Non-renewable groundwater resources: a ... - unesdoc - Unesco

Chapter 1Concept and importanceof non-renewable resourcesJean Margat, Stephen Foster and Abdallah Droubi‘Non-renewable groundwater’ as a conceptGroundwater resources are never strictly non-renewable. But in certain cases the period neededfor replenishment (100s or 1000s of years) is very long in relation to the normal time-frame ofhuman activity in general and of water resources planning in particular. In such cases it makespractical good sense to talk in terms of ‘non-renewable groundwater resources’ (Table 1). Somehydrogeolgical settings which are often associated with non-renewable groundwater are illustratedin Figures 1 A-D.In nature aquifers have a capacity for both water storage and water flow, and thus groundwatersimultaneously accumulates and circulates – the volume of groundwater stored in someof the larger aquifer systems is huge and represents some 97% of our planet’s freshwaterresources (excluding that locked as ice in the polar regions).Groundwater resource renewal is a concept that derives from a comparison between thenatural flow and storage of aquifer systems (Table 2). The range in nature is extreme withrenewal periods of less than 10 years to more than 100,000 years. In cases where present-dayaquifer replenishment is very limited but aquifer storage is very large, the groundwater resourcecan thus be termed non-renewable (Table 1).‘Non-renewability’ does not necessarily imply that the aquifer system is completely withoutreplenishment or entirely disconnected from processes at the land surface (since absolutely zerorecharge is extremely rare). ‘Renewal periods’ (Table 2) are necessarily an approximate averagefor the aquifer under consideration, and may conceal a large range of local variation within thehydrodynamics of the aquifer flow regime – for example with more rapid ‘turnover’ of groundwaterin the upper horizons of a thick aquifer and ‘essentially stagnant’ groundwater at greaterdepth. Moreover, both the drainable storage of an aquifer system and its long-term average rateof recharge are difficult to estimate with precision, and thus it may sometimes be difficult inpractice to distinguish between essentially non-renewable and weakly replenished groundwaterresources.13