Arc Hydro Groundwater: GIS for Hydrogeology describes the groundwater data model, a new geodatabase design for representing groundwater systems using ArcGIS software. The groundwater data model shares a common framework with the surface water components of the Arc Hydro data model, offering a comprehensive overview of water resources. Arc Hydro Groundwater uses sample datasets from the Edwards Aquifer and other locations in Texas to address the data model framework, 3D subsurface representation, geological mapping, 3D hydrogeologic models, time series for hydrologic systems, and groundwater simulation models.