3.4.1 Benefits of Intensive Groundwater Abstraction

The use of groundwater withdrawn from the mega aquifers produces huge benefits to humanity. In the first place, as a source of domestic water for a large part of the 1.6 to 1.8 billion people that live within the boundaries of the 37 mega aquifer systems. Second, as a source of water for approximately 46 million hectares of groundwater-irrigated agricultural land (40 percent of the global area equipped for groundwater irrigation). Third, as a source of water for a wide gamut of industrial, mining, geo-energy development and other human activities. These services contribute significantly to human health and well-being, as well as to job opportunities and economic development of the areas concerned.

The level and intensity of groundwater withdrawal and use vary enormously among the 37 mega aquifer systems. The top three in this respect are the Ganges-Brahmaputra Basin, the Indus Basin and the Greater North China Plain Aquifer system, with around 600, 300 and 300 million people living within their boundaries, respectively, and with approximately 17, 10 and 4 million hectares of land equipped for groundwater irrigation, respectively. These impressive figures explain their estimated combined share of about two-thirds of the cumulative groundwater abstraction rates of the 37 mega aquifer systems.

The benefits accruing from groundwater are not only related to the volumes of groundwater withdrawn, but also to the invaluable buffer function provided by the mega aquifer systems. A notable example is California’s Central Valley, a major agricultural producer exposed to extended periods of drought, most recently during the periods 2006-2010, 2011-2017 and 2020 until present (2021), where temporary increased groundwater pumping has contributed to reducing damages, together with other measures. As presented in Section 3.2 and Table 7, groundwater abstraction intensity is extremely low for several other mega aquifer systems, which suggests that there may be scope for enhancing their profitable exploitation. However, such a hypothesis can only be confirmed after a thorough analysis of the individual aquifer systems, and their current and potential interactions within the local hydrological setting.

3.4.2 Hydrological Responses to Intensive Groundwater Abstraction

Intensive groundwater abstraction modifies the hydrological regime of an aquifer. Direct hydrological responses include:

• depletion of stored volumes of groundwater (accompanied by declining groundwater levels and pressures);
• intrusion of seawater or water from other hydraulically connected water bodies (not only surface water but also groundwater from overlying or underlying strata)
• reduction of natural groundwater discharge (by springs, baseflows of streams, evapotranspiration and evaporation from shallow water tables, outflow into lakes or the sea); and,
• increased recharge from connected components of the hydrological system (streams, lakes, other hydrogeological units).

These hydrological responses, in turn, have their impacts on human society (in particular on groundwater users) and the environment. Figure 23 lists the most common impacts. Some brief explanatory comments will follow, with selected references to mega aquifer systems for which the impacts have been reported.

Figure 23 – Side-effects of intensive groundwater abstraction including hydrological responses and the main impacts on groundwater services and functions.

3.4.3 Impacts of Intensive Groundwater Abstraction

These impacts vary in scale and perspective, ranging from the level and perspective of individual groundwater users to those of aquifers and interconnected environmental systems.

Declining groundwater levels – either caused by hydraulic interference between neighboring wells or by depletion of stored groundwater on a larger scale – occur in all intensively exploited mega aquifer systems and lead to higher cost of groundwater pumped from wells in the affected zones. This higher cost arises because of the increased height groundwater has to be lifted to bring it to the surface, the decline in well productivity, the need to replace the pump with one of higher capacity, or the need to deepen the well to keep it reasonably productive. Under similar abstraction rates, groundwater levels decline much more quickly in confined aquifers than in unconfined aquifers, especially when initial potentiometric levels reach far above the top of the aquifer and the confining layer is thick and poorly permeable, such as applies to much of the Guarani aquifer (Amore, 2018; Hirata and Foster, 2020).

Furthermore, groundwater users who draw their water from flowing wells, springs or qanats experience a gradual reduction in the yield of their source of water when groundwater levels in their area decline. A clear example is the Great Artesian Basin, where between 1878 and 2000 almost 5,000 flowing artesian boreholes were drilled, of which by the year 2000 only some 2,000 were still flowing, with a total yield less than half that of the 2,000 artesian wells that were in flowing conditions around the year 1920 (Habermehl, 2006). Other mega aquifer systems with large zones of artesian conditions where wells may stop flowing under continued current or increased groundwater abstraction rates include the Nubian Aquifer System, the North-Western Sahara Aquifer System, the Iullemeden Basin, the Lower Kalahari Basin, the Cambrian-Ordovician Aquifer System, the Maranhão Basin, the Guarani Basin and the Paris Basin (Margat and Van der Gun, 2013; Hirata and Foster, 2020). In the Paris Basin, deep artesian wells have tapped groundwater from the Albian greensands since the first half of the 19^th century. Although still flowing, their flow rates have declined over time (Margat et al., 2013; Wikipedia, 2021). Regarding the North-Western Sahara Aquifer System, however, conditions are less favorable. Artesian conditions in the Algerian–Tunisian Chott region are likely to disappear under current groundwater withdrawal rates (OSS, 2008; Gonçalves et al., 2013).

Intensive groundwater abstraction over a wide areal extent may eventually lead to significant aquifer zones (or even entire aquifers) becoming incapable of serving as a source of water supply. Often this will be when static water levels fall below critical depths of economically exploitable groundwater, but in other cases it could be due to physical exhaustion (i.e., insufficient groundwater left for withdrawal). For instance, extrapolations by Scanlon and others (2012) predict that the saturated aquifer thickness in 35 percent of the southern High Plains will be less than 6 m by the year 2040, thus unable to support irrigation. For the Central Valley, these authors calculate that between the 1860s and 2003 14 percent of the estimated groundwater in storage before groundwater irrigation started has been depleted; within the Tulare Basin (southern part of the Valley) groundwater levels have declined more than 30 m in the unconfined and more than 120 m in the deeper confined aquifer. Other mega aquifer systems with a long-term depletion history (as mentioned in Section 3.3) are likely to eventually experience similar problems, leading to either groundwater abstraction for several types of water use becoming economically unfeasible, or certain aquifer zones becoming largely dewatered.

Another form of aquifer degradation triggered by intensive groundwater abstraction is the encroachment of saline or brackish water into certain freshwater aquifer zones. A prominent mechanism in this category is seawater intrusion. Since half of all mega aquifer systems are landlocked, seawater intrusion is a potential risk for no more than 18 of them, and only for coastal zones that in most cases occupy only a minor part of the aquifer system. Nevertheless, seawater intrusion is an important potential side-effect of intensive groundwater abstraction, as reported for several coastal zones of mega aquifer systems, such as the Nubian Aquifer System (Sherif et al., 2012), the Gulf and Atlantic Coastal Aquifer System (Barlow, 2003; Rosenshein and Moore, 2013), the Indus and Ganges-Brahmaputra Basins (MacDonald et al., 2016) and the North China Plain Aquifer System (Shi and Jiao, 2014). More widespread are zones where saline or brackish groundwater of connate, transgression, or terrestrial origin may encroach into the freshwater domains as a result of intensive groundwater abstraction, either by upconing or by lateral migration. Such mineralized groundwater is not only common in coastal areas (e.g., in the northern zone of the Nubian Aquifer System, the western half of the Senegal-Mauritanian Basin, the Gulf and Atlantic Coastal Plains, and the Arabian Aquifer System), but also elsewhere, at greater depths in almost all deep sedimentary basins and numerous zones at shallower depths (Van Weert et al., 2009). The third source of groundwater salinization produced by groundwater abstraction (at least partly) consists of irrigation return flows. These have been enriched in dissolved solids content during the irrigation process and contribute to a gradual increase of salinity and agrochemicals in soils and shallow aquifer zones. This source of salinity and pollution is particularly relevant in aquifer systems intensively tapped for irrigation, such as the Central Valley, the High Plains, the Indus Basin, the Ganges-Brahmaputra Basin and the Greater North China Plain Aquifer System. It is not uncommon for several of these sources of salinization to contribute simultaneously, sometimes in combination with other mechanisms unrelated to groundwater abstraction, such as flooding by seawater, dissolution of evaporite layers or high rates of evaporation at the land surface.

As indicated in Figure 23, intensive groundwater abstraction also has environ­mental impacts. In the first place, it produces reduction of natural groundwater discharge, which leads to the degradation or disappearance of wet environmental features such as baseflows of streams, spring flows, wetlands, oases and sabkhas. The Great Artesian Basin, for instance, feeds more than 460 groups of springs, most of them in the marginal areas of the basin, essential for maintaining groundwater-dependent ecosystems with unique fauna and flora. Continued groundwater exploitation by wells has reduced the discharge of many of these springs over time. Many of the springs are therefore protected now under the Environment Protection and Biodiversity Conservation Act of 1999 (Australian Government, 2018; Habermehl, 2020). Other flux-related impacts of intensive groundwater abstraction are the degradation of oases in northern Africa, for example, in the North-Western Sahara Aquifer System (Corsale, 2009; Sghaier, 2010), and wetland degradation, for example, the Azraq wetland in Jordan, at the northern edge of the Arabian Aquifer System (Molle et al., 2017). In areas where crops benefit from shallow water tables for their continuous subsurface water supply, such as in most of The Netherlands, even minor declines of the water table by groundwater abstraction may lead to damage, in the form of crop yield reduction. Intensive groundwater abstraction may, in exceptional cases, lead to positive environmental impacts, such as the reduction of water-logged land area in the Indian state of Haryana (Indus Basin).

A rather different environmental impact of intensive groundwater abstraction is land subsidence. Rather than being related to reduced fluxes, it is triggered by declines of pore water pressures in compressible formations, such as Quaternary clays. It is a major impact of intensive groundwater abstraction in the Central Valley of California (Faunt et al., 2015) and in the North China Plain (Guo et al., 2015), The subsidence-affected area in the North China Plain extends over approximately 120,00 km² (Gong et al, 2018), half of which had subsided more than 200 mm by the year 2010 (Zheng et al., 2010). Other man-induced forms of land subsidence may simultaneously take place, caused by activities such as land drainage, construction works and the development of oil or gas. Hydrocarbons have been discovered at depth in several of the sedimentary basins hosting mega aquifer systems, and in a number of them these energy resources are intensively exploited (e.g., in the Arabian Aquifer System, the US Gulf Coast and offshore, and the West-Siberian Basin).