Factors influencing groundwater inflows in a newly constructed cross-strike tunnel, eastern Massachusetts; 5, Geochemical interpretation of groundwater inflows

Samples of ground and surface waters in and above the tunnel (Curry et al., this volume) were collected to characterize the chemistry of groundwater discharging from fractures and faults. Forty-two water samples were collected: 32 along a transect of the tunnel and 10 from surface waters above the trace of the tunnel. All samples were analyzed for major anions and cations, and delta (super 18) O. Analysis of the anion/cation data indicated that these waters are dominated by sulfate+chloride and calcium+magnesium. However, five sub-classifications can be discerned based on the relative concentrations of ions in the samples. The five sub-classifications are Cl > HCO (sub 3) > SO (sub 4) > NO (sub 3) :Ca > Na+K > Mg (15 samples), Cl > HCO (sub 3) > SO (sub 4) > NO (sub 3) :Na+K > Ca > Mg (7 samples), Cl > HCO (sub 3) > SO (sub 4) > NO (sub 3) :Ca > Mg > Na+K (6 samples), Cl > SO (sub 4) > HCO (sub 3) > NO (sub 3) :Na+K > Ca > Mg (3 samples), and HCO (sub 3) > Cl > SO (sub 4) > NO (sub 3) :Ca > Na+K > Mg (2 samples). Results from statistical analyses indicate that alkalinity, calcium, sodium and potassium do vary as a function of bedrock type and that these differences are significant at the 95% confidence level.In addition, preliminary oxygen isotope data indicate that two large, discrete water producing fault zones located in the eastern part of the tunnel are isotopically enriched (average delta (super 18) O = -7.75) relative to other water producing features in the tunnel (average delta (super 18) O = -8.96). The delta (super 18) O values obtained from all surface water bodies located above the tunnel average -7.56 whereas those values in surface ponds immediately above the fault zones average -6.71. Nitrate levels also show elevated levels in two water producing fault zones (>10 mg/L for some samples) and may result from accidental contamination during sampling, the use of explosives at discrete locations in the tunnel, or from leaking septic systems. The results of the oxygen isotope and nitrate analyses also suggest that some of the fault zones in the tunnel may have a rapid and direct hydraulic connection to the surface.