This entry lists the major (mega) aquifer system(s) that underlie a country, keeping in mind that many of these mega aquifers are so large that they extend under multiple countries. More than 30% of freshwater is held in underground aquifers. There is great variation in the size of such aquifers, but a limited number of very large aquifer systems contain a majority of the World’s groundwater volume in storage.

Topic: AlgeriaLullemeden-Irhazer Aquifer System, Murzuk-Djado Basin, North Western Sahara Aquifer, Taoudeni-Tanezrouft Basin

Topic: AngolaCongo Basin, Upper Kalahari-Cuvelai-Upper Zambezi Basin

Topic: ArgentinaGuarani Aquifer System

Topic: AustraliaGreat Artesian Basin, Canning Basin

Topic: BahrainArabian Aquifer System

Topic: BangladeshIndus-Ganges-Brahmaputra Basin

Topic: BoliviaAmazon Basin

Topic: BotswanaLower Kalahari-Stampriet Basin, Upper Kalahari-Cuvelai-Upper Zambezi Basin

Topic: BrazilAmazon Basin, Guarani Aquifer System, Maranhao Basin

Topic: CameroonLake Chad Basin

Topic: CanadaNorthern Great Plains Aquifer

Topic: Central African RepublicCongo Basin, Lake Chad Basin

Topic: ChadLake Chad Basin, Nubian Aquifer System

Topic: ChinaNorth China Aquifer System (Huang Huai Hai Plain), Song-Liao Plain, Tarim Basin

Topic: ColombiaAmazon Basin

Topic: Congo, Democratic Republic of theCongo Basin

Topic: Congo, Republic of theCongo Basin

Topic: EgyptNubian Aquifer System

Topic: EthiopiaOgaden-Juba Basin, Sudd Basin (Umm Ruwaba Aquifer)

Topic: FranceParis Basin

Topic: GabonCongo Basin

Topic: Gambia, TheSenegalo-Mauritanian Basin

Topic: Guinea-BissauSenegalo-Mauritanian Basin

Topic: IndiaIndus-Ganges-Brahmaputra Basin

Topic: IraqArabian Aquifer System

Topic: JordanArabian Aquifer System

Topic: KenyaOgaden-Juba Basin

Topic: KuwaitArabian Aquifer System

Topic: LibyaNubian Aquifer System, North Western Sahara Aquifer System, Murzuk-Djado Basin

Topic: MaliLullemeden-Irhazer Basin, Taodeni-Tanezrouft Basin

Topic: MauritaniaSenegalo-Mauritanian Basin, Taodeni-Tanzerouft Basin

Topic: MexicoAtlantic and Gulf Coastal Plains Aquifer

Topic: NamibiaLower Kalahari-Stampriet Basin, Upper Kalahari-Cuvelai-Upper Zambezi Basin

Topic: NepalIndus-Ganges-Brahmaputra Basin

Topic: NigerLake Chad Basin, Lullemeden-Irhazer Basin, Murzuk-Djado Basin

Topic: NigeriaLake Chad Basin, Lullemeden-Irhazer Aquifer System

Topic: OmanArabian Aquifer System

Topic: PakistanIndus Basin

Topic: ParaguayGuarani Aquifer System

Topic: PeruAmazon Basin

Topic: QatarArabian Aquifer System

Topic: RussiaAngara-Lena Basin, Pechora Basin, North Caucasus Basin, East European Aquifer System, West Siberian Basin, Tunguss Basin, Yakut Basin

Topic: Saudi ArabiaArabian Aquifer System

Topic: SenegalSenegalo-Mauritanian Basin

Topic: SomaliaOgaden-Juba Basin

Topic: South AfricaKaroo Basin, Lower Kalahari-Stampriet Basin

Topic: SudanNubian Aquifer System, Sudd Basin (Umm Ruwaba Aquifer)

Topic: TunisiaNorth Western Sahara Aquifer System

Topic: United StatesNorthern Great Plains Aquifer, Cambrian-Ordovician Aquifer System, Californian Central Valley Aquifer System, Ogallala Aquifer (High Plains), Atlantic and Gulf Coastal Plains Aquifer

Topic: UruguayGuarani Aquifer System

Topic: Worldsummary statement: aquifers are underground layers of water-bearing permeable rock formations; they include alluvial formations such as unconsolidated sand and gravel aquifers, sedimentary rock formations of sandstone and karst (carbonate rocks such as limestone) aquifers, as well as volcanic aquifers, and basement aquifers (igneous and metamorphic rocks that underlie sedimentary and volcanic rock sequences); groundwater from aquifers can be extracted using a water well; The World Factbook lists 37 major aquifers across 52 countries; of these, 13 are in Africa, 10 in Asia, 5 in North America, 3 in South America, 4 in Europe, and 2 in Australia; although aquifers can vary in size, the major aquifers listed in The Factbook contain the bulk of the stored volume of groundwater; the fresh water held in these aquifers represents more than 30% of the World's fresh water; in the US, groundwater is primarily used for irrigation and globally, 70% of groundwater withdrawn is used for agriculture; groundwater also supplies almost half of all drinking water worldwide

Topic: ZambiaUpper Kalahari-Cuvelai-Upper Zambezi Basin

Topic: ZimbabweUpper Kalahari-Cuvelai-Upper Zambezi Basin