Groundwater reservoirs are natural or man-made formations where water is stored and can be used for various purposes. Most of the groundwater is stored in sands and gravels. These reservoirs are located in stream valleys, interior valleys, and coastal plains. They help store water from precipitation and streams, and can also hold water artificially added to them. Groundwater reservoirs act as pathways to move water from areas where it enters to where it is used. When choosing sites for adding water artificially, factors like the geology of the reservoir, water movement, soil types, water quality, and social and economic factors need to be considered.