In an acquifer extenting over 150 hectare, the water table was 20 m below ground level. Over a period of time the water table dropped to 23 m below the gound level. If the porosity of acquifer is 0.40 and the specific retention is 0.15, what is the change in ground water storage of the acquifer?
67.5 ha-m
112.5 ha-m
180.0 ha-m
450-0 ha-m