The Arc Hydro groundwater data model is a spatial database (geodatabase) design for representing geospatial groundwater information. Concepts and object classes of the data model, which extend the existing Arc Hydro surface water data model, are presented. The data model is designed to achieve four goals: representing data in regional groundwater systems, describing site scale groundwater information, enabling integration of surface water and groundwater data, and facilitating the connection of groundwater simulation models with GIS. Guided by these goals, a set of conceptual classes and relationships between them are identified for representing basic groundwater features and supporting common applications and data analysis. Results of this study outline the main concepts and data structures for representing geospatial and temporal groundwater information, and provide a blueprint for creating a geodatabase for storing groundwater data.