Tips for a short bicycle tour in Spain
From Lonely Planet:
Water
Tap water is generally safe to drink in Spain. If you are in any doubt, ask żEs potable el agua (de grifo) ? (Is the (tap) water drinkable?). Do not drink water from rivers or lakes as it may contain bacteria or viruses that can cause diarrhoea or vomiting.
http://www.lonelyplanet.com/spain/practical-information/health