AQUIFERS, AQUITARDS AND AQUICLUDES

‘Aquifer’ is a useful term that appears whenever groundwater is discussed, but it must be used carefully when discussing groundwater lowering systems. As used by hydrogeologists, an aquifer might be defined as ‘a stratum of soil or rock that can yield groundwater in economic or productive quantities’. Almost all wells used for water supply purposes are drilled into, and pump from, aquifers. Examples of aquifers in the United Kingdom include the Chalk or Sherwood Sandstone. By this definition, strata that yield water at flow rates too small to be used for supply are not aquifers and might be considered ‘non-aquifers’ (in the United Kingdom, the regulatory bodies sometimes use the term ‘unproductive strata’). Examples of non-aquifers might include alluvial silts, glacial lake deposits or unfractured mudstones.

From a groundwater lowering point of view, the hydrogeologists’ definition of an aquifer in terms of ‘productive quantities’ is unhelpful. The groundwater in many strata that yield just a little water (and so are non-aquifers) can cause severe problems for excavation stability (see Chapter 6) - in practice, many large-scale groundwater control schemes have been deployed in strata that hydrogeologists might class as ‘non-aquifers’. For the purposes of this book, the definitions of CIRIA Report C750 (Preene et al., 2016) will be adopted, where the reference to productive quantities is omitted. This definition is given here, together with those for aquiclude and aquitard. The relationship between these strata types is discussed in the following sections.

Aquifer. Soil or rock forming a stratum, group of strata or part or stratum that is waterbearing (i.e. saturated and permeable).

Aquiclude. Soil or rock forming a stratum, group of strata or part or stratum of very low permeability, which acts as a barrier to groundwater flow.

Aquitard. Soil or rock forming a stratum, group of strata or part or stratum of intermediate to low permeability, which yields only very small groundwater flows.

Unconfined Aquifers

An unconfined aquifer is probably the simplest for most people to visualize and understand. The soil or rock of the aquifer contains voids, pores or fractures. These voids are saturated (i.e. full of groundwater) up to a certain level, known colloquially as the ‘water table’ (Figure 3.12a), which is open to the atmosphere. A monitoring well (see Section 11.7.6) installed into the saturated part of the aquifer will show a water level equivalent to the water table. The analogy that can easily be drawn is that of digging a hole in the sand on a beach. Water does not enter the hole until the water table is reached, at which point water will enter the hole and stay at that level unless water is pumped out.

The water table can be defined as the level in the aquifer at which the pore water pressure is zero (i.e. equal to atmospheric); the line of zero pore water pressure is also known as the phreatic surface. Below the water table, the soil voids are at positive pore water pressures and are saturated. Above the water table, the pressure in the voids will be negative (i.e. less than atmospheric), and, at some height above the water table, that height depending on the nature of the soil or rock, they may be unsaturated and contain both water and air (see Section 3.3.5).

Figure 3.12 Unconfined aquifer, (a) Unpumped conditions, (b) During pumping.

If groundwater is pumped (or abstracted) from an unconfined aquifer, it is intuitively apparent that the water table will be lowered locally around the well, and a ‘drawdown curve’ will be created (Figure 3.12b) - which in three dimensions forms an inverted cone. In simple terms, the drawdown curve is the new, curved shape of the water table. Pore water will drain out of the soil above the new lowered water table and in free-draining soils or rock, will be replaced by air - this soil/rock will become unsaturated. The amount of water contained in a soil/rock will depend on the porosity of the soil, but it is important to note that not all of the water in an unconfined aquifer will drain out when the water table is lowered. Some water will be retained in the smaller soil pores by capillary forces. The proportion of water that can drain from an unconfined aquifer is described by the specific yield S_y, which is generally lower than the porosity.

When precipitation such as rain or snow falls on the ground surface above a confined aquifer, a proportion of this water will eventually infiltrate downwards through the unsaturated zone and reach the saturated zone of the aquifer. The percentage of surface recharge that reaches the aquifer will depend on how much of the water evaporates, is transpired by vegetation, is absorbed by any moisture deficit in very shallow soils, or runs off over the top of low-permeability surface layers. However, the important principle is that water levels in unconfined aquifers can vary in response to infiltration from the ground above them.

Confined Aquifers

The distinction between unconfined and confined aquifers is important, because they behave in quite different ways when pumped. In contrast to an unconfined aquifer, where the top of the aquifer is open to the atmosphere and an unsaturated zone may exist above the water table, a confined aquifer is overlain by a very low-permeability layer known as an ‘aquitard’ or ‘aquiclude’, which forms a confining bed. A confined aquifer is saturated throughout, because the water pressure everywhere in the aquifer is above atmospheric. A monitoring well drilled into the aquifer would initially be dry when drilled through the confining bed. When the borehole penetrates the aquifer, water will enter the borehole and rise to a level above the top of the aquifer. Because the pore water pressures are everywhere above atmospheric, a confined aquifer does not have a water table. Instead, its pressure distribution is described in terms of the ‘piezometric level’, which represents the height to which water levels will rise in monitoring wells installed into the aquifer (Figure 3.13a).

If a confined aquifer is pumped, the piezometric level will be lowered to form a drawdown curve, which represents the new, lower, pressure distribution in response to pumping (Figure 3.13b). Provided the piezometric level is not drawn down below the top of the aquifer (i.e. the base of the confining bed), the aquifer will remain saturated. Water will not drain out of the soil pores to be replaced by air in the manner of an unconfined aquifer. Instead, a confined aquifer yields water by compression of the aquifer structure (reducing pore space) and expansion of the pore water in response to the pressure reduction. The proportion of water that can be released from a confined aquifer is described by the storage coefficient S.

If the water pressure in the confined aquifer is sufficiently high, a well drilled through the confining bed into the aquifer will be able to overflow naturally at ground level and yield water without pumping (Figure 3.14). This is known as the flowing artesian condition - artesian is named after the Artois region of France, where such conditions were first recorded. Flowing artesian conditions are possible from wells drilled in low-lying areas into a confined aquifer that is recharged from surrounding high ground. Flowing artesian aquifers are a special case of confined aquifers - confined aquifers that do not exhibit flowing conditions are sometimes known as artesian aquifers or occasionally (and incorrectly) as sub-artesian aquifers.

Aquicludes

An aquiclude is a very low-permeability layer that will effectively act as a significant barrier to groundwater flow, for example as a confining bed above an aquifer. It need not be completely ‘impermeable’ to act as an aquiclude but should be of sufficiently low permeability that during the life of the pumping system, only negligible amounts of groundwater will flow through it. The most common forms of aquitard are layers of relatively unfractured clay or rock with permeabilities of 10-⁹ m/s or lower.

Figure 3.13 Confined aquifer, (a) Unpumped conditions, (b) During pumping.

In addition to having a very low permeability, a stratum should meet two other criteria before it can be considered to act as an aquiclude:

• (i) It must be continuous across the area affected by pumping; otherwise, water may be able to bypass the aquiclude.
• (ii) It must be of significant thickness. A thin layer of extremely low-permeability material may be less effective as an aquiclude compared with a much thicker layer of greater, but still low, permeability. The thicker a layer of clay or rock, the more likely it is to act as an aquiclude.

It is important to remember that no geological material is truly ‘impermeable’, so even an aquiclude can transmit groundwater, albeit very slowly. Therefore, there is no clear

Figure 3.14 Flowing artesian conditions, (a) Schematic cross section through confined aquifer with flowing artesian conditions, (b) Flow from a flowing artesian well. (Courtesy of Dales Water Services Limited, Ripon, UK.)

definition between considering a stratum an aquiclude or an aquitard; the characterization will depend on the hydrogeological setting.

In contrast to unconfined aquifers, little of the precipitation falling on the ground surface above a confined aquifer will infiltrate into the aquifer. Confined aquifers receive their infiltration from sections of the aquifer that are unconfined and are termed ‘recharge zones’. When dewatering pumping is carried out at a location in a confined aquifer, it is possible that the recharge zones may be located many kilometres from the location of the dewatering wells.

Aquitards and Leaky Aquifers

An aquitard is a stratum of intermediate (but still low) permeability with properties between that of an aquifer and an aquiclude. In other words, it is of sufficiently low permeability that it is unlikely that anyone would consider installing a well to yield water, but it is not of such low permeability that it can be considered effectively impermeable. Soil types that may form aquitards include silts, laminated clays/sands/silts, and certain clays and rocks that, while being relatively impermeable in themselves, contain a more permeable fabric of fractures or laminations.

Aquitards are of interest to hydrogeologists because they form part of ‘leaky aquifer’ systems. Such a system (also known as a semi-confined aquifer) consists of a confined aquifer where the confining layer is not an aquiclude but an aquitard (Figure 3.15). When the aquifer is pumped, water will flow vertically downward from the aquitard and ‘leak’ into the aquifer, ultimately contributing to the discharge flow rate from the well. It is apparent that the term ‘leaky aquifer’ is a misnomer, since it is the aquitard that is actually doing the leaking.

Aquitards are relevant to the dewatering practitioner for the following reasons:

• a) If aquitards leak into underlying pumped aquifers, the effective stress (see Section 6.5) will increase, leading to consolidation settlements (see Section 21.4.4). Analysis of the behaviour of any aquitards present is important when assessing the risk of damaging settlements.
• b) While aquitards may not yield enough water to form a supply, construction excavations into aquitards are likely to encounter small but problematic seepages and instability problems. Many applications of pore water pressure control systems using some form of vacuum wells (see Section 9.5.4) are carried out in soils that would be classified as aquitards.

Aquifer Parameters

The concept of permeability k has been introduced earlier (see Section 3.3) and is an important parameter used to describe aquifer properties. The thickness D of an aquifer is also important, since thicker aquifers of a given permeability will yield more water than thinner ones. These two terms can be combined into the hydrogeological term ‘transmissivity’, T.

Figure 3.15 Leaky aquifer system.

For SI units, k and D will be in metres per second and metres, respectively, so T will have units of metres squared per second. Results of pumping tests (see Section 12.8.5) are sometimes reported in terms of transmissivity; Equation 3.8 allows these to be converted to an average permeability over the aquifer thickness. In unconfined aquifers where the aquifer thickness reduces as a result of pumping, transmissivity will reduce in a similar manner.

The amount of water released from an aquifer as a result of pumping is described by the storage coefficient. This is defined as the volume of water released from storage, per unit area of aquifer, per unit reduction in head; it is a dimensionless ratio. Because of the different way that water is yielded by confined and unconfined aquifers, storage coefficient is dealt with differently for each.

For an unconfined aquifer, the storage coefficient is termed the specific yield, S_y. This indicates how much water will drain out of the soil, to be replaced by air, under the action of gravity. Coarse-grained aquifers, such as sands and gravels, yield water easily from their pores when the water table is lowered. Finer-grained soils, such as silty sands, have smaller pores, where capillary forces may retain much of the pore water even when the water table is lowered (see Section 3.3.5); S_y may be much lower than for gravels. Typical values of S_y are given in Table 3.1. Surface tension forces may also mean that the water may not drain out of the pores instantaneously when the water table is lowered; it may drain out slowly with time. This phenomenon is known as ‘delayed yield’ and can affect drawdown responses in unconfined aquifers.

In a confined aquifer, since the aquifer remains saturated, there is no specific yield, and the storage coefficient S is used to describe the aquifer behaviour. Since water is only released by compression of the aquifer and expansion of the pore water, typical values of S will be small, perhaps of the order of 0.0005-0.001. More compressible confined aquifers will yield more water under a given drawdown, and tend to have a greater storage coefficient, than stiffer aquifers. If the piezometric level in the aquifer is lowered sufficiently that it falls below the top of the aquifer, unconfined conditions will develop, and a value of S_y will apply to the unconfined part of the aquifer.

Most of the design methods presented later in this book (see Chapter 13) are based on simple steady-state methods commonly used for temporary works construction applications. The storage coefficient does not appear in those calculations, because by the time steady state has occurred, all the water will have been released from storage. For relatively small construction excavations, this is a reasonable assumption, because steady state is generally achieved within a few days or weeks, so the volumes from storage release are only a

Table 3.1 Typical Values of Specific Yield

Based on data from Sterrett, R, Groundwater and Wells. 3rd edition, Johnson Division, St Paul, Minnesota, 2008 and Oakes, D B, Theory of groundwater flow. Groundwater, Occurrence, Development and Protection (Brandon,T W, ed.), Institution of Water Engineers and Scientists, Water Practice Manual No. 5, London, 1986.

concern during the initial drawdown period. Storage volumes may be more of a concern for large quarrying or opencast mining projects, when steady state may take a much longer time to develop (see Section 9.7).