Groundwater refers to the saturated layer of Earth's crust extending beneath the land surface to a depth where solid rock does not permit the movement or storage of water. The widespread geographical distribution of groundwater and its usually high quality for human consumption, agriculture, and other uses make this a resource of global importance; it accounts for a third of the planet's total volume of water outside the oceans, or close to 98% of all freshwater outside ice caps and glaciers. The spatial occurrence and quality of groundwater are not uniform, which is the result of geology, climate, surface water interactions, and, increasingly, human use and contamination. There is a growing recognition that a comprehensive understanding of groundwater must consider its multiple and interconnected physical, ...