Spanish water workers discovering ancient Roman coins

no need to. It's an incredibly common occurrence in all of Europe. Roman history museums here are literally full of those small treasures, and those coins are not worth much to collectors or researchers, unless they are some really rare types. You can easily buy cheap Romam coins at flea markets, at least in Italy and Spain. At maximum extension the empire had a population >50 million people, so you can imagine how many millions coins circulated and were hidden or lost.