ISOTOPE HYDROLOGY OF THE PAWCATUCK WATERSHED, SOUTHERN RHODE ISLAND

Water Resources management in the Pawcatuck River Watershed, and New England as a whole is complicated by the complex nature of the ground-water/surface-water system. Ground-water occurs in two distinct types of aquifers here: fractured bedrock aquifers, predominantly of crystalline igneous and metamorphic origin, and unconsolidated sedimentary aquifers of glacial origin. These aquifers are spatially heterogeneous and interact to varying degrees with the local surface-water system. Isotopic fingerprinting, coupled with standard geochemical techniques, was used here to gain insight into the hydrologic system including surface-water/ground-water interaction, recharge/discharge relationships and the basin water budget.

Ground-water and surface-water samples were collected in the Pawcatuck River Watershed from June 1999 through December 1999 for major dissolved constituent and stable isotope analysis. Precipitation sampling was initiated in June 1999 and is ongoing to generate a long-term stable-isotope database for precipitation in southern New England. Precipitation data show distinct seasonal variations with winter precipitation yielding isotopic compositions approximately 6 per mil lighter in d¹⁸O and 15 per mil lighter in dD than summer precipitation. Precipitation as heavy as -2.0 per mil d¹⁸O and -8 per mil dD was recorded in September 1999 during tropical storm Dennis, and as light as -31.1 per mil d¹⁸O and -226 per mil dD during a January snowfall. Surface-water samples range from -1.3 d¹⁸O, -21 dD to -6.8 d¹⁸O, -40 dD, and ground-water samples range from -6.2 d¹⁸O, -40 dD to -8.1 d¹⁸O, -49 dD.

The range of observed isotopic compositions permits evaluation of surface-water contribution to ground-water withdrawals, assessment of evaporative stress on the watershed surface water system and identification of hydrologically distinct ground-water flow zones. For example, evapotranspiration produces significant isotopic enrichment of streamflow in portions of the watershed suggesting limited inflow of ground-water along those reaches. Ground water exhibits a relatively narrow range of isotopic compositions consistent with the isotopic composition of precipitation measured during February through June.