Retention time refers to a time frame in which chemicals stay in a certain location. The term is sometimes used interchangeably with residence or renewal time, which in limnology (aquatic ecology ) describes the length of time a water molecule or a chemical resides in a body of water. The residence time of water is called the hydraulic retention time. It may range from days to hundreds of years, depending on the volume of the lake and rates of inflow and outflow, and is often used in calculations of nutrient loading . Retention time can also refer to the length of time needed to detoxify harmful substances or to break down hazardous chemicals in pharmaceutical and sewage treatment plants; the length of time chemicals stay in a living organism; or the length of time needed to detect certain chemicals by instruments such as gas chromatography .