1.3.1 Definitions and Interpretations

According to Theis (1983), the English noun ‘aquifer’ has been derived from the French adjective ‘aquifère’, introduced by Arago (1835), among others, in terms such as ‘couche aquifère’ (water-bearing layer). The term is based on the Latin words ‘aqua’ (water) and ‘ferre’ (to bear). Synonyms in English include water-bearing formation, water-bearing stratum, water-bearing layer and groundwater reservoir. A diversity of aquifer definitions can be found in hydrogeological textbooks and related publications (a selection is presented in Table 1).

Table 1 – Selected Aquifer Definitions

Definition	Source
An aquifer is a rock formation or stratum that will yield water in sufficient quantity to be of consequence as a source of supply.	Meinzer, 1923
Aquifers are permeable geologic formations having structures that permit appreciable water to move through them under ordinary field conditions.	Todd, 1959
An aquifer is a saturated bed, formation, or group of formations which yields water in sufficient quantity to be of consequence as a source of supply.	Walton, 1970
An aquifer is a formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.	Lohman et al., 1972
An aquifer is a geologic formation or group of formations, which (i) contains water and (ii) permits significant amounts of water to move through it under ordinary field conditions.	Bear, 1979
An aquifer is a saturated permeable geologic unit that can transmit significant quantities of water under ordinary hydraulic gradients.	Freeze and Cherry, 1979
An aquifer is a body of rock that is sufficiently permeable to conduct groundwater and to yield economically significant quantities of water to wells and springs.	Bates and Jackson, 1980
An aquifer is a permeable geologic unit that can transmit and store significant quantities of groundwater.	Smith and Wheatcraft, 1992
An aquifer is defined as a single geologic formation or a group of geologic formations that transmits and yields a significant amount of water.	Batu, 1998
‘Aquifer’ means a permeable water-bearing geological formation underlain by a less permeable layer and the water contained in the saturated part of the formation.	International Law Commission, 2008
An aquifer is a hydraulically continuous body of relatively permeable unconsolidated porous sediments or porous or fissured rocks containing groundwater. It is capable of yielding exploitable quantities of groundwater.	Margat and Van der Gun, 2013
An aquifer is a consolidated or unconsolidated (saturated) geologic unit (material, stratum or formation) or set of connected units that yields water of suitable quality to wells or springs in economically usable amounts.	Sharp, 2017

The definitions presented in Table 1 have much in common, but they also reflect differences in views or interpretation among groundwater professionals. As a key characteristic of aquifers, all definitions highlight permeability, or the capacity of aquifers to transmit and yield significant quantities of water. The storage function of aquifers receives less emphasis. Surprisingly, only one of the definitions (Smith and Wheatcraft, 1992) explicitly mentions the capacity of aquifers to store significant quantities of water, and only one (Sharp, 2007) includes water quality as a criterion.

Although the term aquifer has been in use for more than a century, among groundwater professionals there are still important differences in interpretation on several aspects:

• Does the term ‘aquifer’ refer to the lithological matrix (‘container’) only, or does it include also the body of groundwater that fills its interstices (‘content’)?
  There is no consensus on this aspect. Some of the definitions in Table 1 suggest the former (Meinzer, 1923; Todd, 1959; Lohman et al., 1972; Bates and Jackson, 1980; Smith and Wheatcraft, 1992; Batu, 1998) or avoid the issue, while other ones state the opposite (International Law Commission, 2008) or at least demand the presence of groundwater (Walton, 1970; Freeze and Cherry, 1979; Margat and Van der Gun, 2013). Commonly used terms such as unconfined aquifer, confined aquifer, artesian aquifer and fossil aquifer make sense only if the matrix and groundwater are considered together. When talking about a Paleozoic or Mesozoic aquifer, however, or about a sandstone or limestone aquifer, the adjectives refer to the lithological matrix and not to the groundwater in the interstices. In conclusion, there is no generally accepted strict definition of the term ‘aquifer’ and the perception of what it represents depends to some extent on the context in which the term is used.
• The relation between geological formations (stratigraphic units) and aquifers.
  Some groundwater professionals make a strict link between an aquifer and a single geological formation, assuming them to have the same boundaries, even if part of the formation is poorly permeable, or if permeable zones of that formation are in full hydraulic contact with the permeable zone of an adjacent formation. The more common approach among hydrogeologists, however, is to take a hydraulic perspective and consider aquifers as continuous permeable lithological bodies, whose boundaries are defined by contrasts in permeability rather than by stratigraphy, and thus may extend across formation boundaries. Hence, the aquifer may consist of either a single formation or only part of it, or a group of formations or hydraulically continuous parts of them (Lohman et al., 1972: Margat and Van der Gun, 2013).
• Does the water table form the upper boundary of an ‘unconfined aquifer’ or does this type of aquifer extend above the water table and include also the unsaturated zone?
  Authors of reports and other publications on aquifers rarely specify their view on this aspect, but views likely diverge. Among the aquifer definitions in Table 1, only those of Walton (1970) and Freeze and Cherry (1979) link aquifers explicitly to the saturated zone, while the other definitions avoid addressing the issue. If an aquifer is viewed as a container, then it is not illogical to include the unsaturated zone (as long as it is permeable) since it represents the available space to store additional quantities of water and thus potentially may become part of the saturated zone. From a hydraulic point of view, it is convenient to consider a vertically movable water table as the upper boundary of an unconfined aquifer, because the water table marks the divide between substantially different hydraulic regimes.

The intention of presenting these ambiguities is not to make a choice between alternative interpretations or views or to express preferences. The purpose is simply to show the readers that ‘aquifer’ is not an unambiguously defined term, and to make them aware that hydrogeological reports and other groundwater literature may use different interpretations of the aquifer concept, usually without defining it with precision.

Aquifers rarely are completely homogeneous: significant variations can be highlighted by subdividing the aquifer into zones or segments. This can be done either in a vertical sense, for example, by differentiating between layers of different lithological characteristics (analogously to the members of a geological formation), or laterally, for example, based on lateral changes in lithological facies.

Depending on area-specific conditions and on the scale of investigation or mapping, two or more stacked aquifers with intercalated and overlying aquitards may together be called an ‘aquifer system’, as long as they can be considered as interconnected components of one hydraulically continuous system. Table 2 provides a few definitions of aquifer systems. The third definition does not necessarily require the presence of vertically stacked aquifer units, but includes also the option of aquifer systems formed by horizontally connected units. In practice, the distinction between aquifers and aquifer systems is somewhat arbitrary, because distinguishing between aquitards and low-permeability lenses, as well as between permeable and poorly permeable rocks, is subjective. With increasing complexity and size, the term ‘aquifer system’ tends to be preferred.

Table 2 – Selected aquifer system definitions.

Definition	Source
An aquifer system is a heterogeneous body of intercalated permeable and poorly permeable material that functions regionally as a water-yielding hydraulic unit; it comprises two or more permeable beds separated at least locally by aquitards that impede groundwater movement but do not greatly affect the regional hydraulic continuity of the system.	Poland et al., 1972
An aquifer system is a heterogeneous body of intercalated permeable and poorly permeable material that functions regionally as a water-yielding hydraulic unit; it comprises two or more aquifers separated at least locally by confining units that impede groundwater movement but do not greatly affect the regional hydraulic continuity of the system.	Laney & Davidson, 1986 (modification of Poland’s definition)
‘Aquifer system’ means a series of two or more aquifers that are hydraulically connected.	International Law Commission, 2008

Although the majority of what groundwater professionals call an aquifer or aquifer system corresponds to the conceptual description presented above (a hydraulically continuous permeable unit or complex with significant storage capacity), a few somewhat differing variants may be encountered in practice. The first variant consists of assigning a single aquifer name to all occurrences of a single permeable geological formation (or group of formations), although consisting of spatially non-connected parts; these parts show similar hydrodynamic behavior but together they do not form one continuous hydraulic system. An example is the Basin and Range Aquifer System in the US. The second variant assumes that all permeable subsurface rocks present within the boundaries of a river basin form together one single aquifer system. Often such a system – usually named after the corresponding river – is defined without significant information on or knowledge of the permeable rocks included, except for the alluvial sediments directly associated with the river system. An example is the Amazon Basin Aquifer System.

1.3.2 Lithological Aquifer Categories and Aquifer System Settings

Aquifers occur in many different geologic, geographic and climatic settings, which explains the wide diversity of aquifer types and characteristics observed around the globe. Most notable are variations in lithology and geometry (in particular thickness and lateral extent), mainly defined by geological factors. Geography and climate have a major impact on the dynamics of the groundwater bodies inside the aquifers.

Most sedimentary rocks and some igneous and metamorphic rocks are stratified to some degree, whereas most igneous rocks form massive bodies that were intruded or extruded through the stratified rocks (Walton, 1970). Folding and faulting may lead to deformation of originally horizontal or sub-horizontal layers, in exceptional cases (recumbent folds, overthrust faults) even causing older formations to be located on top of younger ones. A distinction can be made between unconsolidated-rock (mainly gravel, sand, silt and clay) and consolidated-rock aquifers. Both classes include rock types that are permeable enough to be aquifers and other rock types that tend to obstruct groundwater flow.

1.3.3 Main Lithological Aquifer Types

Taking lithology as a criterion for classification, the following main categories of aquifers can be distinguished: (1) sand and gravel aquifers; (2) sandstone and conglomerate aquifers; (3) carbonate-rock aquifers (in particular karst aquifers); (4) volcanic rock aquifers; and (5) weathered crystalline and metamorphic bedrock aquifers. A brief description of each of these main categories follows, based on texts such as Todd (1959), Walton (1970), Norum (1973), Freeze and Cherry (1979), Fetter (2001) and Margat and Van der Gun (2013).

Sand and gravel aquifers are widespread and form the most widely exploited aquifers on earth. Most abundant in this category are sands and gravels of fluvial origin (also called alluvial sediments) that can be found in stream valleys, tectonic and intermontane valleys, and on river plains; in downstream zones they usually make way for clays and silts that can form aquitards. Other continental sand-and-gravel aquifers are formed by Pleistocene fluvio-glacial sediments (outwash, glacial fans and lake deltas, eskers, kames, buried valleys) that have been deposited in particular in a significant part of the Northern Hemisphere, together with less permeable glacial tills and fluviolacustrine clays and silts. Dune sands and other sands of aeolian origin consist of rounded grains of uniform size and may form good aquifers, while wind-blown silts (loess) are poorly permeable. Sand formations of marine origin may contain connate saline water (seawater entrapped in the interstices during the formation’s deposition) unless this has been expelled after deposition and replaced by fresh water.

Sandstone and conglomerate aquifers are the consolidated counterpart of sand and gravel aquifers. Their porosity and permeability are in general lower, because of compaction and cementation (part of the original pores has been filled with solid material that cements the grains together). Sandstones represent around 25 percent of all sedimentary rocks on earth (Freeze and Cherry, 1979). Pores facilitate the storage and flow of groundwater in soft, poorly cemented sandstone, while in hard, massive sandstone this role is played by fissures. Flow in many sandstone aquifers is governed by both pores and fissures combined (dual porosity/permeability). Intercalated shales usually function as aquitards. Interbedded layers of coal at shallow depths may behave as aquifers.

Carbonate–rock aquifers consist mainly of limestone and/or dolomite. Like sandstone, limestone occurs in versions ranging from rather soft and porous (chalk) to very hard and dense (massive limestone). The latter qualifies only as an aquifer rock if it has sufficient fissures, fractures and/or karst conduits. Since carbonates are soluble minerals, fissures can become wider over time, which improves the overall porosity and permeability of the formation. In an advanced stage of dissolution, the formation becomes a karst aquifer, characterized by sinkholes, caves and networks of large subsurface conduits that replace surface drainage systems and feed springs, some of them with very high flow rates. In such cases, triple porosity/permeability (pores, fissures, large subsurface conduits) may be present. Karst may also develop in deposits of gypsum or rock salt.

Most volcanic rocks are poorly permeable, but productive heterogeneous volcanic rock aquifers can be found in Cenozoic volcanic rock formations, especially in basalts of Quaternary age. These aquifers, which are scattered over the world’s volcanic massifs, form extensive aquifers on lava plateaus and may occur interbedded in sedimentary basins. Dense lava rock is nearly impermeable, but the formations owe their favorable aquifer properties to blocky rock masses produced by cooling of individual lava flows and to associated gravel interbeds.

Intrusive and metamorphic rocks, outcropping in approximately 30 percent of the area of the continents, are often considered impermeable. Nevertheless, the shallow horizons of these rocks are weathered at numerous locations and thus have storage capacity, which – combined with the transmission capacity of fissures extending to greater depth – results in modestly productive local aquifers. This category of weathered crystalline and metamorphic bedrock aquifers is particularly important for low-cost domestic water supplies in areas where other shallow aquifers are missing.

1.3.4 Aquifer System Settings

Vertical sequences of several permeable sedimentary formations are common and form either heterogeneous aquifers or – if intercalating aquitards are present – multilayer aquifer systems.

The uniqueness of each aquifer or aquifer system arises not only from lithological diversity but also from variations in geographical and geological settings (Figure 3 and Figure 4).

Figure 3 – Two examples of sand and gravel aquifers in different geological settings: a) a hydrogeological cross-section through The Netherlands showing a multi-layer aquifer system in a subsiding geological basin, bordered at its western margin by the North Sea (Dufour, 2000); and, b) a conceptualization of a basin-fill aquifer (USGS, 2000).

Figure 4 – Three lithologically different consolidated aquifer systems in typical geological settings. a) Karstic limestone aquifer with associated land-surface features such as springs, sinkholes, karst valleys and sinking streams (after USGS, 2000). b) A thick complex of overlapping flows of basaltic lava extruded from numerous overlapping shield volcanoes (Southern Idaho). Most flows issued from a central vent or fissure and some are associated with large rift zones in the Earth’s crust (after USGS, 2000). c) Structurally affected groundwater system consisting of sandstones, shale and limestones (after Whitehead, 1996).

Aquifers may be situated along the coast or more inland; in sedimentary basins, in tectonic depressed zones (rift valleys), under plains flanking mountain ranges (piedmont plains), on elevated plateaus, or scattered across mountain areas, often folded or fragmented by faulting; some aquifers are shallow, others are located at great depth and isolated from the active water cycle. Current climatic conditions set their stamp on the dynamics of the groundwater bodies inside aquifers. One product of extreme climatic conditions forms permafrost, observed in polar regions of the Northern hemisphere; frozen soils there prevent groundwater in aquifers from being recharged and thus cause these resources to be non-renewable. Aridity, on the other hand, causes groundwater in other regions (in particular in the Middle East and Northern Africa) also to be non-renewable or only scarcely replenished. In such areas, groundwater development is more likely to become unsustainable than in humid areas or areas with a temperate climate.

Figure 3 and Figure 4 present highly-simplified examples of different types of aquifer systems and their geological setting.

1.3.5 Spatial Patterns of Aquifers and Aquifer Productivity

The spatial distribution of different types of aquifers is not random but follows to a large extent geological macro-structural patterns. This was highlighted by Meinzer (1923) when he divided the territory of the conterminous USA into 21 Groundwater Provinces. Each of these provinces is characterized by a broad uniformity of hydrogeological and geological conditions. The International Groundwater Assessment Centre (IGRAC) adopted this idea of groundwater provinces, applied it on a global scale (resulting in 217 groundwater provinces) and defined additional units at a more aggregated level: Global Groundwater Regions (Van der Gun et al., 2011; Margat and Van der Gun, 2013). The 36 global groundwater regions, each encompassing several groundwater provinces, are shown in Figure 5. Global groundwater regions are less uniform than groundwater provinces, but they still depict a macroscopic pattern of geological environments in which each region can be associated with a certain predominant type of aquifer setting, contrasting with neighboring regions. In this way, a hierarchical system has been created of spatial units related to groundwater systems, all of them identifiable by names and by delineated lateral boundaries. This hierarchy includes, from local to global scale, the following levels: aquifer zones, aquifer segments, aquifers, aquifer systems, groundwater provinces and global groundwater regions.

Hydrogeological maps produced according to the methodology of the United Nations Educational, Scientific and Cultural Organization (UNESCO), described by Struckmeier and Margat (1995), do not delineate discrete spatial groundwater system units but focus on hydrogeological characterization, notably by classifying the hydraulic properties of the subsurface in terms of groundwater productivity and type of interstices. Such maps have been prepared for many areas and countries of the world, as well as for the continents. A compilation in the form of a global map, at a scale of 1:25 million, has been produced and published by the international “World-wide Hydrogeological Mapping and Assessment Programme” or WHYMAP (BGR and UNESCO, 2008). BGR is the German Institute for Geosciences and Natural Resources. A simplified version of WHYMAP’s main map is shown in Figure 6.

Figure 5 – The global groundwater regions as defined by IGRAC (Van der Gun et al., 2011; Margat and Van der Gun, 2013). View a larger version of this figure.

Figure 6 – Simplified version of WHYMAP’s Hydrogeological World Map (BGR and UNESCO, 2008). View a larger version of this figure.

1.3.6 Renewable and Non-Renewable Groundwater Resources

Aquifer recharge corresponds to the inflow of water into an aquifer system¹. The majority of the aquifers located within a few hundred meters of the land surface and containing fresh groundwater are actively recharged, in other words: they contain renewable groundwater resources. Most recharge water comes from natural sources (infiltration of rain, meltwater or surface water), but some regions also enjoy recharge from anthropogenic sources, such as infiltration of excess irrigation water, artificial recharge or pumping-induced recharge. Some aquifers, however, receive very little – if any – recharge due to climatic or geological factors; for that reason, their groundwater resources are called non–renewable. In practice, this qualification is not limited to absolutely zero recharge but is also used in cases of very low recharge. Among groundwater professionals, there is a lack of consensus on clear and numerically consistent criteria for distinguishing between renewable and non-renewable groundwater resources. Non-renewable groundwater may be linked to:

• the rate of contemporary mean annual recharge;
• groundwater age (Döll and Fiedler, 2008: fossil water);
• mean residence time (Margat et al., 2006: > 500 years; Bierkens and Wada, 2019: > 100 years);
• mean residence time combined with low mean annual recharge (Margat and Van der Gun, 2013: > 1000 years and < 5 mm/year, respectively); or,
• transition time required to re-establish hydraulic equilibrium after intensifying groundwater withdrawal (Ferguson et al., 2020: > 50-100 years).

Figure 7 shows the main regions around the globe where aquifers are not or are only weakly replenished due to climatic characteristics. Comparing Figure 7 with Figure 6 reveals that significant parts of the major groundwater basins in Northern Asia are located within a huge zone of continuous permafrost and therefore are not receiving any groundwater recharge. The major groundwater basins of Northern Africa, Southern Africa, the Arabian Peninsula and Australia are all located in arid and hyper-arid regions, which causes their groundwater resources to be non-renewable or weakly renewable. By contrast, aquifer systems containing only non-renewable groundwater resources are less abundant in the Americas, and they are much smaller. None of the very large groundwater basins in the Western Hemisphere belongs to the category of aquifer systems containing only non-renewable groundwater.

Figure 7 – Main regions with only non-renewable or weakly renewable groundwater resources due to climatic characteristics (based on Hanan et al., 2021, and Dolgikh, 2019).

It should be borne in mind that geology is another factor that may cause groundwater to be non-renewable. Many aquifers in deep groundwater basins are confined by impermeable layers and thus effectively disconnected from overlying aquifers and/or from the surface; therefore, they contain non-renewable groundwater. In principle, confining layers at or near the surface may also prevent shallow aquifers from being recharged.

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¹Aquifer recharge appears to be a simple variable, but is complicated in practice by its many potential components (sources) and the variations of omitting/including each particular type of source in the recharge estimate.