Think about your morning coffee, smoothie, or liquid vitamin, what is the main ingredient in all of these? Water!
Even if you take the utmost care to only use the highest purity ingredients, if the water is not pure then all of your effort is wasted. Impure water can introduce high amounts of mercury, lead, arsenic, bacteria, and a litany of other contaminants; because it is the largest ingredient, even a small concentration of impurities will quickly add up.
Hence, asking about water quality is just as important as the origin of any other ingredient. You can check on the purity of the water in your supplements by calling the manufacturer or see if they mention it on their website.