Map of aquifers in Spain

A well gives water if the rock holds it and lets it through. IGME's hydrogeological map (1:200,000) classifies all of Spain by just that.

The classes

Porous (sands, gravels): water fills the gaps between grains. High or medium productivity: the best for a well.
Karstic (limestones): water runs through cracks and caves. They can give a lot, but hitting it is harder.
Low or local: pockets of water here and there; a well may give little.
Impermeable: granites, slates, clays: little water, except in fractures.

A productive rock does not guarantee water under a given plot: a hydrogeologist decides that.

Towns of 20 km² or more, by the share of their area over porous or karstic aquifers of high or medium productivity.

The town finder filters those mostly over productive aquifers. For one plot's water, the wells map shows what the neighbours found.

Source: IGME, Mapa Hidrogeológico de España 1:200,000.