Concept inventories are fundamental in assessing and evaluating student knowledge and understanding across science disciplines. In the earth sciences, existing inventories measure student understanding of introductory geoscience, oceanography, astronomy, meteorology, and mineralogy concepts. This study seeks to develop a concept inventory specific to hydrogeology. First, published literature, textbooks, and consultation with four experts were used to identify 14 key topics for an introductory hydrogeology course, such as hydraulic conductivity, hydraulic gradient and head, Darcy's law, porosity, water budget, and aquifer testing. Next, an initial set of more than 40 possible inventory items was generated from exams, formative class assessments, and other sources. Items were compared, consolidated when redundant, and slotted into the 14 topics with at least one item per topic. Expert review further reduced the instrument to 22 items, approximately half open-ended and half multiple-choice. To convert the open-ended items to multiple-choice, we collected written responses from students (n=21) enrolled in a summer hydrogeology field course. Semi-structured interviews (n=7) were conducted to determine whether students understood the items as intended and to probe their reasoning. Several items were further modified based on interview results. The current version of the instrument has 20 multiple-choice items and only one open-ended item and is in use as part of an expert-novice study of spatial thinking in hydrogeology. Our next steps in instrument development include large-scale deployment across multiple institutions for item response theory (IRT) Rasch analysis. Ultimately, after testing and validation, this hydrogeology concept inventory will provide a valuable tool for instructors and researchers in student assessment.