Humans have a knack for altering the natural environment. Every year, we use enormous quantities of chemicals to make widgets and cure diseases. Our activities also release chemical pollutants that harm ecosystems, change the climate, and make us sick. Solving these problems and mitigating future risk requires understanding how chemicals move, change, and interact in aquatic environments at a variety of scales. This talk will highlight how the tools of environmental chemistry, such as radiocarbon dating and mass spectrometry, can be used to (1) track carbon cycling in the deep Arctic Ocean under changing sea-ice conditions, (2) monitor sewage consumption by microbes in the Hudson River Estuary, (3) fingerprint synthetic estrogens from birth control pills, and (4) design cost-effective strategies for removing estrogens from sewage in Salem, OR.