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Only part of an aquifer's water drains under gravity - the specific yield, typically 10-25% for sand and gravel and under 5% for clay. A 2.5 square kilometre aquifer 18 m thick holds 45 million cubic metres saturated but yields 6.75 million, enough for 13.5 years at 500,000 cubic metres a year.
Saturated volume
volume = area x saturated thickness
Recoverable water
recoverable = saturated volume x specific yield
Because some water clings to grains by capillary and molecular forces. That retained fraction is the specific retention.
No. Sustainable abstraction is limited by recharge, not storage. Mining storage lowers water tables and can cause subsidence.