The toxification of the Hudson River has had a dramatic impact on the health of the river's ecosystem as well as the ability of people living along the river to use and enjoy it. With increasing human population in the last one hundred years, the Hudson has endured high levels of raw sewage, loading of nutrients, and the accumulation of pollutants such as PCBs. In this module, students learn how to monitor a local waterway for changes in water quality, and how the Hudson River has changed over time due to pollutants including nitrates, phosphates, and salt.