Box 2 – Brief Characterization of the Variations in Geogenic Groundwater Mineralization in the Mega Aquifer Systems (as indicated in consulted documents)
Nubian Aquifer System
According to Bakhbakhi (2006), the mineralization level of groundwater in the Nubian Aquifer System increases from a TDS of 500 ppm in the southern part to hypersaline water in the northern part. The confined part of the Nubian Aquifer System contains approximately 150 thousand km3 of saline water, which is 25 percent of all groundwater in the Nubian Sandstone Aquifer System (Nubian and Post-Nubian combined). According to the OSS (Observatoire du Sahara et du Sahel, or in English, Sahara and Sahel Observatory) in 2020, groundwater is saline in a small part of the Post-Nubian aquifer, next to the Mediterranean Sea
North-Western Sahara Aquifer System
Along the northern margin of the North-Western Sahara Aquifer System (NWSAS), shallow evaporites are present in a west-east running zone of chotts (i.e., shallow saline lakes, often dry during part of the year), some of which are more than 5000 km2 in extent. They threaten to salinize shallow fresh waters, and that risk can be exacerbated by intensive groundwater abstraction (Bryant et al., 1994; Mamou et al., 2006; OSS, 2020).
Senegalo-Mauritanian Basin
In the western part of the Senegalo-Mauritanian Basin, in a 100-150 km wide belt parallel to the coast, the Maastrichtian aquifers (dipping deep in a westward direction) contain saline groundwater of NaCl facies, assumed to be of connate origin. Evaporite dissolution and mixing with seawater from Quaternary transgressions have affected water quality in the Continental Terminal and Quaternary aquifers (IAEA, 2017a). Excessive withdrawals led to high drawdowns in the water table and the risk of marine intrusions in coastal areas (OSS, 2020).
Lake Chad Basin
The electrical conductivity level of groundwater in the Quaternary upper aquifer of the Lake Chad Basin is generally low (IAEA, 2017b), but is quite high in the center of the basin (IAEA, 2017). It is somewhat higher in the Lower Pliocene aquifer (IBRD, 2020). The Cretaceous Lower aquifer – still poorly explored – is highly mineralized (GWP, 2013).
Sudd Basin
Salama (1977) reports that groundwater mineralization in the Sudd Basin gradually increases with depth. Furthermore, it varies laterally from 200 to 500 ppm in the peripheral zones to approximately 5000 ppm in the central part of the basin, where water flow is sluggish. A more recent report (RSS, 2015) mentions that groundwater with TDS between 1500 and 5000 mg/L and sometimes more, is encountered in the north-eastern zone of the basin.
Ogaden-Juba Basin
The total dissolved content of groundwater in the complex aquifer sequences of the Ogaden-Juba Basin is variable, but generally rather high to very high, especially at shallow depths. This is true for the Ogaden, Somalian and East-Kenyan shares of the basin (Pavelic et al., 2012). Kebede and Taye (2020) mention high salinity as one of the constraints to using groundwater from sedimentary aquifers in this region, but they are probably referring mainly to conditions in the shallow aquifers. Steyl and Dennis (2018) mention seawater intrusion in the drier countries and connate saline water affecting the Merti aquifer shared by Somalia and Kenya.
Stampriet-Lower Kalahari Basin
In its south-western part (the Stampriet basin) groundwater mineralization generally increases towards south-western Botswana and the north-western Cape in South Africa (i.e., the Salt Block zone). TDS-values are above 1000 mg/L in most of the Kalahari aquifer and reach values above 5000 in the south-western part of the area. Mineralization levels are generally lower in the artesian Auob and Nossob aquifers; in particular in the Auob aquifer (intercalated between the Kalahari and Nossob aquifers) with TDS values less than 1000 mg/L in more than half of its area (GGRETA, 2016).
Karoo Basin
Groundwater in most of the Karoo Basin is fresh (TDS between 450 and 1000 mg/L). Concentrations of dissolved constituents increase from east to west accompanied by decreasing precipitation. High concentrations are limited to the westernmost and southernmost edges of the basin, with mean TDS in the north-western zone (around 10 percent of the total area) exceeding 3400 mg/L. The water is partly of connate origin. Water quality in the sedimentary sequence is regarded as poorer than in the dolerite dykes, due to longer residence time (Woodford and Chevallier, 2002).
Northern Great Plains Aquifer System
Upper and lower Palaeozoic aquifers of the Northern Great Plains Aquifer System are saline to hypersaline (up to more than 300,000 ppm). Saline water from these confined aquifers seeps upward to lower and upper Cretaceous aquifers. The former are most widespread, with limited freshwater zones and TDS mostly above, or far above, 3000 ppm (up to more than 10,000 ppm). TDS in the upper Cretaceous aquifers is mostly between 1000 and 3000 ppm (USGS, 2021; Betcher, 1995).
Cambrian-Ordovician Aquifer System
Groundwater in large areas of the Cambrian-Ordovician Aquifer System is not used because of high salinity (high dissolved solids content). These areas cover half of the confined part of the aquifer system, including parts of Iowa, Missouri Illinois, Indiana and Wisconsin. Dissolved solids concentration tends to increase drastically with depth in these areas, up to 112,000 mg/L (Wilson, 2012).
California’s Central Valley Aquifer System
Kang and Jackson (2016) report that groundwater salinity in California’s Central Valley Aquifer System typically increases with depth. At depths less than 1000 m, TDS concentrations less than 10,000 ppm are more common than those exceeding 10,000 ppm, while at greater depths the latter are more frequently found. The southern counties have a larger proportion of fresher water (< 3000 ppm) at depths less than 1000m than the northern counties. A maximum dissolved solids concentration of 52,000 ppm has been observed in Fresno County in the San Joaquin Valley.
High Plains Aquifer
From north to south, TDS in domestic wells of the High Plains Aquifer tends to increase from less than 250 to more than 2000 ppm. At greater depths mineralization tends to become higher, especially in zones where underlying deep geological formations contain saline water (DeSimone et al., 2014). In the southern part of the plains, upward movement of saline water from deeper aquifers occurs (Scanlon et al., 2012).
Gulf and Atlantic Coastal Aquifer System
Fresh groundwater is present down to considerable depths and is underlain by salt groundwater in much of the Atlantic and Gulf Coastal Aquifer System. Depths below which chloride concentrations of 5,000 mg/L or greater occur increase with distance from the coast. Nevertheless, below this fresh-saline interface there are also zones of fresh water and of brines (Meisler et al., 1988).
Maranhão Basin
Groundwater of calcium bicarbonate type and good quality prevails in the upper 100 to 150 m below ground surface in the Maranhão Basin. Below that is a zone with a diversity of mixed water types, probably partly resulting from chemical reactions with clay layers. Below 1000 to 1500 m of depth, groundwater is usually saline, in some cases with mineral contents (TDS) exceeding 150,000 mg/L (Rebouças, 1976).
Guarani Basin
Natural groundwater quality is generally good, with low mineralization in most areas of the Guarani Basin. Hydrogeochemical and isotopic data show that formations underlying parts of the Guaraní Aquifer System (SAG) – mostly saline aquitards – contribute to observed salinity and significant trace element increases. Downdip increases in groundwater salinity are observed, with high salinities in the extreme southwest of the SAG in Argentina (Hirata and Foster, 2020).
Arabian Aquifer System
Groundwater mineralization in the Paleogene (Umm-er-Radhuma and Damman formations) and Neogene aquifers of the Arabian Aquifer System increases from approximately 1000 mg/L (at the western edge) to saline and hypersaline near the Arabian Gulf Coast. Salinity is related to upward flow of deep saline groundwater (of connate origin or salinized by dissolution of evaporites) and to evaporation at inland and coastal sabkhas (UN-ESCWA and BGR, 2013).
Indo-Gangetic-Brahmaputra Basin
In the Indo-Gangetic-Brahmaputra Aquifer System, 39, 38, 12 and 11 percent of the total volume in the upper 200 m has TDS of < 500; 500-1000; 1000-2500; and > 2500 mg/L, respectively. Groundwater with TDS > 1000 mg/L underlies 28 percent of the aquifer system area. The lower two-thirds of the Indus Basin aquifer area has predominantly saline groundwater (TDS > 2500 mg/L), partly caused by anthropogenic activities. Most of the Gangetic-Brahmaputra plains have fresh to very fresh groundwater, while brackish to saline groundwater (TDS > 1000 mg/L) is found in the northwestern part of the Ganges basin and in the coastal zone of Bangladesh (MacDonald et al., 2016).
West Siberian Basin
According to Foley and others (1994), in the West Siberian Basin, Oligocene to Quaternary deposits (only a minor fraction of the total sedimentary series) form an unconfined aquifer, whereas the Jurassic to Oligocene-age rocks contain confined aquifers and aquitards. These confined aquifers and aquitards are highly mineralized (with brines in the center of the basin), which is in line with the perception that they receive little recharge. Modern and relict permafrost are present in a large part of the basin (as indicated in Box 1).
North China Plain Aquifer System
Below most of the North China Plain (NCP) Aquifer System’s alluvial plain and coastal strip, the hydrogeological sequence includes a brackish-water aquifer of large geographical extent. It overlies confined freshwater aquifers and is locally overlain by thin lenses of fresher groundwater (Foster et al., 2004). The Quaternary series of the NCP (150 to 600 m thick) contains four superimposed aquifers. Groundwater in the first one (unconfined) and the second one (shallow confined) contain fresh water in the piedmont plain zone, but it becomes saline (TDS > 2,000 mg/L) in the central alluvial and littoral plains. The third and the fourth aquifer (both confined) contain fresh water (TDS of 300-500 mg/L and < 1,000 mg/L, respectively) and form the main target for exploitation (Foster et al., 2003; Su et al., 2018). The brackish-water aquifer (locally overlain by thin lenses of fresher groundwater) is present below most of the alluvial plain and coastal strip. It is of large geographical extent and originates from Quaternary marine transgressions. Modern seawater intrusion occurs near the Bohai Sea coast (Shi and Jao, 2014).
Paris Basin
Fresh groundwater is observed down to 1000 m of depth of the Paris Basin (Bodelle and Margat, 1980). Groundwater in the Middle Jurassic and Triassic aquifers is saline, reaching TDS values exceeding 200,000 mg/L in the deep center of the basin, but salinity decreases to only a few hundred mg/L along the edge of the aquifers (Matray and Chery, 1998). The saline Jurassic (Dogger) and Triassic (Keuper, Rhétien) aquifers are used for the production of geothermal energy (Kloppmann et al., 2010).
Great Artesian Basin
Groundwater of the main tapped Lower Cretaceous-Jurassic artesian aquifers of the Great Artesian Basin is characterized by low salinity at 500-1500 mg/L total dissolved solids in most of the area, but it is saline in large zones in the south-western and central-southern parts of the basin (where TDS increases downgradient along groundwater flowlines). The overlying Cretaceous aquifers have higher TDS and are generally brackish. Groundwater in the deeper aquifers in the Jurassic sequence range from fresh in the northern zone of the area to brackish in most of the south-western and southeastern zones (Ransley et al., 2015; Habermehl, 2020).