Answer:
Depletion of ground water as a result of increased usage leading to lower water table.
Explanation:
The presence of small farms nearby coupled with the growing city that is just 50 miles the north, would mean more pumping of water for use by households, farms and even industries located in the growing city. The water level would definitely drop beyond pump intake as more wells are drilled, and water usage increases as well. The lowering of the water table is the most likely reason for the well becoming dry.