1.4.1 Criteria

How is the size of an aquifer defined? For those who see an aquifer as a container (the lithological matrix in which drainable groundwater is stored), it seems logical to define the size of an aquifer theoretically as its volumetric storage capacity, which equals the bulk volume of the aquifer rock times its mean specific yield. Specific yield is the fraction of the bulk aquifer volume occupied by water that can be drained by gravity. Estimating aquifer size according to this concept, however, is in practice possible for only very few aquifers in the world. In most cases, the data required for making such estimates with reasonable accuracy are not available because the aquifers are thick or deep, extend over large areas, and/or have been scarcely or only partially explored.

If an aquifer is perceived as a container (matrix) plus its content (groundwater), then one may resort to a more feasible, semi–quantitative approach for comparing or classifying the size of aquifers or aquifer systems. This approach is based on the following parameters: (i) horizontal area covered by the aquifer or aquifer system (in most cases reasonably known) and (ii) mean cumulative thickness of the hydraulically productive aquifer zones included in the vertical lithological sequence. In many cases, the size of an aquifer is only a rough estimate.

Tentatively, the following aquifer size classification is proposed:

• very small: less than 100 km² in horizontal extent;
• small: 100–500 km² in horizontal extent;
• medium: 500–5,000 km² in horizontal extent and with at least 20 m cumulative thickness of productive aquifer zones;
• large: 5,000–50,000 km² in horizontal extent and with at least 50 m cumulative thickness of productive aquifer zones; and,
• very large: more than 50,000 km² in horizontal extent and with at least 100 m cumulative thickness of productive aquifer zones.

If the thickness criterion is not met, then the aquifer is classified one class lower than the class corresponding to its area.

1.4.2 Small Aquifers

In principle, there is no lower limit to the size of an aquifer, but very small permeable bodies in the subsurface – say, 100 km² or less in lateral extent – are rarely identified as a separate aquifer (with a given aquifer name). Notable exceptions include a few small aquifers that span international boundaries:

• the Abbotsford-Sumas Aquifer, sand and gravel of 100 km² extent that provides water supply to 10,000 people in the USA and 100,000 in Canada;
• the Okanagan-Osoyoos Aquifer, multilayer unconsolidated sedimentary aquifer of 25 km² extent shared by Canada and the USA;
• the Grand Forks Aquifer, unconsolidated sediments 34 km² in extent shared by Canada and the USA; and,
• the Genovese Aquifer, Quaternary fluvio-glacial deposits that are 15-40 m thick of approximately 30 km² extent, from which 15-17 mm³/year of groundwater is abstracted and shared by Switzerland and France. It is the first aquifer in the world with a formal international transboundary aquifer management agreement which has been in force since 1978.

These examples show that even very small aquifers can be important, which is highlighted by such aquifers being included in transboundary aquifer publications and/or agendas (Puri and Aureli, 2009; IGRAC, 2015). Also, myriad small aquifers – often unnamed – that are entirely located within one country (domestic aquifers) are important locally. Because they are numerous, usually shallow, and often closely linked to surface water (alluvial aquifers) thus favorably located to sources of recharge, small aquifers provide a significant share of the world’s exploited groundwater. The smallest aquifers are predominantly tapped by self-suppliers, usually for domestic and agricultural purposes; many of them are vulnerable to seasonal depletion, especially in arid regions. Water utilities usually locate their wells in aquifers that are at least a few hundred square kilometers in extent and have sufficient capacity to buffer seasonal and interannual variations in recharge.

1.4.3 Large and Very Large Aquifers/Aquifer Systems

With increasing horizontal extent and thickness, aquifers tend to become more complex, interbedded with several aquitards, and often hydraulically connected to other aquifers that are usually located above or underneath. In such cases the term ‘aquifer systems’ is appropriate. Large aquifer systems play an important role in hydrogeology and as a source of water because together they cover a significant part of the continents and contain huge quantities of groundwater.

Figure 8 shows the approximate location of 70 large or very large aquifers/aquifer systems scattered over the globe. Table 3 lists their names corresponding to the numbers used to identify them in the figure. The selection consists of two distinct sets: a) 37 so-called mega aquifer systems; and, b) 33 other large aquifers or aquifer systems. The mega aquifer systems are considered to be our planet’s largest aquifer systems (these are discussed in Section 2). They contain a large share of all fresh groundwater reserves on earth. Part of the groundwater they store may be of considerable age (i.e., the time elapsed since water entered the aquifer system), up to the order of a million years. The selected ‘other large aquifers/aquifer systems’, although belonging to the category ‘large’ or ‘very large’, do not necessarily represent the next largest aquifer systems. Rather, aquifers have been selected that are large and also rank among the most well-known in their regions, either because of their importance or as an object of investigation. The selection thus is somewhat subjective.

Figure 8 – Mega aquifer systems and selected other large aquifers/aquifer systems around the globe (Table 3 associates names with the numbers) View a larger version of this figure.

Table 3 – Mega aquifer systems and selected other large aquifers/aquifer systems.

Some well-known aquifer systems are not shown by name in Table 3, because they are implicitly included as components of the listed and mapped large aquifer systems. Examples include the Dakota Aquifer (partly) and the Canadian Paskapoo aquifer as components of the Northern Great Plains Aquifer System; the Floridan Aquifer System and the Mississippi Embayment Aquifer System as components of the Gulf and Atlantic Coastal Aquifer System; and the Saq-Ram, Wajid and Umm er Radhuma-Dammam Aquifer Systems as components of the Arabian Aquifer System.

In principle, this book uses the aquifer and aquifer system names used in the consulted literature, except when that use would lead to confusion or when deemed unsatisfactory for other reasons. A few mega aquifer systems have been renamed, as indicated and explained in Section 2. Sections 2 and 3 focus exclusively on the mega aquifer systems.