Using Stable Isotopes to Assess Surface-Groundwater Interactions and Contaminant Pathways in a Drinking Water Supply Watershed System

The assessment of surface-groundwater fluxes is crucial for understanding pollutant pathways through the natural environment. Several techniques to characterize these surface-subsurface interactions have been applied as an attempt to quantify these fluxes in the humid temperate Northeastern USA. Most recently, stable isotopes are considered to be an important tool to describe the movement of waters through the hydrosphere. This study was conducted in the Quabbin-Wachusett Reservoir System, which supplies water for the Boston Metropolitan Area in Massachusetts and depends on water quality management based on environmental trends. Recent trends indicate that despite efforts to reduce road salt application during the winter, salt indicator trends are still increasing in the watershed. Salt transport characterized by monitoring trends of specific conductivity and chloride across the watershed demonstrate that subsurface water concentrations are significantly higher than the streams and reservoir (for chloride, median value is 204 mg/L for wells and 102 mg/L for streams). The present investigation hypothesizes that salt infiltrates through the subsurface during the cold months (October-March) and then releases back to surface water throughout the year. Since groundwater can act as salt storage, an important question for water management relates to the timeframe needed to observe a reduction of salt presence in the watershed after road salt reduction policies and other mitigation strategies take place. To investigate this, oxygen isotopes are being used to identify the dominant hydrological pathways influencing groundwater recharge patterns.  Stable water isotope compositions for warm precipitation (δ¹⁸O -2.14 to -8.98 per mille), cold precipitation (δ¹⁸O -4.57 to -13.57 per mille), and groundwater (δ¹⁸O -8.27 to -9.66 per mille) were used to assess proportional recharge dominance via local winter and summer precipitation isotope end-members. Preliminary analyses indicate that the groundwater recharge is winter dominant (92% obtained from the winter bias seasonal recharge ratio Rwinter/Rannual; values >= 80% represent winter dominance), thus the applied road salt during cold months can be contributing to sustained increases in conductivity in the groundwater. The results show potential dynamics that explain higher levels of specific conductivity and chloride in the subsurface water and the continued increases in stream and reservoir concentrations. Further investigation is being conducted with larger datasets in order to have a better understanding of sample frequency needed to be representative of the system’s predominant seasonal recharge and runoff generation patterns, as well as, how the water isotopic composition is variable spatially and temporally in the region.