Major aquifers and well fields

6.2.1. Groundwater reservoir characteristics

The groundwater reservoirs could be igneous, metamorphic or sedimentary, depending on the origin of the geological formation. The lithology determines the type of aquifer such as sandstone, limestone or basalt, whilst its porosity determines how water is transmitted such as through intergranular spaces, fractures, channels or karstic pathways. These are further defined by their physiographic setting such as plateau, alluvial fill or fan.

The extent and conditions for natural recharge of aquifers when these are exploited, is of great importance. This is most pertinent for areas of low rainfall, such as in arid and semi-arid regions. The exploitation of aquifers has to be planned on the basis of their “safe yield” or “perennial yield” (see Box 6.2 for pertinent aquifer definitions). Evaluations have to be performed so as to know whether the aquifers contain renewable or non-renewable water. If the water is renewable, at what rate this is renewed? If the groundwater is non renewable it is said that the aquifer contains “fossil” water, as appears to be the case for the Saharan aquifers tapped by the Great Man-Made River project in Libya (Margat and Saad, 1982; Otchet, 2000).

BOX 6.2. Some pertinent aquifer definitions

• Porosity is the ratio of the volume of voids to the volume of aquifer; it depends on many factors such as the cementation of the pores, the fracturing, the packing of the grains, their shape, size and their arrangement.

• The effective porosity or specific yield or storage coefficient of an aquifer refer to the amount of water that can be drained by gravity and is expressed as a percentage.

• The hydraulic conductivity or permeability is the physical property of an aquifer to transmit water and it is expressed in terms of velocity; it is related to the effective porosity.

• Transmissivity of an aquifer is the average permeability times the saturated thickness of an aquifer.

• Aquifer pumping test is the controlled pumping of a well for a specified time during which the rate of pumping and the drop of the water level during and the recovery after ceasing of pumping are monitored; numerous techniques for evaluation of pumping test results for estimation of permeability and specific yield are available in the literature.

• Safe yield of an aquifer can be defined as the water that can be abstracted permanently from an aquifer without undesirable results.

• Perennial yield is the flow of water that can be abstracted from a given aquifer without producing results which lead to an adverse situation.

• Fossil water is the groundwater that entered the aquifer as recharge in past geologic periods, probably under different climatic conditions, and is not renewable.

The porosity of an aquifer determines the storage and to some extent the release of

groundwater by an aquifer. Of greater importance is the effective porosity or specific yield of an aquifer since this controls the quantity of water that can be released by an aquifer.

Another important physical parameter of an aquifer, which is related to the effective porosity, is the hydraulic conductivity or permeability, indicating how easily the water can be transmitted through the aquifer. Both physical properties can be evaluated in the laboratory using undisturbed aquifer samples or by the analysis of pumping tests on wells or boreholes (Box 6.2).

The groundwater storage and exploitation potential of an aquifer is not controlled solely by the lithological conditions. The setting of the aquifer, its structure and geometry should be such as to retain the water and make it available for exploitation. Thus, the dimensions and thickness, the stratigraphy or variation of lithologic units, both vertically and laterally, and the geologic and hydrologic boundaries, together with the effective porosity determine the storage and water bearing capacity of an aquifer.

The hydraulic factors associated with the occurrence of groundwater in aquifers and which affect use of the water are:

ƒ The quantities of water available for natural or artificial recharge (climate, abundance of recharge, infiltration areas).

ƒ The hydrogeological properties, related to the movement of groundwater (hydraulic conductivity, transmissivity, specific yield, storage coefficient, as defined in Box 6.2).

ƒ The hydrological boundaries of the aquifer (coast, contact with waters of inferior quality, contact with a stream or lake).

Depending on whether an aquifer forms a water-table under atmospheric pressure, or it is surrounded by strata of markedly less permeability, the aquifer is called an

“unconfined, water-table, or a phreatic aquifer” in the first case, or “confined or artesian aquifer” in the latter case.

The use that is made of the aquifer, and the quantities of water that may be available in the future, depend on hydro-meteorological factors such as the rainfall, evapotranspiration, and runoff, and on the ability of the aquifer to be recharged. Recharge relates to the infiltration and percolation characteristics of the unsaturated zone and on the distance to the saturated zone. Floodwaters and runoff waters, especially in arid regions, are only partly available for recharge since, usually, their occurrence is brief and there is insufficient opportunity for them to infiltrate to the aquifer.

The chemical quality of groundwater is related to the water bearing materials of the aquifer and the amount of soluble substances they contain. Furthermore, the duration of contact of the groundwater with the lithology of the aquifer influences the concentration of dissolved solids. In coastal aquifers, quality may be affected by sea intrusion, while point and non-point source pollutants can affect phreatic groundwater.

In arid and semi-arid areas, high evapotranspiration is the main cause of increase of salt concentrations in the unsaturated zone. This salt may be transported by water percolating to the aquifer. Deep aquifers often have a high concentration of dissolved solids whilst shallow systems may have low salt content when the latter receive more continuous contributions of recharge from rainfall and surface run-off, replacing volumes extracted or

discharging from the aquifer.

The exploitation of a phreatic aquifer and the increased local use of groundwater for irrigation may affect the quality of the groundwater, especially in areas of high evapotranspiration, due to the accumulation of salts at the surface and their subsequent leaching into the aquifer. Fertilizers and pesticides used in agricultural activities also affect the quality of the local groundwater when proper crop management and good agricultural practices are not adopted. Similarly, pollutants from industrial and urban areas, including heavy metals and organic substances, can produce point source contamination of aquifers when raw wastewater percolates to the groundwater.

The sustainable aquifers are those that receive recharge mainly from surface water, rainfall, streams and lakes in excess of their exploitation. This recharge could be continuous or temporary and intermittent depending on the source of supply.

6.2.2. Discharge, recharge and storage of aquifers

The outflow from an aquifer is discharged through springs, through subsurface flow into the sea, lakes, surface streams or other aquifer systems, rises into surface depressions when the water table is high, and passes into the atmosphere by evaporation and evapotranspiration. Groundwater contributes to the surface flow of streams and is the main contributor to base-flow and to dry-weather flow. Wells and boreholes, drains, quanats and similar, are artificial means for extracting groundwater. In exploited aquifers, they usually account for most of the discharge.

The recharge, which is the downward movement of water, is controlled by the supply of water infiltrating, the vertical permeability between the ground surface and the water table, and the depth to the water table. Subsurface flow originating in upland areas may add to the aquifer recharge. Because the residence time of groundwater within the aquifer is normally very large, periods of drought can be accommodated by an aquifer whereas for surface resources the impact is almost immediate.

The term “sustainability” used in reference to groundwater resources means the use of these resources in such quantities and under such conditions that would allow their renewal at a rate equal to their use. Theoretically, all water resources are renewable. The rate of renewal though, differs for the various types of aquifers. This renewal may occur within periods as short as one year in the case of a shallow minor phreatic aquifer in areas where there is rainfall, to many years as in the case of deep artesian aquifers. In the case of fossil water as found underneath the Sahara and the Arabian Peninsula, this could only be measured in geologic time. Age-dating techniques for the water in these large groundwater reservoirs show that these ground waters infiltrated during the ice age (30000 years ago).

Thus one should distinguish between renewable and non-renewable groundwater resources.

Sustainability thus is a quantitative term and has the meaning of non-depletion of the reserves within the life span of water-works, say 40 to 50 years, rather than geologic times.

In this respect the concept of “safe yield” of an aquifer needs to be applied. This defines the quantities of ground water that can be used which will not create any undesirable results (Box 6.2). Undesirable results could be the lowering of the water table to depths that would be too large for economic pumping, or the deterioration of the groundwater quality (as discussed in section 6.4). However, well planned ‘groundwater mining” is possible under

very specific conditions, and may be economically and socially acceptable. The term groundwater mining is used when the conscious and planned abstraction rate greatly exceeds aquifer recharge (UN, 1992).

The development of groundwater reserves depends on the special climatic conditions in an area, the water balance, the spatial distribution of precipitation and the long-term recharge. The groundwater reserves are also subjected to temporal and spatial variations that may vary on an annual or inter-annual basis. This particularity of ground water, as opposed to other minerals, plays a decisive role in its exploitation since withdrawal should not exceed the aquifer annual or inter-annual recharge. The opposite practice, such as an extended over-exploitation, would lead to the depletion of reserves and the drying up of major parts of the aquifer and its springs. This may result in irreparable damage of the aquifer system, either directly or indirectly, by drawing in water of inferior quality such as seawater as is the case of coastal aquifers, or the compaction of the aquifer granular texture, leading to land subsidence, as discussed further in Section 6.4. An outline of the types of groundwater reserves is given in Box 6.3.

The rational development and management of water resources needs to be based on long-term planning and advanced understanding of their quantitative and qualitative temporal and spatial variations, both at a national scale and to the scale of the individual hydrologic catchment and aquifer.

BOX 6.3. Types of groundwater reserves and their exploitation

• Reserves in live storage which are naturally discharged and could be exploited by pumping.

• Reserves in dead storage which are below the level of natural discharge and which could be exploited only after the live storage is exhausted.

• Local reserves that are pumped by a well or a number of wells and their pumping creates a local cone of depression.

• Reserves in irreversible storage whose pumping leads to a permanent consolidation of the aquifer and loss of water storage space, leading to land subsidence.

On the other hand, the exploitation of non-renewable aquifer systems does not differ from any other mining operation and any such policy should be carefully considered and evaluated on its own merits. Mining of groundwater might be acceptable if environmental values are not compromised and the rights of future generations are duly considered. Such a policy has to be carefully examined beforehand so as not to jeopardize the long term interests of the community and should focus on:

ƒ Economically feasible capital investment, the local and community present and future costs and benefits due to the groundwater development.

ƒ The foreseeable consequences arising from the progressive reduction in the availability of water, including ecological impacts.

ƒ Equity considerations with respect to future generations.

When tapping an aquifer, the pumping schedule and quantities available largely depend on the groundwater in storage, both in terms of the type of reserves (Box 6.3) and the yield capacity of the aquifer (Box 6.4). For an idealized aquifer, in the long run, inflow

and outflow must balance.

BOX 6.4. The yield capacity potential of groundwater

• Mining yield which is the quantity extracted in excess of the recharge and which leads to the depletion of the aquifer.

• Perennial yield that is the extraction that is carried out under specified conditions and which does not create any undesirable repercussions on the groundwater reserves.

• Deferred perennial yield that consists of two different pumping yields. The starting yield is greater than the rate of the perennial yield and results in the depression of the water level. This provides water at lower cost without any detrimental effects on the aquifer. In fact this may reduce initial losses that occur at high water-table conditions such as by evapotranspiration, or subterranean flow to the sea. When the water level is lowered to a predetermined level, then the rate of abstraction is adjusted to the rate of the perennial yield.

• Maximum perennial yield. For this to be accomplished there is need for maximizing the recharge potential, and the aquifer must be treated as one unit. It assumes that surface water is conjunctively used with the operation of the aquifer, and that there is a rational distribution of water demand.

• Sustained yield. This is the maximum extraction rate that could be sustained by the natural recharge, irrespective of any short-term variations of recharge.

• Optimal yield. This is related not only to the safe yield of the aquifer but also to the optimization of the operation and management of the aquifer and includes conjunctive use aspects where these can be applied, and artificial recharge.

A groundwater system holds a certain amount of water in storage. The storage -annual discharge ratio of an aquifer, in the dimension of years is usually quite high compared to the same ratio of a river, where it is usually very small. This feature makes groundwater very attractive in the case of droughts and temporary water shortage conditions.

6.2.3. Exploitation of groundwater storage

The exploitation of the groundwater storage should be a gradual process, starting from an investigation of the aquifer and its characteristics, followed by an evaluation of its potential, and ending with the design and operation of the wells and well-fields. Normally, an aquifer is the first source to be developed by farmers and other private individuals. This usually occurs well before any major water distribution scheme is put into operation because of the ease of access and the relatively low cost of developing a groundwater source. Therefore, in practice, the investigations that are carried out for increased exploitation of an aquifer aim to assess the safe yield and additional potential of the source.

The necessary groundwater investigations required are outlined in many publications and are beyond the scope of this text. The important issue is that before a major water scheme is implemented, all the necessary investigations, surveys and data collection needed for the evaluation of the available potential should be carried out (USBR, 1985).

An exploratory drilling program, which is based on a number of other investigations, is the most direct method for gathering information about a groundwater reservoir, and

provides a basis for designing operational boreholes and well fields after pumping tests.

The location and spacing of wells and boreholes, their depths, diameters, casing and other design specifications (gravel-pack, screen size, and location of pump) have to be determined through the exploratory drilling and pumping test program. Groundwater quality considerations usually determine the type of casing and grain size of filters to be installed. The optimum design of a well field is quite a complicated process that normally requires the use of mathematical simulation models to evaluate and to minimize interference effects between the wells and to provide results to enable yield maximization.

In water scarcity regions, groundwater often constitutes the main source of water.

During droughts, exploitation of large aquifers should be undertaken with care, to avoid irreparable damage to the aquifer and to the quality of the groundwater. On many occasions and in many countries the efforts made to cope with water scarcity are very much bound up with efforts made in conserving groundwater and prevention of over-exploitation.

Obviously, exploitation in excess of the natural recharge will result in the lowering of the reserves and the piezometric head surface. This in turn will have an impact on the yield of the wells and greater energy costs will be required for pumping. Depending on the hydraulic depression created, water of inferior quality may intrude the aquifer, such as seawater or inferior quality water from deeper horizons. Similarly, though depending on the texture of the aquifer material, land subsidence conditions may develop that could result in irreparable damage to the aquifer and even cause damage to properties at the ground surface (see 6.4).

The exploitable reserves of groundwater are those that correspond to the regulated reserves, or live storage. These are included between the highest and lowest water-table level of each hydrologic year and are only a fraction of the geologic reserves that are between the lowest water level mentioned above and the bottom of the aquifer.

Groundwater source development under conditions of water scarcity requires careful quantification of the aquifer potential and the optimization of its use. The exploitation policy should be so formulated as to avoid depletion of the reserves beyond a predefined level that should be defined as the minimum acceptable level which would be expected to cause minimal problems. The main issues that should be considered for a groundwater exploitation policy under water scarcity are listed in Box 6.5.

BOX 6.5. Main issues for aquifer exploitation under water scarcity

• The groundwater yield potential needs to be evaluated.

• The exploitation policy should be designed so as to safeguard the sustainability of the aquifer.

• The distribution of wells should be optimally designed to avoid interference between the wells and the development of water level depressions due to concentrated pumping.

• Water conservation measures should be implemented, such as: less water-demanding cropping patterns, improved irrigation systems and demand management strategies.

• Schemes for rationing of water need to be established.

6.2.4. Management considerations

Managing groundwater reservoirs under the stress conditions of meeting the demand, particularly on the occasion of lack of sufficient water resources, is a very difficult task.

Some of the main issues for groundwater management in water scarcity regions are listed in

Some of the main issues for groundwater management in water scarcity regions are listed in