ARTIFICIAL GROUNDWATER RECHARGE
University of Pisa, Italy
Keywords: artificial groundwater recharge, hydrogeology, map analysis, water management, aquifer, water demand and supply, groundwater, water pollution, water reserve, infiltration basins, recharge wells
2. Artificial Groundwater Recharge (AGR)
3. Influence of Recharge Factors
4. Methods of Artificial Recharge
5. Mixed Systems of Water Recharge
6. Evaluation of Aquifer Recharge Area by Piezometric Map Analysis
7. Advantages and Disadvantages of Artificial Groundwater Recharge
The increasing demand for water in many regions around the world has led to the implementation of more intensive water management measures to achieve more efficient utilization of limited available water supplies. The natural replenishment of groundwater occurs very slowly. If groundwater is exploited at a rate greater than that of its natural replenishment this will cause declining groundwater levels and, in the long term, destruction of the groundwater resource. To augment natural replenishment of groundwater reserves, the artificial recharge of groundwater has become increasingly important in many countries. In artificial recharge schemes aquifers are treated as a naturally-regulated system which may be used to store surface water, thereby leveling out seasonal variations in surface water availability and providing a steady supply of potable water. Furthermore, the soil can be utilized as a reactive agent for improving the quality of the surface water.
The main reasons for carrying out artificial recharge may be summarized as follows:
· promoting recovery of overexploited aquifers;
· storage of surficial waters during flood periods to maintain and improve supply in the dry season;
· storage of local or imported water in the aquifer;
· preventing seawater intrusion by creating freshwater barriers;
· increasing the value of aquifers for water distribution in areas with many wells;
· discharging certain wastewaters, such as cooling water;
· reducing groundwater salinity in agricultural areas;
· reducing subsidence caused by high pumping rates; and
· groundwater quality improvement.
Groundwater resources can be defined as the waters present in the subsurface in a specific area during a specific period of time. These resources can be divided into:
· natural water resources (water resources present in the environment);
· potential groundwater resources (the maximum volume of groundwater resources that can be replaced by artificial methods); and
· available or exploitable groundwater resources (groundwater resources that can be exploited under particular socioeconomic constraints).
Natural groundwater resources include static and dynamic waters. The natural static resources are connate waters, which are contained by exploitable aquifers. Connate waters were formed during periods in which climatic and hydrogeologic conditions were very different from those today, as illustrated by the case of the present-day groundwater resources of the Sahara. Natural dynamic resources are the volumes of moving water on the Earth's surface (runoff) and in aquifers (groundwater).
Potential groundwater resources represent the maximum exploitable natural resource, accessible with or without the use of mechanical devices. Their potential is limited by hydrographic, hydrogeologic, geological, environmental, and ecological limitations. These resources are generally less than the total of all natural resources.
Available or exploitable groundwater resources are less than the total of potential resources because their extraction is subject to socioeconomic restrictions as well as natural physical limitations.
A distinction has also been drawn between conventional and non-conventional water resources. Conventional resources are exploited by traditional methods, while non-conventional resources require the application of new and innovative methods. The boundaries and scales of the two categories require updating continuously in the light of the development of scientific and technical knowledge and the application of new technologies. For example, for many years artificial recharge of aquifers was an innovative experimental method: today it is a well-established technology.
1.1. Augmentation of Water Resources by Conventional Methods
The most widespread methods for resource augmentation rely on artificial storage of water on the land surface using barrages, dams, weirs, and other structures. The major problem with surface storage is the loss of land covered with water, and the ecological, environmental, and social problems generated. These solutions are especially difficult to implement in countries with high population densities and land values.
Storage of water in the subsurface can avoid these problems. Induced recharge of aquifers by artificial means, or artificial recharge, has been used for many years in many different countries and may be considered a proven, conventional method for water resource augmentation. Artificial recharge uses aquifers as reservoirs to store and conserve natural river waters and other surface runoff. Its application depends heavily on the disciplines of hydrogeology and groundwater engineering.
Groundwater reservoirs do not occupy the land surface or lose water to evaporation and plant transpiration: both very important advantages in arid and semiarid regions. In addition, recharged water undergoes slow natural filtration in the subsurface, which tends to clean and purify it. For these reasons, surface waters should increasingly be used to augment groundwater reservoirs and supplies.
1.2. Augmentation of Water Resources by Non-Conventional Methods
Non-conventional water resource technologies that may be used in artificial recharge schemes include:
· desalination of salty or brackish waters
· wastewater recovery or regeneration
· climate modification programs
· schemes to reduce evaporation
In the near future it may also be possible to use water stored in Antarctic or Arctic icebergs. Usually it is only in situations where the need for water is extreme, such as in arid regions, that non-conventional solutions are used in water supply and the storage of water by artificial recharge.
Desalination is a process that removes dissolved minerals (including salt, but also other minerals) from seawater, brackish water, or treated wastewater. A number of desalination technologies have been developed, including reverse osmosis, distillation, electrodialysis, and vacuum freezing.
The desalination of water in small plants has been carried out for centuries, but only in the later twentieth century were substantial technological advances and cost reductions achieved. Cost reductions have allowed this method to be used to supply areas without other water resources, such as deserts and islands, and to be integrated within larger water supply systems in areas where traditional water resources are inadequate.
An essential requirement for the process is the availability of abundant energy in various forms. Desalination plants are generally integrated with power generation plants using a common source of energy. A plant may produce more energy than necessary for desalination and use the surplus for other purposes, for example to produce electric energy.
Worldwide there are more than 7500 desalination plants in operation: the largest, in Saudi Arabia, produces about 128 million gallons of desalted water per day.
1.2.2. Wastewater Regeneration (WWR)
Wastewater regeneration is a process by which wastewater is treated to permit reuse for irrigation, and for industrial and municipal supply. The processes are similar to wastewater treatment prior to discharge to lakes or rivers. However, regeneration is more thorough because it must give the waters special qualitative characteristics to permit direct reuse, and to maximize a groundwater body’s capacity for self-purification.
A major problem with regard to water supply generally is that while water resources are decreasing at a steady rate, water demand for civil and industrial uses are increasing correspondingly rapidly. For this reason natural water bodies become increasingly contaminated, and more advanced decontamination methods are needed as a result.
Wastewater regeneration is common in remote areas, and those where water is very scarce. To a lesser extent, it is also used in areas where water resources are relatively abundant but where industrial water demand and high prices make treatment and reuse of water economically profitable. Limits to regeneration are imposed by the cost of water treatments needed to meet the chemical quality standards established by local laws and regulations.
1.2.3. Climate Modification
Increases in natural meteoric precipitation can been induced by cloud seeding. Rainfall is enhanced by introducing solid particles into the atmosphere that increase the number of natural freezing or condensing particles.
1.2.4. Reduction of Evaporation
Loss of water by evaporation is a very important issue. Its rate depends largely on local climatic conditions. The construction of large artificial storage reservoirs in arid or semiarid areas with very high temperatures may lead to high evaporation losses. Advances in research and large-scale testing have shown that protecting water with a molecular film prevents molecular diffusion into the air and greatly reduces losses.
2. Artificial Groundwater Recharge (AGR)
Traditionally, most water supply systems have been based on either surface water or groundwater. While surface water is being increasingly contaminated by domestic and industrial pollutants, microorganisms, and inorganic solutes, groundwater is generally of better quality and needs only simple, inexpensive treatment to meet the high standards required for water supply. However, the use of groundwater for water supply has led to a situation where natural replenishment cannot match exploitation rates, leading to falling groundwater levels. In view of likely future water shortages the two resources cannot be viewed as isolated bodies, and water resources management will need to give more attention to the combined use of surface and groundwater. Artificial recharge is one method that allows for management of this kind.
Artificial recharge (also sometimes called planned recharge) is a method of storing water underground at times of water surplus to meet demand in times of shortage. It has several potential advantages. They include the use of aquifers for storing and distributing water, and for the removal of contaminants by the natural cleaning processes that occur as polluted rain and surface water infiltrate the soil and percolate through soils and geological formations.
Artificial recharge is a process by which excess surface water is directed into suitable geological formations using infiltration basins, ditches, wells, or sprinkler systems. Bank filtration, or induced recharge, can be used if there is a direct hydraulic connection between an aquifer and a surface water body. In most artificial recharge schemes, aquifers are used for water storage and reuse.
Natural and artificial groundwater recharge involve the same basic physical and geochemical processes, but with some important differences. In artificial recharge the infiltration rate is much higher, thereby decreasing the period over which pollutants are exposed to the natural cleaning processes. This can pose problems, both for the quality of the water and for the operation of the artificial recharge plants. Increased knowledge of the processes involved obtained through research projects will be useful when planning and operating new artificial recharge plants.
The main objectives of artificial recharge are to increase available reserves of water and the water-resource potential of aquifers more generally. It can also be used for the following purposes (the percentages in brackets reflect the main purpose of documented schemes):
(a) promoting recovery of overexploited aquifers (25%);
(b) storage of surficial waters during flood periods to maintain and improve supply in the dry season (20%);
(c) storage of local or imported water in the aquifer (15%);
(d) preventing seawater intrusion by creating freshwater barriers (15%);
(e) increasing the value of aquifers for water distribution in areas with a high density of wells;
(f) discharging certain wastewaters, such as cooling water;
(g) reducing groundwater salinity in agricultural areas (10%);
(h) reducing subsidence caused by high pumping rates; and
(i) groundwater quality improvement.
(Objectives (e), (f), (h), and (i) combined account for ca. 15% of recharge schemes.)
3. Influence of Recharge Factors
Not all aquifers can be artificially recharged. The hydraulic characteristics of the aquifer, the nature of the existing groundwater, and the characteristics of the recharge water can have a major influence on the outcome of a recharge operation.
3.1. Geological Factors
One of the main factors affecting recharge is aquifer porosity. Porosity must be as high as possible, and this factor depends on a uniformity coefficient: where the coefficient is small, porosity is high. Another factor is the hydraulic conductivity of an aquifer: when the hydraulic conductivity is high, recharge is very quick. Hydraulic conductivity values tend to be high when the aquifer grain size is large. However, high conductivity means that an aquifer’s ability to clean recharged water is low, so recharge must be carried out with clean water.
The chemical equilibrium in the aquifer is very important. In most cases the aquifer materials and groundwater are in chemical equilibrium, and care must be taken to ensure that the recharge water does not change this.
Clays tend to be good ion exchangers: if recharge water has a high sodium content then clay flocculation can result, with the consequent blocking of pores. It should also be noted that recharge water can dissolve aquifer materials, and special care should be taken with recharge in rocks that contain carbonates (limestone and dolomite), sulfates (evaporites), chlorides, and iron (pyrite) because of their high water solubility.
3.2. Hydrogeologic Factors
Artificial recharge depends on transmissivity and porosity values, and the uniformity of these parameters is very important. The receiving aquifer must be as homogeneous and isotropic as possible.
The velocity of recharge is directly proportional to transmissivity and hydraulic gradient in the aquifer. For this reason the water table or piezometric surface must be very well defined, so that the depression’s water table or piezometric surface can be identified and used to optimize recharge conditions.
3.3. Physical–Chemical Factors
The chemical and physical characteristics of groundwater and recharge water have a great effect on the results of artificial recharge. The chemical, physical, and biological compatibility of the two kinds of waters must be investigated, as these properties can greatly influence plugging of the aquifer and therefore the rate and duration of recharge.
3.3.1. Physical Characteristics
The main physical characteristics to consider are:
· total dissolved solids (TDS)
· electrical conductivity
· color and odor
Temperature can affect water viscosity, as the two properties are inversely proportional. Since the hydraulic conductivity of the ground is inversely proportional to water viscosity, this means that water that is cold flows more slowly in the aquifer than warmer water. Moreover, if temperature decreases water density grows proportionally, leading to thermal stratification of water in the aquifer. This stratification lead to the obstruction of pores and reduces water infiltration into the aquifer. Because of these factors, it is always recommended that an aquifer be recharged with water that is warmer than the groundwater; thermal effects arising from direct recharge are thus reduced, because surface water balances its temperature with groundwater during infiltration through the unsaturated zone. Even biological life (including microorganisms and bacteria) may be affected by thermal effects, both in the unsaturated and saturated zones.
3.3.2. Chemical Characteristics
The main important parameters affecting artificial recharge are dissolved gas and dissolved salts.
Dissolved or suspended gas (air) has a double function—chemical and physical—inside the aquifer. The presence of small air bubbles inside the pores may cause blockage of the aquifer and reduce percolation into the unsaturated zone. Oxygen in the air causes redox reactions in the ground that can chemically precipitate compounds that block the aquifer, reducing water quality.
Oxidation can destroy pathogenic organisms, prevent the leaching of iron and manganese, and cause the precipitation of iron salts in groundwater. The redox reactions raise the iron content of the water by increasing the solubility of the iron. The different chemical compositions and levels of dissolved salts in recharge water can change the geochemical equilibrium, producing ion exchanges with the aquifer matrix and groundwater. These ion exchanges precipitate salts such as sulfides, sulfates, carbonates, and hydrates. They can cause swelling of clays due to the exchange between sodium and hydrogen, and the coagulation of suspended clay particles in the water. These processes generally reduce porosity and lower the quality of water.
Great care must be taken with regard to the microorganism concentration in the recharge water. Its microbial content is generally higher than that of the receiving groundwater, and pollution of the aquifer may result.
The ground’s ability to cleanse recharged water is very high, and is directly proportional to the residence time of water in the aquifer. Residence time, in turn, is directly proportional to subsurface flow distance and inversely proportional to the grain size of the aquifer matrix.
3.3.3. Groundwater Recharge Precautions
To help prevent groundwater pollution, permits are usually issued by governments to regulate the discharge, disposal, and storage of waste and wastewater. These permits should take into account the vulnerability of the aquifer and make provision for its protection. These provisions should apply to:
· effluents and sludges produced by wastewater treatment plants;
· domestic waste disposal sites;
· subsurface waste containment by deep-well injection or storage; and
· surface storage of wastes that are hazardous by virtue of their chemical composition.
· in permitting activities such as artificial recharge, the hydrogeological environment of the area should be taken into consideration;
· the opinion of qualified specialists should be sought with regard to all the above-mentioned issues;
· continuous monitoring programs should be set up to help control water quality in aquifers; and
· compliance with permits should be periodically checked.
The disposal of wastewater must not pose an immediate or long-term hazard to groundwater. Controlled sites should be equipped with protective installations making use of the best available technology, and should be monitored by competent authorities. Regulations or guidelines should be drawn up for selecting sites for disposal of controlled wastes and the operation, monitoring, shutdown, and eventual rehabilitation of these sites, with particular emphasis on groundwater quality protection. Water that is used for recharge should not contain any compounds that are toxic, persistent, bio-accumulative, or radioactive and may thus put groundwater at risk: such waters should be subject to special treatment. Legislation should ban dumping of solid or liquid wastes at unauthorized sites.
Application of treated wastewater and the resulting sludge on land should be regulated by license and/or conform to nationally agreed codes of practice, and should be restricted to areas where it poses no immediate or long-term hazard to groundwater quality. In this respect, particular care should be taken not to overload the natural self-purification capabilities of the soil filter and the natural processes taking place within it. Special attention should be paid to hazardous substances, such as heavy metals.
In principle, the injection of liquid wastes into the ground should be prohibited altogether. Deep-well injection of liquid industrial waste and other polluted water into the ground should be authorized only on a case-by-case basis, if the necessary precautions and controls for deep-well disposal can be observed and if injected wastes cannot harm nearby aquifers. Control methods should include proper siting, design, construction, operation, decommissioning, and monitoring of deep-well injection sites.
4. Methods of Artificial Recharge
4.1. Choice of Recharge Method
The choice of recharge method depends on various factors. Most important are hydrogeological variables:
· feed flow
· storage coefficient
These are the main hydrodynamic parameters that regulate the efficacy of recharge, above all near the recharge area itself. Whatever artificial recharge system is used, an increase in water load is produced and a "mound" is formed at the water table. In unconfined aquifers this water-level mounding is the result of an increase in the amount of stored water, and its dimensions depend only on the aquifer storage coefficient. In confined aquifers, which have very small storage coefficients, recharge produces a pressure increase over a wide area rather than an observable growth in water volume. In this case, aquifer transmissivity governs the behavior of the recharge system. Recharge generates a velocity field that spreads through the whole aquifer. The velocity intensity decreases with distance from the recharge area.
Other relevant conditions include the following:
· Water available for recharge. Water availability is the first requirement that must be satisfied.
· Appropriate physical–chemical properties of recharge water. The quality of recharged water should be compatible with the chemical characteristics of groundwater to avoid chemical reactions that can reduce the aquifer’s ability to receive and store water. Recharge water must have the lowest possible suspended solids content to avoid subsurface plugging and slowing of the percolation or injection of water.
· Availability of aquifers suitable for recharging. Aquifer properties are very difficult to define in areas exhibiting complex hydrogeological conditions and geological structure, especially where there is a wide range in lithology and grain sizes in the geological formation.
· Thickness and permeability of the unsaturated zone. When the thickness and/or permeability of the unsaturated zone is very high, deep recharge (using recharge or injection wells) is preferable. A surface recharge system is preferable where there is low permeability and the unsaturated zone is relatively thin.
· Thickness and permeability of the aquifer. The aquifer must be capable of absorbing large amounts of water.
· Presence of piezometric surface depressions. Depressions in the water table or piezometric surface below well fields are positive factors, because recharge can then flow towards pumping areas and is not lost to the margins of the aquifer.
· Topography of the recharge area. The phreatic or piezometric head must lie below the elevation of the recharge facility because the head difference is the force that drives the infiltration of water. The topography and morphology of the recharge area are also very important: if recharge has to occur at the surface, the recharge facility will require storage areas of great horizontal extent. If the recharge is carried out using wells, however, morphology is less important.
· Land price and land-use regulations. The morphology of a recharge site is not the only factor influencing choice; the cost of land and regulations in force can also influence the choice of recharge methods.
4.2. Treatment of Water for Recharge
An artificial groundwater recharge facility is economically viable if recharge is continuous and operations and maintenance costs are reasonable. The main costs are those associated with replacement of filtration systems. As far as possible the recharge water must be free of substances that can cause plugging of the aquifer formation. The suspended solids content is the main factor affecting cost, because no economic method exists to remove them from recharge water.
The bacterial quality of recharge water is more important in the case of surface recharge, because the filtration process in the unsaturated zone will remove bacteria.
Suspended solids (40–70%) and bacteria (25–75%) can be eliminated using settling tanks and filters (sand and gravel), which will eliminate approximately 20% of all suspended solids. When the suspended solids are colloids, it is necessary to remove them by flocculation or coagulation using suitable products such as aluminum sulfate and sodium aluminate. Activated carbon filters are mainly used to remove organic material, above all pesticides, herbicides, and other toxic organic compounds.
Removal of algae is another problem. Algae that are present in recharge water should be removed by filters, or they will grow in the recharge basin. Algae grow and die very quickly and the organic matter produced may generate unpleasant odors and consume much oxygen, producing an anaerobic environment. Algae also reduce carbon dioxide content, which leads to precipitation of calcium carbonate and the consequent plugging of the aquifer. It is possible to use chlorine, which is an algaecide, to oxidize part of the organic matter.
While recharge water is percolating through the unsaturated zone it loses oxygen and enriches itself with carbon dioxide. To reduce this effect, recharge water is often oxygenated before infiltration.
4.3. Indirect Artificial Recharge (IAR)
This method involves surface spreading of water in excavated basins. For effective artificial recharge highly permeable soils are required, and it is necessary to maintain a layer of water over these soils.
When direct discharge is practiced, the amount of water entering the aquifer depends on three factors: the infiltration rate, the percolation rate, and the aquifer’s capacity for horizontal water movement or spreading. In a homogenous aquifer, the infiltration rate is equal to the percolation rate. At the surface of the aquifer, however, clogging occurs due to the deposition of particles carried by water in suspension or in solution, to algal growth, to colloidal swelling and soil dispersion, and to microbial activity. Recharge using spreading basins is most effective where there are no impeding layers between the land surface and the aquifer and where clear water is available for recharge (although relatively turbid water can be tolerated with this method better than with well recharge). The most common problem in recharging by surface spreading is clogging of the surface material by suspended sediment in the recharge water or by microbial growth. Removal of fine suspended sediment is difficult in coarse-grained materials.
Recharge methods may be classified as follows:
· basins and lagoons
Basins are large shallow depressions where recharge water is stored. Their shape and dimensions are generally adapted to local topography. Many basins need scheduled maintenance to improve the capacity of the bed for infiltration.
If rainwater is used for recharge, one basin is usually sufficient. If stream water is used, it is better to use a series of basins (Figures 1–3). In this case water is taken from the river by a single channel where it arrives at the first basin, and then slowly flows into the others. Some water returns to the river, and in this way the basins operate like settling basins. By using a bypass between basins, it is possible to avoid interruption of water recharge during scheduled maintenance.
Figure 1. Scheme of artificial groundwater recharge in the Ruhr Valley, Germany (by permission of Ruhrverband, D-45128 Essen, Germany)
Artificial recharge using basins is the most widely used method, on account of its practicality and ease of associated maintenance. Basins fall into three categories, depending on their floors, as follows:
· with natural bottom (after excavation)
· with bottom covered with vegetation
· with bottom covered with an artificial bed of sand
Figure 2. Aerial view of waterworks in the Ruhr Valley (Germany), showing slow-rate sand filtration basins
Figure 3. Infiltration basins
The infiltration potential of rivers and channels is increased by deepening the bed and constructing barrages to enlarge the areal extent of the riverbed. Barrages must be removed before flood seasons to prevent damage to the courses of rivers and channels. They are reconstructed later to resume recharge.
Parallel ditches (Figure 4) do not distribute water deeply, and are usually located very close to one another. They are usually arranged in three different ways:
· with a "zigzag" course following the contours and land slope;
· with branches from a main channel; or
· with ditches arranged perpendicular to the main channel.
The excess of recharge water flowing from the ditches is collected in a discharge channel. Ditches vary in width from 0.3–1.8 m; the slope must be very slight to minimize erosion and to increase the wetted section. These methods may differ in areas with irregular topographic surfaces.
Figure 4. Artificial groundwater recharge using ditches
Holes are generally small and relatively shallow, and are used in areas where the ground has low permeability. Abandoned quarries are often used. Suspended solids in the water settle on the bottom of the hole while the walls remain permeable to the recharge water.
This method is used on level ground, where waters are spread directly on the natural surface to produce a thin water layer. The ground must retain its natural vegetation cover.
4.4. Direct Artificial Recharge (DAR)
This is usually carried out using wells. Radial wells and (to a lesser extent) infiltration galleries are also used in direct recharge. While an artificial recharge well can be compared with a pumping well operated in reverse, in fact the hydraulic behavior is very different.
A pumping well is considered to be in hydraulic equilibrium if its pumping level is stationary over a fixed period, and if the cone of influence remains unchanged over time. In recharge wells the recharge cone (the inverse of the pumping well cone) spreads, even if the water level inside the well remains stationary. Near the recharge well the saturated thickness increases, due to the formation of a recharge cone. Another difference between pumping wells and recharge wells is that recharge tends to reduce ground subsidence, rather than promoting it.
From a structural viewpoint, too, these wells operate under different conditions. In a pumping well the flow of water tends to pull fine sand or other fine materials into the well, removing these elements from the ground and increasing hydraulic conductivity nearby.
4.4.1. Water Recharge Wells
There are two factors to consider in DWR by wells: efficiency and cost. The cheapest process is to inject recharge water by force after removing the valve from the bottom of the pump. In this way the flow resistance through the pump impeller is very high. Another method is to alternate recharge periods with pumping periods in order to flush the well; while this operation is not expensive it is potentially dangerous because air contained in recharge water may block the aquifer. A well filter may be located some meters above the top of the water recharge level or aquifer to eliminate any danger of aquifer blockage by the air carried into the system in the recharge water. In this way air may free itself easily through the top of filters and arrive at the surface through little breather pipes. Breather pipes around 5–7 cm in diameter may start at the top of filters and end a few centimeters below ground level. Using this system it is possible to alternate recharge operations with pumping, because air is conveyed along the space between the covering and the pump pipes.
It is always preferable to recharge the aquifer with a flow lower (perhaps 85%) than the pumping flow: to control the system it is sufficient to install a regulation valve in the recharge pipe. The duration of recharge and pumping phases is not always the same: usually a pumping period of two hours for every 24 hours’ recharging gives good results.
5. Mixed Systems of Water Recharge
These represent the best recharging methods, because many of the problems caused by transport of fine materials are eliminated: in this way the risk of aquifer obstruction is reduced. Figure 5 shows a mixed system in which the first aquifer carries out a filtration function and the second is at the final level of recharge.
Figure 5. Mixed system of water recharge: basins and recharge wells in combination
When basins and recharge wells work in combination the basins recharge the first aquifer; wells pump water from this aquifer and then inject it into a lower one. A ring zone of impermeable material of radio R0 is located around every well. This is necessary to slow water flow to ensure minimum circulation in the aquifer, so that recharge water arrives into the well sufficiently filtered and mixed. R is the radius of the basin joined to every well; this radius is calculated so that its area is equivalent to the area calculated by Thiessen polygons.
By way of example, Figure 6 illustrates a recharge scheme at Cavaillon-Cheval Blanc (Durance River, France). Recharge water arrives in the well through a removable sand and gravel filter. The basin is 12 m deep and measures 70 m2 in plan. Five recharge wells are located inside the basin: the total flow rate is 700 L sec–1.
Figure 6. Mixed system of water recharge at Cavaillon-Cheval Blanc, France
6. Evaluation of Aquifer Recharge Area by Piezometric Map Analysis
Aquifer recharge is affected by the amount of water that infiltrates the groundwater flow. Groundwater represents only one step in the hydrologic cycle; for this reason measurements of other components of the cycle may be useful in estimating the scale of water resources by calculating the hydrologic balance. The hydrologic balance supposes that the amount of water entering the aquifer is the same as that leaving it, less an amount that represents the water storage fluctuation:
I = O ± storage variation
where I (income) is rainfall, recharge from surficial water, other aquifers, wells, or other sources; and O (outflow) is superficial streamwater, runoff, evapo-transpiration, surficial and underground drainage, catchments, springs, and other hydraulic elements.
The method depends on knowledge of every element of the water balance in a particular area, and on their quantification. In practice it is necessary to find all terms of the equation through direct measurements or estimation, leaving aquifer recharge (which is more difficult to obtain) as a value to calculate indirectly. Stoertz and Bradbury described a method for calculating the spatial distribution of water income and outflow, using as starting points a piezometric map and some estimates regarding hydraulic conductivity and the thickness of the aquifer. The application of a two- or three-dimensional mathematical model as a basis for water balance model analysis is essential to the process.
Water-balance processes lead to groundwater flow between cells, considering all nodes have a fixed load that may be calculated by piezometry. The difference between income and outflow for individual cells is the recharge, or drainage, through the water table. It must be emphasized that flows calculated by this way depend very much on the conductivity values used, and on values for loads obtained by piezometric interpolation. In many situations (for example, with regard to pollutant management) it is sufficient to know the direction of water recharge flow during dry and rainy periods. The resulting iso-fluctuation maps, when integrated with other data, permit the evaluation of the volume of water stored into the aquifer as a water balance factor, and the estimation of local pumping effects. Figures 7a and b show piezometric results for two different years. (The pictures are superposed in Figure 7c.) Differences in value (positive or negative) have been attributed to every crossing point, and the value contours then mapped (Figure 7d).
Figure 7. Evaluation of aquifer recharge area by piezometric map analysis
In this example the recharge area is located in the southwest corner of the map and the drainage area in the northeast corner; this distinction is less clear if we consider only Figures 7a and 7b.
7. Advantages and Disadvantages of Artificial Groundwater Recharge
Artificial recharge offers several potential advantages:
· Aquifers may be used for the storage and distribution of water, and for removing contaminants by natural cleaning processes that take place as polluted rain and surface water infiltrate the soil and percolate down through the various geological formations.
· The technology is sound and generally well understood, both by technicians and the general population.
· Very little special equipment is needed to construct drainage wells.
· In rock formations with high structural integrity, few additional materials (concrete, soft stone, coral rock blocks, or metal rods) may be needed to construct the wells.
· Groundwater recharge stores water during the wet season for use in the dry season, when demand is highest.
· Aquifer water can be improved by recharging with high-quality injected water.
· Recharge can increase the sustainable water yield of an aquifer significantly.
· Recharge methods are environmentally attractive, particularly in arid regions.
· Most aquifer recharge systems are easy to operate.
· In many river basins, controlling surface water runoff to provide aquifer recharge reduces sedimentation problems.
· Recharge with less saline surface waters or treated effluents often improves the quality of saline aquifers, facilitating use of their water for agriculture and livestock.
· In the absence of financial incentives, laws, or other regulations to encourage landowners to maintain drainage wells, they may fall into disrepair and ultimately become sources of groundwater contamination.
· There is potential for contamination of the groundwater from injected surface runoff water, especially from agricultural fields and road surfaces. In most cases, surface runoff water is not pre-treated before injection.
· Recharge can degrade the aquifer unless quality control of the injected water is adequate.
· Groundwater recharge may not be economically feasible unless significant volumes can be injected into an aquifer.
· A very full knowledge of the hydrogeology of an aquifer is required before any full-scale recharge project is implemented. (In karstic terrain, dye tracer studies can assist here.)
· Disturbances of soil and vegetation cover during the construction of water traps may cause environmental damage to the project area.
:Artificial groundwater recharge.
:A lithologic unit (or combination of units) having an appreciable capacity to store and transmit water that is recoverable in economically usable quantities.
:A potentially advantageous method of storing water underground in times of water surplus to meet demand in times of shortage.
Available or exploitable groundwater resources
Water resources that can be exploited if certain socioeconomic conditions are satisfied.
:Artificial processes that increase meteoric precipitation.
:Direct artificial recharge.
:Removal of dissolved minerals (including salt, but also other chemicals) from seawater, brackish water, or treated wastewater.
:Exploitable groundwater resources.
:Indirect artificial recharge.
:Natural groundwater resources.
Natural water resources
:Water resources present in the environment.
:Potential water resources.
:Contaminant materials or organisms. Not all pollution is caused directly by human activities. Natural water pollution may be regarded as changes in the constituents and properties of water independent of human activities that are significantly greater than the expected normal variations.
Potential groundwater resources
:The maximum volume of groundwater that can be replaced by artificial methods.
:Increase in groundwater salinity at a given location and depth as a result of human impact(s). Intrusion can result from a variety of influences. However, a particular groundwater problem worldwide is saltwater intrusion in coastal aquifers. Here excessive pumping may cause lateral or upward movement of saline water into wells.
:The volume of water, measured as a fraction of a cubic foot, released from storage in each "column" of an aquifer (with an equivalent base value and a height equal to the full thickness of the aquifer).
:Total dissolved solids.
Wastewater recovery or regeneration
:A process by which wastewater is treated to permit reuse for irrigation, industry, and municipal supply. Regeneration is similar to wastewater treatment prior to discharge to lakes or rivers.
Chiesa G. (1992). La Ricarica Artificiale Delle Falde, 293 pp. GEO-GRAPH. [In Italian. Describes the different methods of artificial groundwater recharge, and the principles and methods of controlling and calculating the volume of water necessary for aquifer recharge.]
Custodio E. and Llamas M.R. (1996). Hidrologia Subterranea. (2 vols. 2359 pp.) Barcelona: Ediciones Omega, S.A. [This handbook provides a wealth of useful information on virtually all areas of hydrology, hydrogeology, and hydrochemistry. It contains authoritative coverage of the basic theories and principles, and indicates the data required for study and management of groundwater resources.]
National Research Council (1994). Groundwater Recharge Using Waters of Impaired Quality. 283 pp. Washington DC: National Academy Press.
O'Hare M.P., Fairchild D.M., Hajali P.A. and Canter L.W. (1997). Artificial Recharge of Groundwater. Lewis Publishers [This describes existing groundwater recharge projects. Examples are used to illustrate the common techniques used, show the variety of purposes for which recharge is planned, and provide concrete examples of the problems these projects sometimes face.]
Pérez-Paricio A. and Carrera J. (1998). Preliminary study for deep injection experiments at the Cornellà site, Barcelona. Third International Symposium on Artificial Recharge of Groundwater (TISAR). 21–25 September, pp 325–330. Amsterdam: A.A. Balkema. [Describes the deep artificial groundwater recharge method applied at a site in Spain.]
Pyne R.D.G. (1995). Artificial Recharge of Groundwater. 2: Groundwater Recharge and Wells. 376 pp. Boca Raton, FL: Lewis Publishers. [This book reviews the technical constraints on recharge, and various issues that have been addressed and resolved through research and experience at many sites. The book presents aquifer storage recovery (ASR) technology and traces its evolution since 1975 in the United States. The suggested procedures outlined should help achieve success with groundwater recharge through wells. Selected case studies are examined.]
Water Research Association (1970). Artificial Groundwater Recharge. Reading Conference, 21–24 September. 2 volumes. 481 pp. Marlow: Water Research Association.
Dr. Roberto Spandre was born on 30 April 1950. He took his M.Sc. in Hydrogeology at Complutense University, Madrid, Spain, and his Ph.D. in Geology at the University of Pisa, Italy. Since 1979 he has been Professor of Hydrology at UNAM (Universidad Nacional Autonoma de Mexico), Mexico City. In 1980 he was appointed Senior Geologist (Libya) at the Geological Research Center, Florence, Italy. From 1981–91 he was Research Hydrogeologist at the Universidad Autonoma of Madrid, and was also employed by the Foreign Office Ministry of Italy. Since 1991 he has been Research Hydrogeologist and Titular Professor in the Earth Sciences Department at the University of Pisa.
Dr. Spandre has undertaken research projects for the European Community, the Italian Ministry of Universities and Scientific and Technologic Research, the National Research Council of Italy, the Italian Foreign Office, and UNEP. He has worked throughout Spain and in Cuba, El Salvador, Ecuador, Colombia), Mexico (DF, Aguascalientes, San Luis Potosi, Michoacan), Angola), Libya), India), the Czech Republic, the United States), and Argentina). He is a member of AGID (Association of Geoscientists for International Development), IAH (International Association of Hydrogeologists), IAEH (International Association of Environmental Hydrology), UNCCD (Convention to Combat Desertificacion),ALHSUD (Asociación Latinoamericana de Hidrologia para el Desarrollo), and the CST (Committee on Science and Technology) of the United Nations. He has more than 70 publications to his credit.