What happens to ground water when the rate of infiltration is less than the rate of water being pumped out of the ground

The water table lowers because this means that you have a negative rate of change in water, lowering your available groundwater because you are taking more water than is going back. 

If the rate of infiltration is less than than rate of water being pumped out of the ground than the supply of ground water will decrease. The speed it decreases depends on the difference between the rate of infiltration and the rate of water being pumped out of the ground.

They both determine the exact age of rocks.