The study looked at water in the Mekong Delta to understand where it comes from and how it moves. They found that the isotopes of oxygen and hydrogen in the water change depending on the season and how close the water is to the sea. Rainfall and river flow affect the isotopes and salt levels in the water. Evaporation is important in changing the isotopes, while mixing with seawater doesn't have a big impact. Groundwater in different areas has different isotopes and salt levels, showing where it comes from and if seawater is mixing in. Shallow groundwater mostly comes from rain and rivers, while deep groundwater might be very old water with little seawater mixing.