A layer of rock that is sufficiently porous to store water, and permeable enough to allow water to flow through it, is called an aquifer. Consolidated porous and permeable rocks, for example, sandstone and limestone, can form important and extensive aquifers (e.g. Figure 15). Unconsolidated sands and gravels may also be good aquifers because they are relatively porous and very permeable, but in the UK their saturated thickness is usually quite small and they have limited storage, so they are not important aquifers.
Although the porosity of an aquifer is a measure of the amount of water stored within the pores or fissures, it does not provide a direct measure of the amount of water that may be recovered by pumping or drainage. This is because a proportion of the water is always retained around the individual grains by surface tension, and this is known as the specific retention.
The specific yield is the maximum amount of water that can be recovered. Figure 16 illustrates how specific yield, specific retention and porosity vary with grain size for unconsolidated sediments. The relationship between specific yield, specific retention and porosity is expressed by the equation:
All three terms in the equation are expressed as percentages of the total volume of the rock. The specific retention decreases with increasing grain size in unconsolidated sediments (Figure 16).(A few large particles would have a smaller total surface area than a lot of smaller particles occupying the same volume, and a smaller surface area retains less water by surface tension.) This means that less water is retained in coarse-grained sediments. However, the specific yield is greatest for medium-grained sediments (sands), rather than for coarse-grained sediments, because porosity decreases with increasing grain size.
The exploitable storage of water in an aquifer is the volume of water it will yield:
where V is the volume of the aquifer that is being exploited, and Y the specific yield. It is important to distinguish between the specific yield (a percentage of the volume of the rock) and the exploitable storage (a volume of water).
Pumping from an unconsolidated aquifer lowered the water table by an average of 5 m over an area of 8 × 105 m2.
a.If the porosity of the aquifer averages 37% and the specific retention is 7%, calculate the specific yield of the rock.
b.From this value, calculate the volume of water that was actually removed.
c.From Figure 16, what type of rock is indicated by the data in (a)?
a.specific yield = porosity − specific retention = (37 − 7)% = 30%
b.Essentially, 'free' water was removed from an area of 8 × 105 m2 of water-saturated rock to a depth of 5m; a volume of 8 × 105 × 5m3. Therefore, using Equation 3.5:
volume removed (the exploitable storage)
= (8 × 105 × 5 × 30/100)m3
= 1.2 × 106 m3
c.Figure 16 suggests that the aquifer consists of fine gravels.
There are two types of aquifer, unconfined and confined, distinguished on the basis of their geological location in relation to the position of the saturated zone.
Unconfined aquifers crop out at the ground surface. The water table is the top of the saturated zone in an unconfined aquifer, and water normally has to be pumped to the surface except where the water table actually intersects the surface of the ground and forms a spring (Figure 17). A thin impermeable layer sometimes occurs locally in an aquifer, and this may support a small perched aquifer, separated from the main water table (see Figure 17).
Confined aquifers are separated from the ground surface by an impermeable layer (as in most of Figure 18) and are generally at greater depths than unconfined aquifers. Pressures in confined aquifers may be sufficient for the water in wells that penetrate the aquifer to discharge naturally at the surface without pumping.
Water in confined aquifers is called artesian water and a well that penetrates a confined aquifer is called an artesian well. The height to which water will rise in a well is called the potentiometric level and the potentiometric surface is an imaginary surface joining the potentiometric levels for a confined aquifer. For an unconfined aquifer, the potentiometric surface is the water table. The gradient of the potentiometric surface in a confined aquifer can be used to calculate groundwater flow rates, just as water table gradients are used to work out flow rates in unconfined aquifers using Darcy's law (Equation 1). The potentiometric surface is usually curved, with a convex upper slope, because the saturated thickness is decreasing in the direction of groundwater flow and so the hydraulic gradient has to steepen to maintain constant flow through a smaller saturated thickness. Even if the head is insufficient for water to rise to the surface, water in artesian wells rises above the top of the aquifer.
For artesian pressure to be maintained, the water that flows from the well must be replaced by water that infiltrates into the aquifer where it crops out and is thus unconfined (that is, the same aquifer can be confined in one area and unconfined in another — see Figure 18). This area of outcrop is called the recharge area of the aquifer.
Naturally flowing artesian springs occur where the potentiometric surface is above the ground surface. An oasis in the desert is a natural spring, where groundwater is discharged at the surface (Figure 19). Oases can occur where the crest of a fold in a confined aquifer is intersected by the ground surface, or as an artesian spring where water can rise to the surface along a fault where the potentiometric surface is at or above ground level (Figure 20). The water discharged at oases is often recharged in mountainous areas, which may be a great distance away. Therefore the groundwater in large, confined aquifers may be of considerable age; i.e. much time has elapsed since the water fell as rain. An example is the Nubian sandstone aquifer, a confined aquifer that underlies a large part of northern Africa. Artesian water in this aquifer has been dated using carbon isotopes, giving ages of up to 40,000 years. Such dates make it possible to calculate groundwater speeds and, from Darcy's law, the hydraulic conductivity of the aquifer.
Because groundwater flows so slowly, the water in large artesian aquifers is not always renewed as fast as it is extracted. Also, the water may have accumulated when the climate was very different: the oldest Nubian artesian water, for example, fell as rain at a time when the climate was cooler and wetter. Since then the area has become much warmer and drier, so the aquifer is recharging much more slowly. Under such circumstances, water in some large artesian aquifers in semi-arid regions is in practice a non-renewable resource.
All the same, in the short term, it is possible to 'mine' water from an aquifer (which will be replaced very slowly or not at all), because the volume of water stored in an aquifer is usually large compared with the amount being removed. In the longer term, however, mining water will have adverse effects, including a lowered water table, a need for more expensive pumping from greater depths, a reduced flow to springs and rivers, and possibly a deterioration in groundwater quality.
In general, the rate at which water is removed from an aquifer should not be allowed to reduce the average water table level or have other adverse effects. The maximum quantity of water that can be safely removed from an aquifer annually is termed the safe yield for the aquifer. The consequences of extraction of groundwater above the safe yield from the aquifer below London are examined in Box 2.
Box 2 The fall and rise of groundwater under London
London is underlain by a confined aquifer, the Chalk, which is a limestone of Cretaceous age, and the overlying Tertiary Basal Sands. The aquifer is folded, with the deepest part below central London (Figure 21), and is overlain by Tertiary clay. This aquifer is recharged on the Chalk outcrops in the Chiltern Hills, Berkshire Downs and North Downs. Under natural conditions, groundwater flows through the confined aquifer and discharges through wells in the Thames Valley. The Trafalgar Square fountains in London used to rise naturally by artesian flow.
During the 19th and 20th centuries water was pumped from the aquifer for the public water supply, industrial and commercial uses. Long-term extraction and high extraction rates caused the potentiometric surface in the aquifer to fall to around −80m OD by the mid-1960s (Figure 22). The fall in the potentiometric surface:
stopped the natural discharge to the Thames estuary below London;
led to saline intrusion from the Thames estuary below London;
reduced the flow of springs from the Chalk outcrop and reduced river flows;
increased the rate of flow of groundwater through the confined aquifer, because of the steeper hydraulic gradient;
allowed water to drain from the clays overlying the confined aquifer, causing shrinkage, and as a result the ground surface subsided by up to several tens of centimetres.
Some of these consequences may seem to be undesirable but they were offset by the ready availability of groundwater below London during the 19th and 20th centuries, which played an important role in the economic development of the city.
As London expanded, higher buildings with deep basements and foundations were constructed. Also, expansion of the public transport system and other services led to the construction of deep tunnels to avoid using surface land. These foundations and tunnels were designed for the low groundwater levels and clay properties at that time.
Since the mid-1960s reduced extraction rates have led to a steady rise in groundwater levels (Figure 22). The reduction is attributed to: industrial decentralization from London, the introduction of licensing controls on private extractions, and the high cost of pumping.
It became a concern that if groundwater levels continue to rise, underground structures in London could be endangered by flooding of basements and tunnels, increased water pressures, swelling of clay and chemical attack on buried concrete and steel — an environmental problem for the future. However, by 1999 the groundwater level seemed to have stabilized (Figure 22).