FRACTURE CONTROLLED GROUNDWATER FLOW TO THE METROWEST WATER SUPPLY TUNNEL, MA: CONSTRAINTS ON SOLUTE EVOLUTION

The MetroWest Water Supply Tunnel, when completed, will carry drinking water from the Quabbin and Wachusett reservoirs in central Massachusetts to the greater Boston area. The 28 km long, 5 m diameter tunnel will extend from Marlborough to Weston at an average depth of 70 m below surface. It traverses highly fractured and faulted terranes assembled by three compressional orogenic events spanning the Middle Ordovician to Late Paleozoic time. A series of microcontinents were accreted onto the North American craton by an east-dipping subduction zone. This tectonic activity is responsible for the north-east trending faults and fractures in the gneissic granites, myolinitized quartzite, and metavolcanic rocks which compose the terrane. The construction of the tunnel provides a unique opportunity to analyze the chemistry of groundwater flowing through fractures and faults in these compositionally distinct, crystalline rocks.

Previous work at the University of Massachusetts mapped groundwater flowpaths between surface features and subsurface fracture sets in the MetroWest tunnel. During each of three field sessions, approximately forty water samples were collected at fractures along the 9 km completed section of the tunnel. Initial results show a range in conductivity from 229 to 1086 µS. d¹⁸O isotope data from tunnel water range between -9.8 and -5.5 ‰ (SMOW) and are typical for groundwater in this region. High nitrate concentrations of over 50 µeq/L were found in contiguous fractures within the tunnel. Two likely sources for these anomalous values are septic systems and agricultural runoff.

The tunnel passes directly beneath Dudley Pond located in the towns of Wayland and Natick. Residents have observed and expressed concern over the drop in water level of Dudley Pond since the construction of the tunnel. Interestingly, tunnel water flowing from fractures directly below the pond are those which record the highest concentrations of nitrate. Continuing analyses of water samples taken from Dudley Pond and the tunnel fractures will be used to correlate fluid flow between the surficial and groundwater reservoirs.