parts: 1) a program to characterize the geographic variation in water quality throughout the State, and 2) a program to characterize the temporal variation of water quality in selected ground-water basins. The program will enhance the MGS water-well database, which contains information about well location, depth, and yield for over 16,000 drilled wells, with information on water quality.

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Last updated on April 9, 2012

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