along fracture traces. Locating a well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or subsidence of the ground surface that can create a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers as shown in the accompanying image to the left. For example, in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make karst aquifers much more sensitive to groundwater contamination than porous aquifers. In the extreme case, groundwater may exist in underground rivers (e.g., caves underlying karst topography. Fractured If a rock unit of low porosity is highly fractured, it can also make a good aquifer (via fissure flow), provided the rock has a hydraulic conductivity sufficient to facilitate movement of water. Transboundary aquifer When an aquifer transcends international boundaries, the term transboundary aquifer applies. Transboundariness is a concept, a measure and an approach first introduced in 2017. The relevance of this approach is that the physical features of the aquifers become just additional variables among the broad spectrum of considerations of the transboundary nature of an aquifer: social (population); economic (groundwater productivity); political (as transboundary); available research or data; water quality and quantity; other issues governing the agenda (security, trade, immigration and so on). The discussion changes from the traditional question of “is the aquifer transboundary?” to “how transboundary is the aquifer?”. The socio-economic and political contexts effectively overwhelm the aquifer's physical features adding its corresponding geostrategic value (its transboundariness) The criteria proposed by this approach attempt to encapsulate and measure all potential variables that play a role in defining the transboundary nature of an aquifer and its multidimensional boundaries. Human use of groundwater Challenges for using groundwater include: overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, groundwater becoming saline, groundwater pollution. By country or continent Africa Aquifer depletion is a problem in some areas, especially in northern Africa, where one example is the Great Manmade River project of Libya. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the