what is the world's largest aquifer? what is the world's second largest aquifer?

Answer 1

The Ogallala aquifer located in the midwestern United States is believed to be the world's largest aquifer.

This aquifer is named after a Sioux Indian tribe. It is estimated to be more than two million years old and to hold about 650 trillion gallons (2,500 trillion liters)! It underlies parts of eight states, stretching about 800 miles (1,288 km) from South Dakota to
Texas. The Ogallala aquifer supplies vast amounts of water to irrigate the crops in this vitally important agricultural area.

The Guaraní Aquifer System (GAS), perhaps, is the world’s second largest fresh groundwater reservoirs, due to its extent and volume. It underlies portions of Argentina, Brazil, Paraguay and Uruguay, stretching over an area of approximately 1.2 million km2, with almost 15 million inhabitants. The capacity of the GAS is estimated at around 40,000 km3.