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It's a LOT of calcium in the urine.
How old are they?
What exactly do they eat? What kind of hay, and how much? Which brand of pellets, and how much? and which veggies, and how much of each?
No, the tap water may be contributing to it, but it's not the only thing going on.
Nothing with a seed mix is good for guinea pigs. They have extremely narrow throats, and can easily choke on hard seeds/nuts. Additionally, the main ingredient in pellets should be hay, and the main ingredient in Living World pellets is peas. They also use calcium carbonate as the calcium source, and that can cause some guinea pigs to excrete large amounts of urinary calcium, and that has been implicated in bladder stone formation.
At five months of age, he does need some extra calcium, but depending on what veggies you're giving him and how many, he may be getting too much. See https://www.guineapigcages.com/foru...vy-Nutrition-Charts-amp-Poisonous-Plants-List for what you can feed and how often, and what the calcium (and other nutritional) content is.
At his age, he needs no more than a cup of vegetables per day, and possibly not that much. Hay is the most important thing in his diet, because it's what keeps his molars ground down and his gut moving properly. If you're feeding too many veggies, cut back on them and see if the urine clears up any. Even normal urine will be slightly milky, but not nearly as much as the picture above.