Investigations that integrate concepts from geological sciences with biology and chemistry are rare. The authors present an investigation that introduces high school students to microbe-mineral interactions by tying together anaerobic respiration, reduction reactions, metal ion solubility, and groundwater pollution. During the investigation, students build a simple Winogradsky column to model a surface-water environment. Students first add iron (III) chloride to the mud-water environment and then observe the changes in color and in the concentration of dissolved oxygen in the column over a period of at least four days. From these changes, students infer that bacterial respiration shifts from aerobic to anaerobic, causing the iron (III) to become the electron acceptor in place of the oxygen. During this process, the reduced iron (III) becomes soluble iron (II). The investigation demonstrates how toxic substances, such as arsenic, that are bound to iron-containing minerals in the soil can become soluble and pollute the groundwater through interactions between bacteria and an iron-containing mineral. (Contains 4 figures.)