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

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CHAPTER 3 ■ AQUIFER CHARACTERISATION TECHNIQUESFigure 7Vadose zone isotopic and chemical investigations of groundwater recharge in arid regionsAn estimation of local contemporary recharge rates and an indication of the severity of the Sahel drought in the late1960s can be obtained from these deep vadose zone profiles from northern Senegal (Edmunds, 1991 and Enrich,2001). The 3 H peak corresponds to 1963 infiltration and the enrichment in 18 O, together the high Cl concentrationsat shallower depth are a consequence of very low recharge rates and soil fractionation during the drought years.Detailed field research on the applicability of isotopic and geochemical methods in the vadose(unsaturated) zone for groundwater recharge estimation was coordinated by the InternationalAtomic Energy Agency during 1995-99, with results being obtained from 44 sites mainly in aridclimates (IAEA, 2001). Detailed information on the physiography, lithology, rainfall, vadose zonemoisture content and chemical and isotopic determinands were collected at each profiling siteand reliable contemporary recharge estimates obtained (Table 12).Table 12. Summary of physical, isotopic and geochemical investigationsof contemporary groundwater recharge at sites with arid climateCOUNTRYRegionMEANPRECIPITATION(mm/a)VADOSE ZONEDEPTH(m)RAINFALLCl CONTENT(mg/l)MEAN RATEOF RECHARGE(mm/a)JORDANJarashAzraq480672171061282SAUDI ARABIAQasim 133 18 13 2SYRIADamascus Oasis 220 21 7 2–6EGYPTRafaa 300 20 16 18–24NIGERIAMfi 389 10 —
NON-RENEWABLE GROUNDWATER RESOURCESGeochemical and isotopic techniques can also be applied to the study of groundwater in thesaturated zone to identify recharge processes and sources. Age dating through radiometricmethods can provide the groundwater age-gradient down the vertical profile in the saturatedzone which have been used to estimate recent recharge rates. Such an approach using tritium( 3 H) was reported to give an estimate of 3–7 mm/a for the average natural direct recharge to themain aquifer system in Qatar (Yurtsever, 1999), but one of the main assumptions inherent in thelumped parameter modelling approach needed for interpretation is steady-state flow conditionsin the aquifer system and this is often likely to be invalid.Aquifer numerical modelling44Lumped-parameter aquifer models can provide useful bulk assessments of aquifer systems – astheir name suggests they lump each set of aquifer parameters by a single averaged value. Theirapplication to reconnaisance-level investigation of non-renewable groundwater resources is generallyfor rapid assessment of the ‘resource life’ or an indication of the bulk response which canserve as a cross-check for more detailed numerical modelling.Another category of aquifer models are the analytical groundwater radial flow models.These are of great utility at the scale of waterwell testing for local aquifer properties and wellhydraulic diagnosis. They can include consideration of complexities, such as aquifer anisotropyand vertical property variations.In general, however, when reference to aquifer modeling is made it relates to the applicationof distributed-parameter numerical aquifer models, since these are readily capable ofrepresentingthe spatial variations of the aquifer system. The simulation of groundwater flow involves iterativesolution of the relevant partial differential equation, using either finite difference or finiteelements. There is an extensive scientific literature on numerical aquifer modelling and a largenumber of modelling codes/programmes are available, both free and as commercial packages.The user has a very large choice, but the selection of an appropriate code requires a considerableappreciation of the hydrogeological problem to be solved, and the pros and contras of the variouspackages.Initially numerical aquifer models are used to examine the validity of the conceptual modelof the aquifer system. As additional data become available and the aquifer is better characterised,the model can be used to identify regions of greatest uncertainty where exploratorydrilling is most needed. At the aquifer development stage, models are first used to test potentialwellfield configurations and subsequently to optimise their performance (Chaleb, 1999). Systemcharacterisation will inevitably be subject to considerable initial uncertainty and it is thusrecommended that ‘worst-case parameter values’ be selected if numerical aquifer modeling isused as a basis for planning. Once reasonably calibrated there is potential for the outputs ofnumerical aquifer models, which have been subjected to appropriate sensitivity analysis, to beused by water regulatory agencies for resource administration , providing they are capable ofwithstanding legal challenges on the validity of their basic assumptions.In general aquifers respond only slowly to groundwater extraction. And this is well demonstratedthrough numerical modelling analysis of a shared aquifer system in Jordan and SaudiArabia. The model study (Schmidt et al., 1999), which considered the aquifer response to climaticchange over the 5000-year period following the last pluvial recharge in the region, showedthat the aquifer could have been discharging to the Dead Sea at rates of almost 500 Mm 3 /a,
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