Detailed knowledge of the groundwater flow regime and interactions between groundwater and surface waters is a pre-condition for an understanding of geochemical processes in the subsurface. Groundwater flow velocities and directions, for example, are essential parameters for delineating trends in As concentrations and to quantify reaction rates. The hydrogeology core laboratory provides tools and expertise for collection and analysis of hydrogeologic data for the research projects. The following types of data are obtained and interpreted: hydraulic head, grain size and porosity, geophysical data (resistivity, ground penetrating radar, and land seismic),and tracer data (SF6, 3H/3He, 14C, 13C, 18O, D). The equipment provided and operated by the facility includes multilevel wells, in situ groundwater monitoring devices, sample preparation systems for isotopic analyses, a multi-channel seismic system, a multi electrode resistivity meter, a frequency em conductivity system, ground penetrating radar, differential GPS systems, gas chromatographs, and noble gas mass spectrometers. The research core also provides 3-D modeling capabilities for goundwater flow simulations. Analysis of tritium, noble gas isotopes, SF6, carbon isotopes and stable water isotopes performed by gas chromatography and mass spectrometry is used to derive groundwater residence times for a range of time scales (months to 10,000s or years). In addition, The combination of stratigraphic, hydraulic and tracer data is being used to build and constrain groundwater flow and transport models.