Final answer:
A single water molecule takes approximately 4,000 years to complete the water cycle, revealing the vast difference in residence times across different parts of Earth's water reservoirs.
Step-by-step explanation:
Scientists estimate that it takes a water molecule approximately 4,000 years to complete one full cycle through the water cycle. The water cycle involves various processes such as evaporation, condensation, precipitation, and collection, and water molecules can have different residence times in different parts of the cycle, such as oceans and seas, groundwater, glaciers For instance, in the oceans and seas, a water molecule might stay for about 4,000 years, whereas in groundwater, it could be as quick as two weeks or as long as 10,000 years. Glaciers could keep water frozen for 1,000-10,000 years, illustrating the wide range in the cycle time for individual molecules of water.