The Water Environment

Before any attempt is made to assess the likely impact of surface mineral extraction on the water environment, or to control or mitigate such effects, it is essential to understand how groundwater flows through different materials, and how it interacts with surface water in streams, rivers, lakes and wetland areas.

Whilst mineral operators, mineral planners, or the general public would not be expected to have a detailed understanding of these processes, it is important to be aware of the basic concepts and potential issues. It is equally important to be able to recognise where specialist information and appropriate expert advice is needed in order to address these various issues, as and when they arise.

Part One of the original Guide to Good Practice⁷⁸ includes a detailed review of the basic concepts relating to hydrological and hydrogeological processes, and explains the meaning of most commonly used terms. Some of the key points from that review are shown in Figure 1.



All natural surface water and virtually all water within the ground originates from rainfall. When rain falls onto the surface of the land, much of it runs away as surface run-off or evaporates from bare soil and rock surfaces, vegetation surfaces or paved areas.

Water that is not intercepted by these surface processes infiltrates into the soil. From here, a further proportion is taken up by plants via their root systems and transpired back into the atmosphere. Some near-surface soil moisture is also evaporated. The combined process of evaporation and transpiration is commonly termed evapo-transpiration.

Water that remains at the surface, and that which rejoins it from beneath the ground in the form of springs and seepages, forms the most obvious part of the hydrological cycle: the surface water system. This comprises a series of topographically-controlled drainage basins ,within which surface water flows, through progressively larger streams and rivers from upland headwater areas to the coast. Figure 2 shows a number of different drainage basins in an upland area, and indicates how each point on a water course will have its own particular catchment area. The further you go downstream, the larger the drainage basin will be.



Except in the steepest headwater areas, most streams and rivers flow within a recognisable floodplain . Precipitation that is not converted into run-off or evapo-transpiration enters the groundwater system, gradually percolating down through cracks and pore spaces until it reaches the water table, i.e. the upper surface of the permanent groundwater store.

In the zone above the water table, only a proportion of the voids within the rock contain water. This is known as the unsaturated zone.

Below the water table is the saturated zone where all of the void spaces are filled with water. Saturated rocks through which water can flow easily and which contain sufficient water to be used for water supply are referred to as aquifers.

Within the saturated zone, groundwater no longer flows vertically downward under gravity. Instead it flows, more or less horizontally, from areas of higher hydraulic head to those of lower head. (Figure 3).



For an "un-confined" (or water table) aquifer, i.e. one that is exposed at the ground surface, or that is overlain only by soils and rocks through which water can easily flow, piezometer water levels are equal to the actual water level within the aquifer. In this situation the water table is the surface connecting all such water levels, and the slope of this surface represents the hydraulic gradient.

In "confined" aquifers, the water is kept under pressure by an overlying layer of clay or other impermeable rock. The pressure causes piezometer water levels to rise above the top of the aquifer until the weight of water counterbalances the pressure. Thus, the level to which the water rises in a piezometer is a measure of the aquifer pressure. Where the pressure is sufficient to cause the piezometer water level to rise above the ground surface, such that water would overflow without pumping, the conditions are described as "artesian". The imaginary surface connecting the piezometer water levels is known as the "piezometric surface", whose slope represents the hydraulic gradient in the confined aquifer.

Most sedimentary rock types and all unconsolidated sediments (including soils) are formed from small particles packed closely together with voids in between. Groundwater moves along irregular pathways through these void spaces, as intergranular flow (Figure 4).



Other rock types are characterised by an interlocking crystalline (rather than granular) structure, in which pore spaces are rare or even absent. These rock types include most igneous and metamorphic rocks (such as granite, basalt, gneiss and marble) but also many of our older sandstones, limestones and other sedimentary rocks. Where voids do exist within these rocks they are not usually well-connected and, as a consequence, very little (if any) intergranular flow can take place.

However, these and most other types of rock also contain a variety of fractures or fissures through which groundwater may be able to move by fissure flow.

In hard limestones and sandstones there are invariably well-developed joints and bedding planes, as well as fractures induced by subsequent 'tectonic' folding of the rocks, which can allow substantial flows of groundwater.

In soluble rock types, such as limestone, gypsum and halite, the joints and fissures may have been widened by dissolution of the rock in flowing groundwater and, in some cases, this can result in the formation of well-developed cave systems. Under such conditions, a very rapid type of fissure flow, known as "conduit" or karstic flow may take place.

In extreme cases, well-developed karstic systems may develop, comprising a series of interconnected caves, as found in the Carboniferous limestone of the Mendip Hills. These, however, are the only circumstances in which the popular, but usually misconceived idea of 'underground rivers' may actually be found.

Some materials, such as the Chalk in the south and east of England, and the Sherwood Sandstone in the English Midlands, are characterised by what is often referred to as "dual porosity" or "dual permeability". That is, they clearly exhibit both intergranular and fissure flow (as shown in Figure 4).

The ability of an aquifer to yield water in economic quantities depends on two principal properties - how much water the rock unit can hold (porosity) and how easily water flows through it (permeability). The Environment Agency in England and Wales, sub-divides aquifers into major or minor aquifers , depending largely upon the extent to which they are utilised for public and private water supply within a given area.

Strata such as clays and hard, non-fissured igneous rocks, have a very low permeability and are capable of acting as a barrier to groundwater flow. These are termed non-aquifers or aquicludes (by the Environment Agency). Rocks and soils which have a low, but not insignificant, permeability may be termed aquitards (Figure 5).



The significance of these distinctions to surface mineral extraction is that, in highly permeable materials, groundwater flow is relatively rapid, and large quantities of water will enter an excavation, if this extends beneath the water table. Dewatering (discussed in detail later) in such circumstances to allow mineral extraction to continue may be difficult or (in extreme cases) impractical. Where dewatering is carried out in such conditions, it is likely to have a widespread impact on groundwater levels within the surrounding area.

By contrast, groundwater flow rates in lower- permeability materials are slow so that relatively little water will enter an excavation. In such circumstances, the impact of dewatering, if any, is likely to be limited. In strata which have very low permeability, the rates of flow are such that it may be possible to extend a quarry well beneath the regional water table with little or no active dewatering. In such cases, in addition to run-off, water may still issue from high level fissures or minor perched water tables intersected by the quarry, especially following heavy rains, so that intermittent pumping may be required from the base of the excavation.

The Environment Agency has adopted the 1992 NRA document "Policy and Practice for the Protection of Groundwater" (PPPG) to "provide a framework for decision making" on matters concerning groundwater exploitation, conservation and protection, including mineral extraction. This document is supported by a series of regional appendices which set out more detailed information on the individual geological formations and hydrogeological units within each area.

In order to produce consistent, workable policies, the PPPG categorises aquifers according to their vulnerability to contamination. Groundwater vulnerability is determined by the physical, chemical and biological properties of the soil and rocks, which control the ease with which an unprotected hazard can affect groundwater.

In addition to the concept of resource vulnerability, which considers the whole groundwater unit, is the concept of groundwater source protection, which seeks to protect individual groundwater sources (including springs, wells and boreholes).

The proximity of an activity to a groundwater source is one of the most important contributors of risk to an existing groundwater source, whether by contamination or by derogation. A nested hierarchy of three groundwater Source Protection Zones (SPZs) are therefore defined, the orientation, size and shape of which are determined by the hydrogeological characteristics surrounding each individual site:

The concept of groundwater source protection originally applied only to sources used for public water supply, but now extends to larger private abstractions needed (for example) for industrial processes and agricultural purposes.

Smaller, private abstractions, both licensed and unlicensed, generally do not have formally identified source protection zones, though they do have a right to protection. Where this right is threatened by proposals for mineral extraction (or any other form of development), the owners may raise objections to the development as part of the normal planning process.