Which aquifer is the largest in the southwestern United States and is being depleted by irrigation?

• A. The Guarani Aquifer
• B. The Ogallala Aquifer
• C. The Great Artesian Basin
• D. The Floridan Aquifer

Answer: (B)

This item tests how groundwater resources are used and what happens when withdrawals outpace natural recharge. The aquifer in question is the largest groundwater reservoir underlying the central United States, sometimes called the High Plains Aquifer. It spans a wide area across several Great Plains states and holds very large volumes of ancient recharge water.

Because it supports extensive irrigation for crops, water withdrawals have exceeded the rate at which it can be replenished, leading to steadily falling water levels and long-term concerns about supply for farming and local ecosystems. That combination—huge size plus heavy irrigation-driven depletion—fits the scenario described.

The other aquifers mentioned are real but located in different regions and aren’t the largest in the United States nor the primary source of irrigation-driven depletion in that area, so they don’t match the description as well.