To better inform teaching, curricular decision-making, and management of college-level environmental programs (EPs), we are investigating environmental students' foundational knowledge, alternative conceptions, and their ability to grasp complex systems-level concepts. Specifically, we are creating a Next Generation Concept Inventory (NGCI) by applying an established machine learning method of evaluating constructed response (CR) (e.g., short answer questions). Multiple choice items have been the chosen assessment tool used for previous concept inventories. However, numerous studies show that CR items reduce students' ability to use testwiseness to correctly answer exam questions and CR items test students on higher levels of Bloom's Taxonomy than multiple choice items. Concept inventory development follow these steps: (1) selection of concepts considered important to educators in the field of study, (2) identification of student thinking about the concepts and common alternative conceptions, (3) development of multiple-choice questions using these alternative conceptions as distractors, (4) validation interviews of novices and subject experts that were not involved with instrument development, and (5) statistical validation. For this presentation, we share preliminary results from step 2, understanding how students think about EP concepts and identifying alternative conceptions. Specifically, we are interested in student understanding of the Food-Energy-Water (FEW) Nexus. FEW Nexus concepts are interdisciplinary and commonly covered in introductory EPs. We are conducting approximately 100 semi-structured interviews with students in introductory EPs across ten higher education institutions. Institutions were chosen as a representative sample of EPs across the US and include baccalaureate colleges (4), master's college and universities (3), and doctoral/research universities (3). Interview data were analyzed thematically in NVivo 12. Preliminary results will show areas of overlap in student thinking and inaccurate understandings of FEW Nexus concepts. The NGCI will be an important assessment tool for EP educators and results will be valuable for evidence-based course content and program development.