Acadia National Park - FLOW: Groundwater

Groundwater on Mount Desert Island is stored in both surface deposits of porous soil and in fractured bedrock. The surface deposits are of glacial origin. They include unsorted glacial till, glacial stream deposits, end moraines, marine deposits, and glacial stream deltas. These groundwater storage sites are often in the vicinity of sand and gravel pits where the storage formation is mined for construction purposes. The water yield from wells drilled in such formations is low, often below 10 gallons per minute.

Water in bedrock resides in fractures, joints, and faults. Wells tapping these deeper groundwater storage areas produce about the same flow of water, roughly 10 gallons per minute. Drawn by gravity, groundwater moves downward through fractured bedrock, but it also flows sideways (laterally) for hundreds of miles, and can move upward by capillary action. The water table is groundwater"s upper surface, which tends to lie parallel to the ground surface above it.

A layer of clay (pulverized rock) deposited by the last glacier often covers or caps other more porous surficial deposits, creating perched reservoirs of groundwater unable to penetrate through the dense and impassable layer of clay. Most reliable supplies of groundwater on Mount Desert Island are tapped by wells driven into the fractured bedrock.

Groundwater on Mount Desert Island is characterized as "soft" and of generally good quality for domestic use. By rough estimate, half the residents of the Town of Bar Harbor, Maine, get their water from private wells tapping the local groundwater, while the other half uses water drawn from Eagle Lake supplied by the Bar Harbor Water Company.

The local granite can contain radon, which dissolves in the groundwater, and can be released into homes, particularly when the shower is turned on. This can release radioactive radon gas into poorly ventilated spaces, possibly reaching concentrations that are a potential risk to human health. When inhaled, radon can cause lung cancer.

Groundwater can be polluted by liquids poured or spilled on the ground filtering down through the soil into aquifers below. Since human taste buds can detect a single molecule of gasoline in a million molecules of water, one gallon of spilled gasoline can make a million gallons of groundwater unsuitable for drinking.