The site owner chose to focus on a passive closure objective for the site, which would require minimal management and achieve long‐term environmental stability. Ensero was retained to conduct an assessment to inform the design of passive treatment technologies for site runoff water in the post-closure period for the Mine. Ensero successfully designed and tested a constructed wetland treatment system (CWTS) to remove cadmium, copper, molybdenum, selenium, and zinc during the remediation of site runoff water in the post-closure period for the Mine. Off-site pilot systems provided the data needed for full-scale wetland design and sizing and confirmed treatment rates and performance of several design options. After commission the on-site demonstration-scale CWTS is consistently reducing Se to <0.001 mg/L and Cd to non-detectable levels.