Detailed knowledge of groundwater flow is essential for understanding geochemical processes in the subsurface. Identification of recharge and discharge areas and mechanisms, as well as flow lines are essential for delineating trends in geochemical evolution, including changes in As concentrations, and to quantify reaction rates. Hydrogeology Core D provides the tools and expertise for collection and analysis of a broad range of hydrogeological data in the US and in Bangladesh under Projects 4, 5, and 6 and the Research Translation Core. The following types of data are being obtained and interpreted: high-precision position and elevation by differential GPS; surface permeability by coring and frequency EM conductivity, resistivity/SIP surveys also to characterize the subsurface, aquifer stratigraphy from drill cuttings, core samples, and downhole geophysical logging; determination of hydraulic properties including water levels, conductivity, storativity, and porosity; groundwater dating and pathway tracing from SF6, 3H/3He,18O, 2H, and Br. Field activities supported under Core D include the installation of multilevel wells equipped with pressure loggers, deploying a Geoprobe system and a new freeze-shoe coring device, as well as cross-borehole resistivity and induced polarization surveys. The Core supports in situ forced gradient experiments, with a focus on characterizing the zone of injections and tracking injected fluids with geophysical methods and tracers. Laboratory activities include sample preparation systems for isotopic analyses, gas chromatographs, and noble gas mass spectrometers. Tracer data are also being used to derive groundwater residence times for a range of time scales (months to 10,000s of years), to identify recharge and discharge areas and mechanisms and to track groundwater mixing in the aquifers. Modeling activities supported under Core D include three-dimensional flow modeling using hydraulic and tracer data as calibration targets and reactive transport simulations for As and other relevant aquifer constituents.