No scientist here either! However I'm going by the fact that the underlying geology doesn't change, ie rainwater filters through the same rocks at the same rate today as it did 100 or 1,000 years ago and those rocks are the aquifer which feeds the water supply, so the isotope content should be unchanging, so much so that here in the UK archaeologists can use comparisons between modern and ancient remains to determine where a body grew up. It's used, for example, to map the origins of historic and prehistoric migrants into and around the UK, such as determining which parts of the Roman Empire migrants came from.