Environmental chemistry is the study of the chemistry of the atmosphere, groundwater, and soil. It is the study of how man-made chemicals and natural chemicals produced via human activity affect the cycles of the natural environment. We can study how gas pollutants affect the global air temperature (global warming). We can study how pesticides affect groundwater. We can also study how chemical spills can affect local animal and plant populations.
Keep in mind that environmental chemistry is a relatively new area of study. I finished college in 1997 and my senior year was the first year that a new course called environmental chemistry was offered. You have probably also heard the term "green chemistry." This is a totally new area of chemistry that focuses on utilizing more environmentally friendly chemicals in industry and manufacturing.