In many places, water for municipal and household use comes from underground aquifers. If the groundwater in such a place became contaminated, the consequences could easily affect entire cities or more. How does something like this happen? This classroom lab resource offered by Science Friday opens with a ten-minute video explaining one example of this very situation, which occurred in 2015 at the Gold King Mine in Colorado, and the efforts to mitigate it. The lesson provides an introduction to groundwater hydrology with helpful diagrams and a downloadable worksheet to check students' understanding. The most valuable part of this resource may be its detailed instructions for building reusable classroom groundwater models from easily obtained materials, as well as its accompanying instructions for using the models to simulate the contamination of an aquifer. This standards-aligned, hands-on activity was written with high school students in mind and is estimated to take two hours of class time. Downloadable handouts and data spreadsheets are included. This resource was created for Science Friday by Ariel Zych, Ryan Hollister, and Xochitl Garcia and published in December 2018.