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The use of stable isotopes in groundwater studies is not a new concept. Stable isotopes have been used as far back as the 1970s for N-source identification, and since the 1980s to distinguish between N-cycling processes. Current groundwater studies have combined these applications of stable isotopes (N, O, and C) to trace the forms of N found in the groundwater back to the source at the soils surface. The majority of these studies involve measuring the isotopic signature of the expected source and comparing it to the signature measured in the groundwater. Source can be grouped by land use and include conventional fertilizers, septic and sewage waste, manure, and treated wastewater. Isotope fractionation can occur via chemical processes (different processes that make up the N-cycle) as the source passes through the soil before reaching the groundwater, and can affect the end members of the isotopic signatures. The occurrence of fractionation in the soil can alter the signature of the source before it reaches the groundwater, leaving the potential for misidentification of the source from groundwater measurements alone. Because there has been an increase in the levels of nitrate in the Silver Springs springshed over the last 100 years, accurate source identification is an important component to groundwater remediation; if the source can be identified in the groundwater, then proper action can be taken at the surface. This study focuses on the stable isotope fractionation of 15N and 18O in NO¬¬3- as it passes through selected soil profiles that include top soil, vadose zone and aquifer material, and that differ with land use. The expected outcome for this study is the 18N and 15O signature will change as NO3- undergoes N-cycling processes in the soil. The end members measured at the surface will change and will not match the end members measured in the aquifer material and groundwater. |