https://phys.org/news/2018-07-deep-groundwater-coastal-deltas-resilient.html

Understanding how deeper groundwater is replenished and how vulnerable it is to contamination from saline and arsenic groundwater from shallow depths is critical to safeguarding public drinking water supplies.

"Our results reveal no modern components in deep groundwater with ages ranging from 1,000 to 10,000 years. Traces of modern groundwater are, however, evident in a few pumping wells where they are associated with short-circuiting of vertical leakage within inadequately sealed boreholes," said lead author, Dr Dan Lapworth (UCL Geography and British Geological Survey).