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 northeast trending faults and fractures in the gneissic granites, myolinitized quartzite, and metavolcanic rocks that 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 units.

Previous work at the University of Massachusetts mapped groundwater flowpaths between surface features and subsurface fracture sets in the MetroWest tunnel. For this study approximately forty water samples were collected during three field sessions at fractures along a 9-km section of the MetroWest tunnel.

Groundwater samples ranged from 229 µS to 1086 µS in conductivity. d¹⁸O isotope data for tunnel water range between -9.8 and -5.5 ‰ (SMOW) and are typical for groundwater in this region. High nitrate values of over 50 µeq/L were found in contiguous fractures within the tunnel. Two likely sources for this spike are septic systems and agricultural runoff. Carbon isotopic composition of the DIC in these waters suggests an organic source for the nitrate.

The tunnel passes directly beneath Dudley Pond located in the towns of Wayland and Natick. Interestingly, tunnel waters flowing from fractures directly below the pond are among those that record the highest concentrations of nitrate. Further sampling in the region, along with USGS water quality data, will be used to correlate fluid flow between the surficial and groundwater reservoirs.