Groundwater has not been a traditional area of application of geographic information systems (GIS) to water resources, in part because groundwater resources are less visible and readily mapped compared to streams, rivers, lakes, and watersheds. Also, groundwater is inherently a 3D phenomenon because the depth at which water is found in a well is a critical measure of its accessibility. As a well is drilled, its borehole passes through many subsurface strata laid down in layers over geologic time. The spatial extent of these layers is much larger than is their vertical thickness, much like sheets of paper, so 2D GIS mapping of well locations and aquifer boundaries is the normal point of departure for groundwater projects. The core framework of the Arc Hydro data model supports 2D mapping of groundwater resources, and the extensions of this model support 3D representations of boreholes and hydrogeologic strata.