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

CHAPTER 1 ■ CONCEPT AND IMPORTANCE OF NON-RENEWABLE RESOURCESTable 2. Hydrogeological concept of groundwater resource renewalPARAMETER SYMBOL DIMENSION UNITS“Groundwater resource renewal is the replacement or displacementof groundwater in aquifer storage through aquifer recharge ”Total drainable aquiferS Lstorage reserves*Mm 3 **Average annual aquiferrecharge rateR L 3 /T Mm 3 /aRate of aquifergroundwater renewalR/S L 3 /TL 3 x 100 %/aRenewal periodof aquiferS/R L 3 T/L 3 year“Renewal is a comparative (not absolute) concept – relative to both aquifer storage and rechargeAND thus subject to very wide variation due to geological factors (aquifer thicknessand drainable porosity) and climatic parameters (especially rainfall regime) ”Notes:* It is recognised that the definition of the term 'drainable aquifer storage' will be rather subjective in practice.** Mm 3 (10 6 m 3 ) also referred to as hm3 in many countries, and km 3 (10 9 m 3 ) also frequently used for large aquifers.L = lenght, T = time.Most importantly the development of non-renewable groundwater resources will imply the‘mining of aquifer storage reserves’ (Table 1), such that they will not be available for future strategicuse. As such it has special social, economic and political sensitivity compared to otherwater resource development.The occurrence of non-renewable resources in a variety of different hydrogeological settingsresults in a number of important consequences for their management :● some non-renewable aquifers sustain (directly or indirectly) important aquatic ecosystems(eg. Figures 1A and B);● non-renewable groundwater can be vulnerable to pollution from inadequately-controlledhuman activities at the land surface, especially the discharge of polluted and/or salinewastewater (eg. as would be the case in Figures 1 A and C);● the clearance of natural vegetation could reactivate deep groundwater recharge under certainconditions (eg. Figure 1C), but would also be associated with the mobilisation of salts●previously accumulating in the vadose zone;if a source of surface water is made available through a major pipeline transfer or othermeans, then it will be possible to artificially recharge many non-renewable aquifers but greatcare is needed to avoid the mobilisation of saline water.The development of a non-renewable groundwater resource usually involves the extraction ofso-called ‘fossil groundwater’ (Table 1), which originated as recharge in past, more humid,climatic regimes. But to speak synonymously about ‘fossil groundwater’ and ‘non-renewablegroundwater resources’ is misleading since there are many aquifer systems containing largevolumes of fossil (usually Holocene) groundwater at depth which if extracted is replaced (orrenewed) by more modern recharge.17