STRUCTURAL GEOLOGY OF HIGH-YIELD FRACTURED ROCK AQUIFERS, MASSACHUSETTS

Geologic mapping, aquifer tests, and ground water flow modeling 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. Well-foliated, biotite- to sillimanite-grade metasedimentary rocks underlie the West Newbury (yield=251 gpm), Maynard (780 gpm), and Paxton well sites (148 gpm). Possible pathways for ground water flow include sub-horizontal sheeting fractures along gently dipping layer-parallel foliation, younger steeply dipping cleavage or schistosity, fault-related mineralized fractures, or post-metamorphic joints. Drawdown during aquifer tests occurred parallel to the trend of deep water-bearing zones along the foliation in the two sites with gently dipping foliation (West Newbury and Paxton) and parallel to the trend of fault-related mineralized fractures in the site with steeply dipping foliation (Maynard). During the aquifer tests, ground water in the bedrock showed a connection to ground water in the overburden or surface water, presumably along steep fractures, at two sites (West Newbury and Maynard). In Paxton, however, ground water in shallow bedrock wells showed direct connection to water in the overburden and to surface water, but deep bedrock wells showed limited connection, perhaps because of limited vertical fracturing. Numerical modeling for zones of contribution at the three sites incorporated directional data acquired during geologic mapping and adequately simulated aquifer properties. These findings illustrate the importance of pre-existing fabrics in foliated metamorphic bedrock to fracture flow anisotropy. Where foliation dips gently, foliation-parallel 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 based watershed-scale ground water flow models in glaciated metamorphic bedrock aquifers.