{"id":53,"date":"2022-12-27T14:50:07","date_gmt":"2022-12-27T14:50:07","guid":{"rendered":"https:\/\/books.gw-project.org\/large-aquifer-systems-around-the-world\/chapter\/recoverable-volumes-of-stored-groundwater-groundwater-reserves\/"},"modified":"2022-12-29T01:13:52","modified_gmt":"2022-12-29T01:13:52","slug":"recoverable-volumes-of-stored-groundwater-groundwater-reserves","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/large-aquifer-systems-around-the-world\/chapter\/recoverable-volumes-of-stored-groundwater-groundwater-reserves\/","title":{"raw":"2.4 Recoverable Volumes of Stored Groundwater: Groundwater Reserves","rendered":"2.4 Recoverable Volumes of Stored Groundwater: Groundwater Reserves"},"content":{"raw":"
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2.4.1 Total Stored Volume Versus Groundwater Reserves<\/h1>\r\n

Before focusing on the recoverable volumes of groundwater stored in the mega aquifer systems, it is useful to give some thought to what is meant by the term \u2018volume of stored groundwater<\/em>. The most obvious interpretation is that this includes all water present in the interstices of the saturated rock formations in the area or aquifer system concerned. In its most simple form: the volume of groundwater storage equals the bulk volume of saturated rock times mean porosity. Although usually not explicitly mentioned, this seems to be the idea underlying the various estimates of the global volume of groundwater made since the 1960s (e.g., Nace, 1969; NRC, 1986; Shiklomanov and Rodda, 2003; Margat and Van der Gun, 2013; Gleeson et al., 2016). These estimates are no more than educated guesses, based on generic assumptions about the subsurface rather than on area-specific geological data and information, but they have contributed to a widely accepted belief among the global groundwater community that the total volume of groundwater on Earth is approximately 23\u00a0million\u00a0km3<\/sup>, of which between 8 and 9\u00a0million\u00a0km3<\/sup> is fresh.<\/p>\r\n

However, one should be aware that a considerable share of all groundwater cannot be abstracted by wells, nor drained under gravity, due to matrix forces that keep a fraction of the water trapped in the pores of saturated formations (represented by the parameter specific retention<\/em>). These matrix forces are particularly strong and effective in fine-grained formations such as clays and shales that form aquitards. For this reason, more relevant than the total volume of groundwater is the \u2018theoretically recoverable groundwater volume<\/em>\u2019 (or \u2018groundwater reserves\u2019<\/em>), which is calculated as the product of the bulk volume of saturated rock and its specific yield. Estimates of stored groundwater volumes made in the framework of regional or aquifer-specific studies commonly are based on this interpretation of groundwater storage; although often not explicitly mentioned, this can usually be concluded from the text or data presented in the report or paper. When producing their estimates for aquifer systems that contain both aquifers and aquitards, hydrogeologists usually disregard the aquitards, since their contribution to the total theoretically recoverable volume of groundwater is minimal. Permafrost in several of the Russian mega aquifer systems creates special conditions that significantly reduce the aquifer volume occupied by drainable groundwater, as illustrated in <\/a>Box 1<\/span><\/a>. In practice, only a minor part of the groundwater reserves is available for exploitation, due to technical, financial and water quality constraints as well as the need to avoid undesired side effects such as land subsidence and harmful impacts on surface water and aquatic ecosystems (Alley, 2007).<\/p>\r\n\r\n

2.4.2 Assessing Groundwater Reserves<\/h1>\r\n

The volume of recoverable groundwater stored in the mega aquifer systems is difficult to assess, given the vast extent and considerable thickness of the systems. Nevertheless, several scientists attempted to do so. For instance, MacDonald and others (2012) have mapped groundwater storage in Africa, based on their assessment of saturated aquifer thickness and effective porosity (Figure 11). The African mega aquifer systems are visible on this map. The authors estimate total recoverable groundwater storage in Africa to be 0.66\u00a0million\u00a0km3<\/sup>, and the greatest mean equivalent water depths (>\u00a025\u00a0m) are found in the Nubian Aquifer System, the North-Western Sahara Aquifer System, the Murzuk-Djado Basin, the Senegalo-Mauritanian Basin and the Lower Kalahari-Stampriet Basin. Richey and others (2015a) estimated recoverable groundwater storage for all 37 mega aquifer systems considered in this book, but their estimates seem to be poorly underpinned by geological data and they are presented for each aquifer system in the form of minimum and maximum values that differ mostly by two orders of magnitude. Therefore, for a closer look at groundwater reserves, the focus here will be on estimates produced by hydro\u00adgeologists investigating individual mega aquifer systems and assumed to be familiar with the geology of these systems.<\/a><\/p>\r\n

\"Map<\/p>\r\n

Figure <\/strong>11<\/strong>\u00a0<\/strong>-<\/strong>\u00a0<\/strong>Estimated equivalent depth of recoverable water stored in Africa\u2019s aquifers (MacDonald et al., 2012).<\/p>\r\n

Groundwater assessments of aquifers and aquifer systems only rarely include estimates of recoverable groundwater storage. Consequently, reports and publications presenting such estimates are not abun\u00addant, and therefore estimates of recoverable groundwater storage have only been found for approximately half of the mega aquifer systems, in some cases including only parts of these systems. These estimates are presented in Table 6, expressed in thousands of cubic kilometers (third column), and\u00a0\u2013\u00a0to facilitate comparison and analysis\u00a0\u2013\u00a0converted (in the fourth column) to an equivalent mean depth of water over the entire horizontal area occupied by the aquifer system. Although detailed explanations are missing in the majority of the publications, it is assumed that all estimates refer to fresh groundwater reserves stored in aquifers.<\/p>\r\n

<\/a>Table\u00a0<\/strong>6<\/strong>\u00a0<\/strong>-<\/strong>\u00a0<\/strong>Estimates of theoretically recoverable groundwater storage (\u2018groundwater reserves\u2019) for selected mega aquifer systems (or parts of these systems).<\/p>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
#<\/strong><\/td>\r\nAquifer System<\/strong><\/td>\r\nGroundwater\u00a0storage<\/strong><\/td>\r\nSource<\/strong><\/td>\r\nComments<\/strong><\/td>\r\n<\/tr>\r\n
10<\/strong>3<\/strong><\/sup>\u00a0km<\/strong>3<\/strong><\/sup><\/td>\r\nEquivalent depth<\/strong>\u00a0(m)<\/strong><\/td>\r\n<\/tr>\r\n
1<\/td>\r\nNubian Aquifer System (NSAS)<\/td>\r\n457<\/td>\r\n208<\/td>\r\nBakhbakhi, 2006<\/td>\r\nOnly fresh groundwater (saline water\u00a0=\u00a068\u00a0m)<\/td>\r\n<\/tr>\r\n
<\/td>\r\n542<\/td>\r\n247<\/td>\r\nSalem, 2005<\/td>\r\nOnly fresh groundwater<\/td>\r\n<\/tr>\r\n
<\/td>\r\n500<\/td>\r\n227<\/td>\r\nOSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
2<\/td>\r\nNW Sahara (NWSAS)<\/td>\r\n31<\/td>\r\n30<\/td>\r\nBaba Sy, 2010<\/td>\r\n<\/td>\r\n<\/tr>\r\n
<\/td>\r\n60<\/td>\r\n59<\/td>\r\nZektser & Everett, 2004; OSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
3<\/td>\r\nMurzuk-Djado Basin<\/td>\r\n4.8<\/td>\r\n11<\/td>\r\nSalem, 2005; OSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
4<\/td>\r\nTaoudeni-Tanezrouft<\/td>\r\n10<\/td>\r\n3.0<\/td>\r\nOSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
5<\/td>\r\nSenegalo-Mauritanian<\/td>\r\n9\u00a0\u2013\u00a010<\/td>\r\n30\u00a0\u2013\u00a033<\/td>\r\nDiagana, 2005b<\/td>\r\n<\/td>\r\n<\/tr>\r\n
<\/td>\r\n1.5<\/td>\r\n5.0<\/td>\r\nOSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
6<\/td>\r\nIullemeden-Irhazer<\/td>\r\n4.95<\/td>\r\n7.8<\/td>\r\nBaba Sy, 2010; OSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
7<\/td>\r\nLake Chad Basin<\/td>\r\n5.8<\/td>\r\n3.0<\/td>\r\nOSS, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
8<\/td>\r\nSudd Basin<\/td>\r\n11.5<\/td>\r\n32<\/td>\r\nRSS, 2015<\/td>\r\nArea 432,700\u00a0km2<\/sup><\/td>\r\n<\/tr>\r\n
16<\/td>\r\nCentral Valley<\/td>\r\n0.86<\/td>\r\n16.5<\/td>\r\nScanlon et al., 2012<\/td>\r\nPredevelopment (1860s): 1000\u00a0km3<\/sup> (19.2\u00a0m)<\/td>\r\n<\/tr>\r\n
<\/td>\r\n1.02<\/td>\r\n19.6<\/td>\r\nKang & Jackson, 2016<\/td>\r\nFresh in upper 1000\u00a0m (TDS\u00a0<\u00a03000 ppm)<\/td>\r\n<\/tr>\r\n
<\/td>\r\n2.7<\/td>\r\n51.9<\/td>\r\nKang & Jackson, 2016<\/td>\r\nFresh down to 3000\u00a0m (TDS\u00a0<\u00a03000 ppm)<\/td>\r\n<\/tr>\r\n
17<\/td>\r\nHigh Plains<\/td>\r\n3.67<\/td>\r\n8.2<\/td>\r\nScanlon et al., 2012<\/td>\r\nPredevelopment (1950s): 4000\u00a0km3<\/sup> (8.9\u00a0m)<\/td>\r\n<\/tr>\r\n
19<\/td>\r\nAmazon Basin<\/td>\r\n32.5<\/td>\r\n21.7<\/td>\r\nRebou\u00e7as, 1999<\/td>\r\nOnly Cenozoic strata<\/td>\r\n<\/tr>\r\n
20<\/td>\r\nMaranh\u00e3o Basin<\/td>\r\n17.5<\/td>\r\n25.0<\/td>\r\nRebou\u00e7as, 1999<\/td>\r\n<\/td>\r\n<\/tr>\r\n
21<\/td>\r\nGuarani Basin<\/td>\r\n40<\/td>\r\n33.5<\/td>\r\nTujchneider et al., 2010<\/td>\r\n<\/td>\r\n<\/tr>\r\n
<\/td>\r\n30<\/td>\r\n25.2<\/td>\r\nHirata & Foster, 2020<\/td>\r\n<\/td>\r\n<\/tr>\r\n
22<\/td>\r\nArabian Aquifer System<\/td>\r\n253<\/td>\r\n171<\/td>\r\nChowdhury & Zahrani, 2013; Frenken, 2009<\/td>\r\nWater Atlas of Saudi Arabia (only share of Saudi Arabia)<\/td>\r\n<\/tr>\r\n
<\/td>\r\n338<\/td>\r\n228<\/td>\r\nChowdhury & Zahrani, 2013; Frenken, 2009<\/td>\r\nMinistry of Planning SAU (only share of Saudi Arabia)<\/td>\r\n<\/tr>\r\n
23<\/td>\r\nIndus Basin<\/td>\r\n10.4<\/td>\r\n33<\/td>\r\nMacDonald et al., 2012<\/td>\r\nOnly upper 200\u00a0m of 3000\u00a0m of sediments<\/td>\r\n<\/tr>\r\n
24<\/td>\r\nGanges-Brahmaputra Basin<\/td>\r\n19.6<\/td>\r\n33<\/td>\r\nMacDonald et al., 2012<\/td>\r\nOnly upper 200\u00a0m of 3000\u00a0m of sediments<\/td>\r\n<\/tr>\r\n
29<\/td>\r\nGreater N. China Plain<\/td>\r\n3.96<\/td>\r\n29<\/td>\r\nCao et al., 2013<\/td>\r\nOnly Hai Plain (136,000\u00a0km2<\/sup>)<\/td>\r\n<\/tr>\r\n
32<\/td>\r\nParis Basin<\/td>\r\n>\u00a00.68<\/td>\r\n>\u00a03.6<\/td>\r\nK\u00f6nig, 2015 & LeCompa, 2010<\/td>\r\nOnly Lower Cretaceous Albien (425\u00a0km3<\/sup>) and N\u00e9ocomien (230\u00a0km3<\/sup>), plus Tertiary Nappe de Beauce (20\u00a0km3<\/sup>)<\/td>\r\n<\/tr>\r\n
<\/td>\r\n0.70<\/td>\r\n4.0<\/td>\r\nSIGES, 2021<\/td>\r\nAlbien and N\u00e9ocomien<\/td>\r\n<\/tr>\r\n
36<\/td>\r\nGreat Artesian Basin<\/td>\r\n87<\/td>\r\n51<\/td>\r\nHabermehl, no date<\/td>\r\nEarliest estimate<\/td>\r\n<\/tr>\r\n
<\/td>\r\n64.9<\/td>\r\n38<\/td>\r\nHillier & Foster, 2002<\/td>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

2.4.3 Comparison and Analysis of the Estimates of Groundwater Reserves<\/h1>\r\n

Purposefully, Table\u00a06 shows alternative estimates for several of the aquifer systems. This is done not only to avoid choosing between alternatives without careful analysis but more to emphasize that the estimates are subject to a large degree of uncertainty, caused by the scarcity of geological data and the unavoidable subjectivity of their hydrogeological interpretation. Despite the uncertainties, the set of estimates suggests that most of the mega aquifer systems have groundwater reserves equivalent to a mean depth of water of tens to hundreds of meters. The area-weighted average equivalent thickness for all the mega aquifer systems is 69\u00a0m. That value would certainly have been 10 to 20 percent more if the total thickness of the Amazon, Indus and Ganges-Brahmaputra basins had been included in the table.<\/p>\r\n

By extrapolation one may estimate that the groundwater reserves of the 37 mega aquifer systems together are between 2 and 3\u00a0million\u00a0km3<\/sup>. Among the mega aquifer systems listed in Table\u00a06, the largest reserves are present in order of their magnitude in the:<\/p>\r\n\r\n