According to:
The Absorbency of Rock
Chalk can absorb 32% of its volume in water. So the 65 million gallons of chalk added to the original hill could hold about 20 million gallons of water.
There are 2 different ways the reservoir could be filled:
1. Precipitation
2. A bucket brigade from at least the bottom of the hill.
Precipitation seems the more likely. This suggests a climate with a wet season and a dry season (as opposed to a country that exports raincoats).
A mound such as this would not necessarily have had a 15 year plan. Rather, past experience would have led the village founders to look for a small hill with reservoir potential. Then a ditch would be dug to trap water seeping out of the hill during the dry season. Then the hill would be incrementally enlarged in hopes of increasing its capacity. At some point, diminishing returns or a change in climate would cause construction to stop.