DIRECTIONAL ANISOTROPY IN THREE HIGH-YIELD FRACTURED METAMORPHIC BEDROCK AQUIFERS OF MASSACHUSETTS

Geologic mapping and aquifer tests at three high-yield municipal water systems in fractured metamorphic bedrock in eastern Massachusetts indicate distinct structural controls on ground water flow at each site. The West Newbury well sites (yield=250 gpm) are located in phyllite of the Silurian Eliot Formation. Ground water flow occurs along sheeting fractures parallel to a sub-horizontal foliation (S1), a steeply dipping foliation (S2), and steep fractures. Drawdown during aquifer tests shows an elliptical trend that correlates with the strike of the steep foliation and a principal fracture trend. During the tests, ground water in the bedrock shows a connection to ground water in the overburden and to surface water. The Maynard well site (750 gpm) is located in schist of the Ordovician Nashoba Formation. Secondary porosity in the rock is the result of intense fracturing between the Spencer Brook and Assabet River faults. Fracturing in the vicinity of the well site is complex and has associated sulfide mineralization. Drawdown during aquifer tests shows an elongate trend that correlates roughly with the strike of a penetrative foliation, the faults, and a principal fracture trend. During the tests, ground water in the bedrock shows a direct connection to ground water in the overburden. The Paxton well site (180 gpm) is located in schist and granofels of the Silurian Paxton Formation. Here, rocks contain a pervasive, gently dipping foliation that exhibits excellent sheeting but limited vertical fracturing. Drawdown during aquifer tests occurs along a major water-bearing zone parallel to the foliation. During the tests, ground water in shallow bedrock wells shows direct connection to water in the overburden and to surface water, but deep bedrock wells show limited connection.

     These findings illustrate the importance of pre-existing fabrics in foliated metamorphic bedrock to fracture flow anisotropy. Where foliation dips gently, fracturing is enhanced during isostatic unloading. Where foliation dips steeply, subsequent fracturing may create vertical pathways and along-strike directional drawdown. The highest yield well sites exhibit vertical pathways between deep ground water and shallow ground water in the overburden, locally along fractures parallel to foliation.