Trans-boundary water systems are hot issues as pressures on water resources are enormous especially shared trans-boundary systems. Utilization of surface water and groundwater resources differs in many aspects; also the negative impacts resulted from using these two water resources may differ. While surface water, e.g. rivers and lakes, are renewable, some groundwater are known to be of fossil origin, i.e. not renewable. In general, the rest of all water resources have specific residence times depending on the nature, structure and composition of their drainage areas as well as the hydrology of the systems. Rivers have the shortest residence times, typically few days, while lakes can vary from days or months for small ones up to several decades for large lakes. Sallow gravel aquifers may have residence times of few days, while deep aquifers may have very long residence times of some millions of years. Residence time of water, and particles therein, is simply the average time that a water/particle spend in a given system. Residence times of water have many important engineering applications in particular time-space hydrological, hydro-chemical processes, e.g. water budget, water balance and flow-rates as well as contamination and pollution studies. Also, in other water engineering disciplines such as biological, biochemical, biogeochemical, environmental and geological engineering as well sd rehabilitation actions.
For the Nubian Sandstone Aquifer System (NSAS), shared by Chad, Egypt, Libya & Sudan, which is one of the largest aquifer systems in the world, an agreement were conducted during the 1989-1999 “Constitution of the Joint Authority for the Study and Development of the Nubian Sandstone Aquifer Waters”. This agreement is more an institutional agreement than merely a water management one. However, the outcome of two projects that were conducted following this agreement, 1998-2006, identified key transboundary concerns what regards declining water levels, damage or loss of ecosystems and biodiversity, and deterioration of water quality. Based on these studies common management actions with adopted vision for the NSAS were concluded.