Influence of Fracture Trends on Ground Water Flow Directions in Metamorphic Bedrock Aquifers, Eastern Massachusetts

Geologic mapping and aquifer tests at three high-yield municipal water systems in glaciated fractured metamorphic bedrock in eastern Massachusetts indicate distinct structural controls on ground water flow directions in the bedrock at each site. The West Newbury well sites (yield = 251 gpm) are located in biotite-grade phyllite. Possible pathways for ground water flow include sub-horizontal sheeting fractures along a layer-parallel foliation, steeply dipping cleavage with quartz-calcite veins and associated vugs, and steep fractures. Borehole geophysical logs indicate sub-horizontal water bearing zones within 100 feet of the land surface and steeply dipping water bearing zones to a depth of 200 feet. Drawdown during aquifer tests shows an elliptical northeast trend that correlates with the strike of the steep cleavage and a principal fracture trend. Numerical modeling suggests, however, that the northeast trend is best simulated by a laterally extensive sub-horizontal transmissive zone rather than aquifer anisotropy along steeply dipping fractures. During the tests, ground water in the bedrock shows a connection to ground water in the overburden and to surface water, presumably along steep fractures. The Maynard well site (780 gpm) is located in sillimanite-grade schist. Secondary porosity in the rock is the result of intense fracturing between the east-northeast trending Spencer Brook and Assabet River faults. Fractures near the well site have sulfide mineralization and show complex orientation trends. Borehole geophysical logs show wide variations in water-bearing fracture trends. Drawdown during aquifer tests shows an elongate east-west trend that potentially correlates with a sulfide-mineralized principal fracture trend. Numerical modeling with an east-west transmissive zone adequately simulates aquifer properties. During the tests, ground water in the bedrock shows a direct connection to ground water in the overburden, presumably along steep fractures. The Paxton well site (148 gpm) is located in sillimanite-grade schist and granofels. Here, rocks contain a pervasive, gently dipping foliation that exhibits excellent sheeting but limited vertical fracturing. Drawdown during aquifer tests occurs parallel to the trend of a deep water-bearing zone along the foliation. Two contrasting numerical models, a 2 layer model and a gently dipping 5 layer model, adequately simulated aquifer properties, with the latter providing better approximation of heads in observation wells. 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 flow dynamics in fractured rock aquifers. Where foliation dips gently, fracturing is enhanced during isostatic unloading. Where foliation dips steeply, subsequent fracturing may create vertical pathways and potential 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 steeply dipping fractures parallel to foliation or in high-angle fault zones. In all cases, these findings support geologically acceptable watershed-scale ground water flow models in glaciated metamorphic bedrock aquifers.