Watershed: Groundwater

Although surface waters are the most visible manifestation of runoff and available water supply, they are derived primarily from groundwater via subsurface flow. In a 1978 report it was stated that 60% of stream flow comes from groundwater.

The principal aquifers in the Merrimack River watershed are composed of unconsolidated sand and gravel that were deposited in meltwater streams during the glacial period. Aquifers that can sustain well yields of more than 300 gallons per minute (gpm) lie primarily along the Merrimack and its major tributaries.

The largest area of glacial deposits capable of yielding more than 300 gpm to single wells is located in northwestern Amesbury, MA near the NH border. Many small aquifers in stream valleys and wetlands throughout the basin sustain wells yielding less than 100 gpm.

Igneous and metamorphic bedrock throughout the basin provides a sufficient quantity of water for most domestic wells. Yields range from less than 1 gpm, with the median yield of 41 bedrock wells at 10 gpm.