It all depends on.. the amount of water you need, the quality of water, contamination by man or natural.
If you need clean drinking water it's a good idea to go through a couple aquifers, to find the cleanest possible water. You could driĺ and hit water pit also a methan pocket at the same time, don't want to drink that. Over time local drillers have accumulated knowledge on there local geology and have a very good understanding where water is.
Additional-Rhubarb-8 OP t1_j9zqqho wrote
Reply to comment by Rubbery_Elbow in how accurate is the greenland ice core oxygen isotope study in regards to earth's climate history ? by Additional-Rhubarb-8
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001GL014580