ASSESSING WATERSHED-SCALE HYDROLOGIC RESPONSE USING d18O AND dD: PAWCATUCK RIVER WATERSHED, RHODE ISLAND, USA

Stable isotopes d¹⁸O and dD were used as isotopic tracers in the Pawcatuck River watershed of southern Rhode Island.  The isotopic composition of surface and ground water was studied to assess the temporal variation in precipitation, spatial and temporal variation in surface-water bodies, spatial variation in ground water, and the relationship between isotopic composition and solute concentrations. 

Precipitation was sampled during June 1999 – August 2000 yielding a local meteoric water line of dD=7.5 d¹⁸O +9.3.  Distinct seasonal variations are evident with winter precipitation yielding isotopic compositions approximately 6 per mil lighter in d¹⁸O and 15 per mil lighter in dD than summer precipitation.  Ground-water and surface-water samples were collected for major dissolved constituent and stable isotope analysis from June 1999 through December 1999.  Ground-water samples range from –6.2 d¹⁸O, -40 dD to –8.1d¹⁸O, -49 dD, consistent with a recharge occurring predominantly during the period October to March.  Surface-water samples range from –1.3 d¹⁸O, -21 dD to –6.8 d¹⁸O, -40 dD with a strong evaporative enrichment trend (dD=4.4 d¹⁸O –11.8) observed in the pond data.  Significant shifts in the isotopic signature of river samples during the period August to October reflect the transition from baseflow-dominated to stormflow-dominated streamflow.  August streamflow is isotopically similar (d¹⁸O=-7 to –6 per mil) to shallow ground-water.  Heavy precipitation during September, however, produced a marked shift of both stream and pond samples toward the isotopic composition of the precipitation events (9/10-11/1999: 3 inches, d¹⁸O=-7.1 per mil; 9/16-17/1999: 2.5 inches, d¹⁸O=-3.4 per mil).  Ground-water isotopic compositions suggest a tendency toward isotopically lighter water at depth in both the surficial and bedrock aquifers.  A weak correlation ( R²=0.50) is present between NO₃-N and isotopic composition for surface-water samples; high nitrate concentrations are associated with isotopically light surface water.  This association may reflect nitrate source (septic or ground-water derived irrigation return flow) or the uptake of nutrients and coincident evaporative enrichment of surface water during the growing season.