Editorial on the Research Topic Groundwater systems worldwide Groundwater, with a total volume of 23.4 × 10 6 km 3 , represents 30% of the world's freshwater or 2.5% of the total global water storage. Thus, it is an important area of research and a valuable resource for humankind (Oki and Kanae, 2006). Groundwater is an essential component of the global hydrological and biogeochemical cycles and plays a major role in ecosystems sustainability (