I will test the use of boron and lithium isotopic variation as hydrologic tracers at the Fresh Kills Landfill on Staten Island, NY. Fresh Kills provides a complex hydrologic environment because of the mixing of three distinct end-member waters—landfill leachate, sea water, and fresh water. The chemical composition and mixing of these waters is already well understood, providing an opportunity to test the conservative nature of the isotopes and their ability to trace the movement and mixing of groundwaters. We have collected a preliminary set of samples comprising the different end-member waters and mixing between them. The use of base-cation concentrations and ratios of base cations allows an independent conformation on the boron and lithium isotope data. Analysis of boron and lithium isotopic composition is performed using thermal ionization mass spectrometry (TIMS) after concentration and chemical separation.