In many catchments, there are groundwater bodies that are important for water supply and the maintenance of low flows in rivers. The proper management of groundwater is therefore an important topic in hydrology and all hydrologists need to understand the basic principles that govern the analysis and prediction of groundwater. Some of the necessary theory has already been covered in Chapter 5 where the basic principles, parameters and terminology were introduced in considering the monitoring of soils and ground waters. Here we will look at how the way in which groundwater bodies might react to changes in recharge or pumping conditions can be predicted. In doing so, we will recognise that such predictions are now commonly made using modelling programs such as MODFLOW and ASM (see Section 15.5). In applying such models, however, it is important to have a proper understanding of the concepts and assumptions on which they are based.
Groundwater is an important source of water supply in the UK. The principal aquifers of the UK are shown in Fig. 15.1a, with water table levels in the major aquifer that underlies East Anglia shown in Fig. 15.1b.
We may distinguish between confined and unconfined aquifers. The former are fully saturated permeable layers beneath a confining impermeable layer. They are particularly important for water supply purposes in synclinal basins where the groundwater system is recharged at the edges of the basin but where there is a useful storage of water in the lower part of the syncline. There is such a basin under London, where a confined aquifer in the chalk is overlain by impermeable London clay. Wells drilled into the chalk provide water of good quality and, before the aquifer was extensively developed for water supply purposes, wells penetrating into the confined part of the chalk aquifer used to flow without pumping in some places. This was because the pressure in the groundwater (the piezometric head) was greater than that needed to raise the water to the surface. Such wells are called artesian wells (Fig. 15.2).
Abstraction for water supply purposes from the confined chalk under London in the late nineteenth century and early twentieth century caused the piezometric head to decline steadily (Fig. 15.2). In the second half of the twentieth century, however, abstraction started to decrease until a balance of recharge and abstraction was reached in about the 1960s, with abstractions at a level of about 480 ML day-1. By this time, groundwater levels in the centre of the London Basin had fallen by about 65 m.
With declining demand for groundwater for industrial uses in London, abstractions continued to decrease and the piezometric head started to rise again, with potential consequences for flooding of tunnels and building foundations. Abstractions are now managed through the Environment Agency's London Catchment Abstraction Management Scheme (see Chapter 17).
An unconfined aquifer is a saturated permeable layer overlain by an unsaturated zone. Unconfined aquifers are found in the recharge areas for the chalk in the London Basin (Fig. 15.2), but also in the shallow aquifers in the fluvioglacial deposits of the London gravels nearer to the surface in the Thames Valley. In unconfined aquifers, patterns of recharge from the surface will have an important effect on the shape of the saturated zone, the upper boundary of which is called the water table. The water table represents a surface where the pressure in the water in the pore space is zero relative to atmospheric pressure. In Chapter 5, the concept of capillary potential in an unsaturated soil (negative with respect to atmospheric pressure) was introduced. In the saturated zone, all the pores will be saturated and the local pore pressures will be positive with respect to atmospheric pressure. Pressure is also commonly expressed in terms of 'head' or energy per unit weight of water. This is convenient because head has units of length (see also use of head in describing river flows in Section 14.3.2).
In all groundwater bodies, water will move from regions of high total head to low total head. Total head will be sum of the local pore pressure (negative with respect to atmospheric pressure in unsaturated conditions and positive in saturated conditions) and elevation above some datum level, where elevation is defined to be zero. It does not really matter what elevation datum is used but it is important that it is the same for all points in the analysis. Thus, mean sea level is a suitable datum, but the ground elevation is generally not (only if all the points considered are in the vertical profile below a particular point on the surface).
Expressing total pressure in terms of head makes it easy to draw diagrams, called flow nets, to show how the groundwater should move, particularly for vertical slices. In confined aquifers, water will move from regions where the piezometric head is high to regions where it is low. In unconfined aquifers, water will tend to flow from regions where the water table is high to regions where it is low. Flow in aquifers is often of low velocity and laminar in nature. Flow velocities will in these conditions vary linearly with the head gradient, as described by Darcy's law. Flow velocities are usually expressed in this context as Darcian velocities, the flux per unit cross-section of the aquifer, q which can then be expressed as:
where Ks is the saturated hydraulic conductivity, h is total head and x is distance in the direction of flow. In a porous medium, of course, not all the cross-section is flowing water; part of it is solid material that forms the matrix of the aquifer. In fact, in the distribution of pore spaces in the cross-section, there may be many different flow velocities. Any solute or pollutant flowing with the water will locally follow this distribution of velocities and, as a result, be subject to significant dispersion (see Section 15.7 on transport processes in groundwater below). Thus, another descriptive velocity that is used is the mean pore water velocity. This is related to the Darcy velocity as q = vpn (15.2)
where q is the Darcy velocity, vp is the mean pore water velocity and n is the water-filled porosity of the porous medium.
Models based on Darcy's law have been the basis for nearly all models used in groundwater management when we need to understand questions like what will be the effects of adding a new water supply well on the yield of other wells or low river flows; what is the capture zone for a particular well and are there potential sources of pollution at the surface; what will be the travel pathways and times of a pollutant; and what will be the impacts of climate change on the groundwater resource if natural recharge rates are changed?
It is important, however, to recognise the limitations of the simple groundwater theory that will be presented here. In deep aquifers, the pressures may result in deformation of the pore space and changes in the density and viscosity of the water. It may then be necessary to take account of the compressibility of the aquifer materials. There may also be circulations developing, not only as a result of head gradients but also because of temperature and density gradients. Density-driven flows are also important where a freshwater aquifer interacts with sea water in a coastal or island aquifer with direct connections to the sea. Finally, there are karstic aquifers and fractured aquifers, where the flows may not be laminar and will then not be well described by Darcy's law. Karstic aquifers are typically found in limestones, where solution of the carbonate rock by percolating rainwater results in channels and caves that can provide complex networks of rapid pathways through the relatively impermeable solid rock. Similarly, fractures in otherwise impermeable rock can also provide storage and permeability. The chalk aquifers that are so important for water supply purposes in many parts of the world are again a particularly interesting case. They tend to exhibit a form that consists of high porosity but relatively low permeability blocks of matrix (originally formed from the carbonate remains of diatoms). Between the blocks there are joints and fractures, that provide little additional storage but add greatly to the permeability, and, occasionally, the chalk also exhibits karstic features that give rise to very rapid flow pathways.
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