Despite the superior energy efficiency of geothermal heat pump systems, widespread adoption of the technology is hindered by the higher initial capital cost relative to conventional heating and cooling systems. A promising avenue for reducing first costs is better characterization of salient subsurface properties and the subsequent implementation of more optimal designs for the ground heat exchange component. This study evaluates the potential for geological and pedological models to facilitate optimal design of vertical- and horizontal-based ground heat exchange systems, respectively. Data from well borings and subsurface monitoring sites in Indiana are used for geological model development and the assessment of existing pedological models. Results from performance simulations illustrate how the total design length of a conceptual, commercial-scale vertical ground heat exchange installation could be reduced by 17% as a result of better characterization of the variability in thermal properties gleaned from geological modeling. Given the abundance of publicly available well bore data, the explicit integration of geological modeling during the system design phase appears to be a feasible and promising approach for improving affordability. Results from horizontal ground heat exchange modeling at sites in Indiana that contain in situ monitoring data indicate that the use of design parameters derived from existing pedological models generally leads to overbuilt ground loops that were on average 45% longer than necessary to achieve design specifications. Despite the apparent limitations of existing pedological models, such models have utility in identifying sites that exhibit stable thermal properties and are thus most amenable for optimal design of horizontal ground heat exchange systems.