8.2 Solutions Pertinent to Exercises Related to Section 2
Solution Exercise 5
None of the mega aquifer systems are located in shield regions, large igneous provinces or high-relief folded mountain zones (although the Guarani Basin sediments are covered by volcanic rocks). Such structural zones are less suitable for the accumulation of large volumes of porous sediments.
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Solution Exercise 6
Aquifer systems with their oldest sediments being of Infra-Cambrian age include the Taoudeni-Tanezrouft Basin and the Russian Platform Basin. Aquifer systems with their oldest sediments of Cambrian age include the Nubian Aquifer System, North-Western Sahara Aquifer System, Murzuk Djado Basin, Iullemeden-Irhazer Aquifer System, Cambrian-Ordovician Aquifer System, Arabian Aquifer System, Tunguss Basin, Angara-Lena Basin and the Yakut Basin. Almost all these mega aquifer systems are located in Northern Africa or the northern part of Eurasia.
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Solution Exercise 7
- ‘Stored groundwater volume’ includes all water present in the subsurface below the groundwater table. In other words: the stored groundwater volume includes all subsurface water at pressure exceeding atmospheric pressure.
- The term ‘groundwater reserves’ refers to the theoretically recoverable part of the total stored groundwater volume; it excludes water that cannot flow freely under gravity (because it remains adsorbed within the interstices of the solid matrix or it is locked inside impermeable formations), and usually, it also excludes groundwater of such a quality that it is unsuitable for human use.
- Due to technical, economic and environmental constraints only a fraction of the groundwater reserves can be exploited in practice. The volume of this fraction is called ‘exploitable groundwater reserves’. The constraints are time-dependent and case-specific, and partly depend on policy, thus the corresponding volumes are subjective.
- The term ‘exploitable groundwater resources’ takes into account both the groundwater reserves and groundwater recharge. It is usually expressed as the maximum volume of groundwater that can be abstracted on an annual basis, given physical, economic and environmental constraints. Annually exploitable groundwater resources of an aquifer usually correspond – if sustainable groundwater exploitation is pursued – to a fraction of the long-term mean annual recharge volume.
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Solution Exercise 8
The eight mega aquifer systems in the Northern half of Africa are all visible in Figure 11. In terms of mean equivalent water depth, the top three systems in the table coincide with the top three systems in the figure: the Nubian aquifer system, the North-Western Sahara Aquifer System and the Senegal-Mauritanian Basin (which has a rating similar to the Murzuk-Djado Basin in Figure 11). The lowest two systems in the figure (Iullemeden and Taoudeni-Tanezrouft aquifer systems) are among the lowest three in the table (Lake Chad, Taoudeni-Tanezrouft and Iullemeden). The rankings thus are in reasonable agreement, but the estimated values diverge, in some cases by an order of magnitude (Lake Chad). This is no surprise, given the arbitrary assumptions underlying the estimates.
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Solution Exercise 9
The so-called ‘historical estimates’ were obtained by allocating the total global storage estimates of Nash and Korzun entirely to the 37 mega aquifer systems, with shares in proportion to their areas. The implicit assumption is that no groundwater storage would be present outside these mega aquifer systems, thus groundwater storage would not occur under three-quarters of the global land surface. This is a very unrealistic assumption thus the results are not useful.
The second category, called ‘regional estimates’, includes estimates reported by researchers who studied individual mega aquifer systems or who cited the outcomes of such studies. Some of the papers mention only the estimate, others present extensive additional information on the aquifer system geology and hydraulic parameters. This category of estimates is likely to be based on area-specific information and knowledge, which contributes to confidence in the outcomes. There are a few flaws due to mismatches between areas: either by adopting an estimate for the mega aquifer system that pertains to a larger area of which it is only a part (e.g., the California estimate was assigned to the Central Valley, the Sahara estimate was assigned to the NWSAS), or by attributing an estimate made for only part of the mega aquifer system to the entire system.
The third category is called ‘revised estimates’, but these are not based on aquifer-specific information and knowledge, except for values of the area covered by the aquifer systems. There are two sub-categories, calculated according to the paper’s equations 6 and 7, respectively. The former derives the mean specific yield of the aquifer from a soil map (which is meaningless, because geology varies with depth and soil maps reflect the top meter of material); and the latter assumes an average porosity of 0.01 over the total depth range for all aquifer systems considered. Values determined using equation 6 are calculated for a 200 m aquifer thickness along with minimum, mean and maximum specific yield values; while those determined using equation 7 are calculated for aquifer thicknesses of 20, 50, 100, 200, 500 and 1000 m. It is not clear why the authors use the term ‘revised estimates’ given that the estimates for each aquifer vary over two orders of magnitude, and there is no clear link with aquifer characteristics.
In summary, unlike what the authors seem to suggest, the so-called ‘regional estimates’ are the better estimates of those presented in the paper.
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Solution Exercise 10
Pronounced examples of systems with shallow brackish or saline groundwater overlying fresh groundwater are the North-Western Sahara Aquifer System (NWAS), the North China Plain Aquifer System (NCP) and the Great Artesian Basin (GAB).
The Chott region in the northern part of the NWSAS is characterized by salt flats (chotts), discontinuously fed by scarce rainfalls, and present-day formation of evaporites. Consequently, shallow groundwater in this region is saline and forms a potential risk if intensive exploitation of the underlying fresh artesian aquifers would lead to a reversal of the vertical flow component from upward to downward.
The Quaternary aquifer system of the NCP is traditionally divided into shallow and deep aquifers. The shallow aquifer (near the coast hundreds of meters thick) is largely filled with brackish water of recent and paleo-marine origin in the zones of coastal and alluvial plains, but not in the piedmont zones. The deeper aquifers, recharged only in the piedmont zones, contain fresh groundwater.