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# Well Construction, Cones of Depression, and Groundwater Sharing Approaches

## Summary and Keywords

The importance of groundwater has become particularly evident in the late 20th and early 21st centuries due to its increased use in many human activities. In this time frame, vertical wells have emerged as the most common, effective, and controlled system for obtaining water from aquifers, replacing other techniques such as drains and spring catchments.

One negative effect of well abstraction is the generation of an inverted, conically shaped depression around the well, which grows as water is pumped and can negatively affect water quantity and quality in the aquifer. An increase in the abstraction rate of a specific well or, as is more common, an uncontrolled increase of the number of active wells in an area, could lead to overexploitation of the aquifer’s long-term groundwater reserves and, in some specific contexts, impact water quality. Major examples can be observed in arid or semi-arid coastal areas around the world that experience a high volume of tourism, where aquifers hydraulically connected with the sea are overexploited. In most of these areas, an excessive abstraction rate causes seawater to penetrate the inland part of the aquifer. This is known as marine intrusion. Another typical example of undesirable groundwater management can be found in many areas of intensive agricultural production. Excessive use of fertilizer is associated with an increase in the concentration of nitrogen solutions in groundwater and soils. In these farming areas, well design and controlled abstraction rates are critical in preventing penetrative depression cones, which ultimately affect water quality.

To prevent any negative effects, several methods for aquifer management can be used. One common method is to set specific abstraction rules according to the hydrogeological characteristics of the aquifer, such as flow and chemical parameters, and its relationship with other water masses. These management plans are usually governed by national water agencies with support from, or in coordination with, private citizens.

Transboundary or international aquifers require more complex management strategies, demanding a multidisciplinary approach, including legal, political, economic, and environmental action and, of course, a precise hydrogeological understanding of the effects of current and future usage.

# Introduction

Among all water resources, aquifers are one of the most important for humanity’s sustainable development. They represent 99% of accessible fresh water and are being used for multiple purposes by 2 billion people around the world (UNESCO-IHP, 2009). The majority of groundwater volumes are obtained from spring catchments and vertical wells, which represent a global abstraction rate between 600 to 800 km3/year. Groundwater is also the source of drinking water for many people worldwide, especially in rural areas. Inasmuch as groundwater provides drinking water, the quality of this resource is of the highest importance (Fetter, 1992).

The catchment of spring water at the source was the main groundwater system until the Industrial Revolution, when the possibility of using vapor energy (first) and petroleum (later) for pumping great quantities of water from the ground, in addition to drilling more and deeper wells, vastly increased abstraction. However, the main hydraulic difference between these two systems is the generation, during pumping, of a groundwater-level depression around the well, which has several negative environmental impacts and requires a more complex water management framework.

The actual worldwide augmentation of abstraction rates has directly affected headwater levels in some aquifers and the level of surface water bodies in relation to groundwater depletion zones. One typical example of these negative effects is the reduction of flow or the physical disappearance of some sections of rivers around the world.

Increases in groundwater abstraction rates may also impact water quality, which affects the availability of water resources as well as the quality of related ecosystems and has significant economic consequences. One typical example is the reduction of adult fish and shellfish captures due to excessive pumping in the inland part of coastal aquifers, which exist in many parts of the world. Excessive abstractions also decreases fresh groundwater discharges in related mangrove areas, wetlands, and coastal marine springs and changes the hydrochemical relationship between fresh water and marine water in fish breeding and shellfish coastal areas, thus diminishing biological productivity in these littoral zones.

# Basic Key Concepts

Many researchers have presented key hydrogeological concepts, highlighted by the contributions of Davis and de Wiest (1966), Bear (1972), Castany (1975), Freeze and Cherry (1979), Custodio y Llamas (1983), De Marsily (1986), and, more recently, Schwartz and Zhang (2003), Morris et al. (2003), Misstear et al. (2006), and Fundación Centro Internacional de Hidrología Subterránea (FCIHS) (2009).

Almost 90% of Earth’s surface is covered by water, but over 90% of that is seawater, with an average salinity close to 35 g/L. The remainder is fresh water, grouped in solid phases in polar areas and in a liquid state, basically running across geological formations that extend for tens, hundreds, or even thousands of square kilometers (groundwater). A small part remains in rivers and lakes (surface waters). These different water sources are interdependent and their relationship constitutes Earth’s hydrological cycle, which controls their distribution and magnitude around the globe.

## Aquifer Types

From a geological point of view, the great diversity of geological formations that are able to contain water in sediments or rocks can be classified according to their consolidation grade, but for hydrogeological management purposes, it is more useful to classify different geological bodies in aquifers, aquitards, and aquicludes according to their capacity to contain and transmit water.

An aquifer is a geological formation capable of storing and transmitting water in significant quantities. For that reason, discovering and characterizing aquifers is the main objective of different groundwater users, with the goal of obtaining the most water for the least expenditure of energy and money.

Aquicludes are geological formations that are able to store water, normally in very low quantities, and do not transmit it. Aquicludes are not suitable for significant groundwater exploitation, but may be a key factor toward groundwater quality protection.

Finally, the third hydrogeological formation is the aquitard. These geological bodies are capable of storing water in moderate quantities and slowly transmitting it under natural conditions. Normally, depending on their hydrogeological local parameters, aquitards are not suitable for intensive exploitation but play an important role in the exploitation of some aquifers.

Aquifers can also be classified according to their predominant water pressure regime. Two main types are free (unconfined) and confined aquifers.

Unconfined aquifers receive the main part of their water recharge directly from the surface through rain infiltration and from hydraulically connected surface water bodies such as rivers, lakes, and ponds. They usually have a shallow water table that is at atmospheric pressure (Figure 1A). In many free aquifers, it is possible to distinguish an unsaturated or vadose zone, above the groundwater table, in which air fills their porosity, and a saturated zone below the groundwater level where water fully occupies the space between solids, grains, or fractures. Under these conditions, it is possible to dewater this aquifer.

Figure 1. Main types of aquifers. (A) Free or unconfined aquifer. (B) Confined aquifer.

Confined aquifers are limited at their tops by aquitards or aquicludes, which indicates that the groundwater is under higher pressure than atmospheric pressure (Figure 1B). In this type of hydrogeological unit, the saturate zone coincides with the aquifer’s thickness. This aquifer class does not receive its main water from surface percolation and its main recharge zones are usually lateral or vertical groundwater inputs from other aquifer areas.

When a well penetrates the top of a confined aquifer, the water level rises inside the drill, and in some cases could reach the topographic surface (artesian conditions). Under these hydraulic conditions, pumping does not dewater the aquifer, but only diminishes its water head. However, if the aquifer’s abstraction rate increases with time, it is possible to change the pressure regimen from confined to unconfined.

## Hydrogeological Parameters

Water in geological formations occupies the free space or porosity between different sediment grains or filling fractures and rock discontinuities. The part of this porosity that is hydraulically connected and is capable of producing flow is called effective porosity.

The value of effective porosity in a geological body depends mainly on the pore size and its distribution. Good aquifers show a high percentage of interconnected pores or discontinuities, where groundwater circulates through them.

Hydraulic conductivity, also called permeability (k), is a constant of proportionality (with LT-1 units) that was defined in the 18th century by Henry Darcy (De Rooij, 2016). Permeability values depend on the characteristics of the geological bodies as well as on temperature, density, and viscosity of the fluid. For fresh water at 25 ºC, the currently analytical expression of k is

$Display mathematics$

where Q is the circulating flow through a known section (S) of a porous granular medium, and the applied hydraulic head difference (Δ‎h) along the length (L) is the hydraulic gradient (i). The hydraulic gradient plays a critical role in defining the spatial distribution of the water head potential at every point in the porous media. During pumping, a specific area around the well is generated where the gradient is progressively higher according to the proximity of the abstraction.

Hydraulic conductivity is one of the most commonly used parameters in hydrogeology to quantify groundwater flow velocity, water residence time in an aquifer, or the behavior of groundwater during pumping. The product of the values of aquifer permeability and saturated aquifer thickness is called transmissivity (T). More effectively than hydraulic conductivity, this parameter describes the rate of flow through the real water-saturated aquifer section.

Another important parameter for understanding the behavior of a pumping well is storativity (or storage coefficient, S). S is a dimensionless parameter that is defined as the water quantity released or added to aquifer storage when one unit of water level declines or increases.

Finally, the relationship between transmissivity (T) and the storage coefficient (S) is called hydraulic diffusivity (D). The diffusivity parameter shows the approximate effect of pumping depletion and propagation across the aquifer (Barlow & Leake, 2012). Water head level changes and flow effects propagate faster through groundwater bodies with higher D values than with lower D values.

# Pumping Wells: Main Characteristics and Analysis of Their Effects

Historically, humanity has developed many imaginative technological solutions for extracting water from the ground. Some of them, such as rock galleries, drains, direct catchments of springs, or ditches, are still in use in certain geological media or for specific purposes. However, vertical wells are the most popular catchment solution in the 21st century.

A vertical well consists of a drill or a vertical excavation of a circular, square, ellipsoidal, or irregular section, with the capacity to attract groundwater inside it; the groundwater is removed to the surface manually or, more usually, with a pump, according to the piezometric level of the aquifer. A vertical well is a hydraulic infrastructure designed specifically according to local aquifer characteristics and its pumping rate is set to obtain maximum hydraulic efficiency. In a modern well, three main construction parts are commonly differentiated (Figure 2):

1. 1. The surface well zone includes the shallower parts of the well. It must be designed to protect the upper part of the well and prevent undesirable infiltration from the surface toward well screens across the annular space between the drill and the casing. A common construction solution is to inject a cement seal of about 3–5 meters deep, filling this part of the annular space.

2. 2. Nonproductive parts of the well include sections that are necessary for drilling, casing, cementing, and sealing in order to access deeper productive levels. These areas do not provide water or contain bad quality water. The drill is normally coated with a blind casing of steel or PVC, which prevents the fall of sediments or rocks inside the perforation. Low grain-size materials or cements normally fill the space between the blind casing and the drill in order to stabilize the casing and prevent sloughing.

3. 3. The productive well zone is the area of the well where water is expected to be obtained. In these zones (screen or filter zone), the casing is not blind and is made with a large number of holes or slots that permit water entrance into the well. The total area of holes or slots related to the total area of the screen casing is an important parameter in well design and controls the well’s future yield. In the admission zones, the space between the drill and the screen casing is usually filled with a granular sediment such as gravel or coarse sand in order to protect the screen, stabilize the drill walls, and reduce the incorporation of solid material into the well. These materials, called pre-filter or filter pack, need to be selected taking into account the width of the screen orifices.

Figure 2. Principal components of a groundwater well.

Depending on the objective and geological context, many types of vertical wells can be designed (Driscoll, 1986). The most representative are the following: (1) pressured vertical wells for artesian confined aquifers; (2) gravitational vertical wells for unconfined aquifers or non-flowing artesian confined aquifers: (3) shallower dug wells; (4) large-diameter wells (up to 800 or 900 mm); (5) very small-diameter wells (20–40 mm well points); (6) vertical wells with radial collectors at different depths or in the bottom of the drill; and (7) injection or deep recharge wells, used to introduce water into aquifers.

The correct selection, design, and construction quality of a well has a major influence on how water depletion in the aquifer occurs, on the well’s extension and maximum depth, and on the hydraulic and energy efficiency of the abstraction, which controls the well’s economic cost. Water well design and construction has been thoroughly researched in many languages; note the work of Driscoll (1986), Harlam et al. (1989), and Moss (1990).

## Hydraulic Tests

Various types of tests are used to obtain aquifer hydrogeological parameters (De Rooij, 2016). Some of them, such as pulse or slug assays, are specifically adapted to low permeability media (usually aquitards). Others, such as the pumping–recovery test at constant flow, are able to obtain T and S values. Finally, the pumping test with variable flow (step-drawdown test, SDT; Custodio y Llamas, 1983) is used to establish hydraulic well efficiency.

The SDT assay analyzes water well drawdown evolution(s) with different abstraction rates. Part of this drawdown is due to the hydrodynamic aquifer response and shows a linear relation with flow at close to ideal conditions, which is easy to reproduce using common analytical equations (Hantush, 1964; Davis & de Wiest, 1966; Lohman, 1972; Custodio & Llamas, 1983). However, in real wells, additional drawdown exists which is controlled by non-linear factors such as the age of the well, a bad design or well execution, or a bad pump selection. The flow rate (called critical flow), where the relation between flow and drawdown begins to describe a non-linear evolution, is considered a good indicator of the limit of well abstraction in reaching maximum well efficiency with optimal energy costs.

Due to higher energy costs and expanding dependency on groundwater in many areas, well efficiency has become a cause for concern in water management. Well efficiency may be defined as the ratio of the actual to the theoretical specific capacity (TSC), or:

$Display mathematics$

where Q is the flow rate and s is the well’s dynamic drawdown.

In practice, it is not easy to solve the TSC equation for a real well. For that reason, Rorabaugh (1953) defined efficiency in terms of flow (Q) as:

$Display mathematics$

where B and C are constants and n is an exponent having a value usually between 1 and 3. Using SDT assay methodology, the constants B and C can be obtained to solve the equation according to different values of Q and also fix the well’s critical flow point.

Figure 3 plots the relation of well efficiency between six wells. The comparison of the different curves allows establishing a hierarchy of wells according to their efficiency (FCIHS, 2009). Additionally, the analysis of successive SDTs in the same well permits an evaluation of the aging of the well over time and prepares for future maintenance activities.

Figure 3. Relation between hydraulic efficiency (Ef, %) and discharge rate (Q, m3/day) of several wells, obtained from their respective step-drawdown tests (Rorabaugh, 1953). The white rings mark the position of their respective critical flow points.

## Pumping Well Affections

As discussed (see “Pumping Wells: Main Characteristics and Analysis of Their Effects”), good construction and a hydraulically efficient well, together with good management of the aquifer, are the basis of sustainable use of groundwater resources. Otherwise, two kinds of generally undesirable effects may occur:

1. 1. A reduction in the quantity of available water: The depletion of water levels when natural rates of replenishment are exceeded can result in an increment in economic costs and may induce the aquifer’s partitioning, land subsidence, sinkholes, and terrain-compaction processes, sometimes associated with a drastic reduction in the aquifer’s storage capacity.

2. 2. A degradation of the quality of the groundwater reservoirs: Such degradations may be due to (a) the augmentation of water mineralization with the exploitation of deeper aquifer levels, (b) an increment of contaminant concentrations from surface recharge (such as sewage from urban areas or industrials activities), (c) marine intrusion processes, and (d) the increment of infiltrations of bad quality waters from agricultural runoff (Morris et al., 2003).

The possibility that pumping rates can create undesirable effects depends on a combination of several factors such as: (1) the aquifer’s geology, (2) a correct design and proper execution of the well, (3) the intensity of the abstraction rate and its temporal persistence, (4) the number of wells and their spatial distribution in the aquifer, and (5) the existence of a proper management exploitation plan.

When a pump is turned on, it provokes a depletion of the water level inside the well and, consequently, a local modification of the hydraulic gradient around the well which causes the introduction of groundwater in the catchment, across the admission zones.

At the beginning of pumping, the static water level experiences a fast and transient descent, augmenting its depth related to the surface. In normal situations, the drawdown velocity decreases progressively with time until the water head level in the well (and in the aquifer) reaches a new depth of equilibrium and is more or less stationary; this is called the dynamic well water level (DWWL; Figure 2). Well diameter plays an important role in the evolution of DWWL. Considering equal pumping rates and permeability values, smaller well diameters create larger dynamic depletions inside the catchment.

To compensate for the quantity of water that has been extracted, a radial descent of the aquifer’s piezometric level around the catchment occurs. That effect is more intense in the immediate vicinity of the well but tends to disappear with well distance.

The distance where the effects of pumping are negligible is called the influence radius (R), and it is useful in groundwater management to fix appropriate distances between wells. The R values can be obtained from hydraulic assays, and a number of references describe and calculate the spatial evolution of the cones of depression for main aquifer types (free, confined, or semi-confined) and for transient or stationary conditions (Thiem, 1906; De Glee, 1930; Theis, 1935; Copper & Jacob, 1946; Hantush, 1964).

A geometric 3D figure that describes a spatial radial drawdown is similar to an inverted conical form where the vertex is the dynamic well water level (see several 2D sections in Figure 4 and a 3D numerical simulation in Figure 10). This conical form deepens and is wider with time until it reaches a stationary state, where the expansion stops. During this transient time, the cone lateral extension (R) depends on the hydrogeological parameters of the aquifer: its transmissivity (T) and storativity (S), the pumping time (t), and the aquifer type (confined or unconfined).

The different effects of the hydrogeological parameters in the shape of the cone can be seen in Figure 4. Shallower descent cones will occur in aquifers of high transmissivity (T) and high storativity values (S), and for a same T value, lower S values provoke deeper cones.

Figure 4. The effect of transmissivity (T) and storativity (S) relations in the shape of well cone depressions. For the same flow and pumping time (400 m3/day and 1 day), high T/S ratios define shallower and wider cones more than low T/S ratios. Shapes were calculated using Jacob’s logarithmic approximation of the Theis equation (Jacob, 1963).

When different wells are pumped at the same time, each develops cones of depression, depending on each well discharge. The spatial combination of the different cones creates a larger tridimensional zone of depletion, with an extension that is the lineal combination of their individual piezometric drawdowns (Custodio & Llamas, 1983).1 In addition to the abstraction rate and the hydrogeological parameters of the aquifer, the distance and the hydrogeological position between multiple wells is one of the most critical parameters that defines the shape of the depletion area and can be modeled by mathematical methods (Figure 5).

Figure 5. Example of the effect of a multiple well abstraction in a free aquifer, calculated using mathematical codes (MODFLOW package; Harbaugh et al., 2000). In this case, the numerical simulation reflects the combined abstraction of a pumping group (B) that creates a reconcentration of the flow lines around the wells (gray arrows), and also the “practical” partition of the aquifer in two areas, with a significant distortion of the previous natural flow direction (A, blue arrow).

As stated (see “Introduction”), when an aquifer and a surface water body (such as a river or a lake) are hydraulically interdependent, it is possible to modify their respective water flow system. In this situation, usually pumping of an aquifer creates an associated water volume reduction of the related surface mass. In the case of a river, this process is called “capture” or, more specifically, “streamflow or river depletion” (Barlow & Leake, 2012), and it is generally found in most free alluvial aquifers hydraulically connected by rivers.

The most important parameters that control depletion evolution of surface water bodies by pumping are hydraulic diffusivity (D) and the distance between wells and rivers, in addition to the dimensions of both water bodies.

In most river–aquifer systems, the reduction of river flow generated by an excess of pumping rates at the early stages of overexploitation is not heavily reflected in declining aquifer storage (Figure 6). This effect is not permanent and can be maintained until the river dries, whereupon the piezometric levels suffer drastic descent.

Figure 6. Evolution of a river–aquifer water level relationship in an intensive groundwater exploitation context. (A) Natural conditions. (B) Early pumping stages: modification of hydraulic gradient conditions between the river and the groundwater body. (C) Persistent intense exploitation context. Q, pump flow. 1,2,3: evolution of the well’s dynamic water level.

For instance, the Cubeta de Sant Andreu is a small alluvial unconfined aquifer, with no more than 30 km2, connected by the Llobregat River in an industrial zone close to Barcelona’s metropolitan area. This aquifer had a very intensive abstraction rate beginning in 1950, with a maximum in the 70th decade (with more than 15 hm3/year in 1975), when the river suffered severe water declines that forced a drastic reduction in well abstractions. During the years 1988 through 1997, a surface artificial recharge determination (see “Augmenting Groundwater Inputs: Expanding Use of Artificial and induced Recharges of Aquifers”) was implemented to increase river infiltration. If the water aquifer’s balance is used to compare both periods (Figure 7), it is seen that this recharge actuation allowed extracting of between 2 and 4 hm3/year additional groundwater volumes from wells.

Figure 7. Water balance between abstraction rates and surface recharge (natural and induced by basins) in the Cubeta de Sant Andreu aquifer between 1975 and 2005. The blue diamonds show the evolution of the aquifer’s storage.

## Long-Time Intensive Groundwater Exploitation: Over-Abstraction or Groundwater Mining

In many aquifers around the world, an increase in the number of pumping wells has generated wide areas affected by deep depression cones and significant aquifer depletions. In some cases, regional descents of water levels appear episodically, and they are naturally re-equilibrated by natural groundwater recharge periods when the abstraction period ends or diminishes. However, in other situations, groundwater depletion is, for the most part, permanent and is accompanied by a constant long-term water head level tendency toward descent. In some aquifers, a related depletion of the river base flow of wetlands occurs. In general, the definition of an intensive development (or use) of groundwater is when a significant proportion of inter-annual renewable resource is withdrawn from aquifers and modifies the aquifers’ original hydrogeological behavior (Custodio & Llamas, 2001). If this new “state” causes significant and negative ecological, political, social, or economic changes, it is defined as an overexploitation situation.

Recognizing these problems, the safe yield concept was first developed and the sustainability concept later evolved; both have long been discussed (Morris et al., 2003). The aquifer safe yield concept was defined as the quantity of water that can be withdrawn from an aquifer without producing an undesired effect. More recently, the concept of sustainability has been defined as an aquifer’s development level that meets the needs of the present generation without compromising the ability of future generations to reach their needs.

Groundwater mining is an aquifer management method that has been applied at some of the world’s biggest aquifers, such as the Nubian sandstone aquifer in North Africa and Arabian sandstone aquifer systems in Asia. This strategy treats the water of these aquifers as if it is a non-renewable resource.

In many cases, confined geological formations contain higher proportions of “fossil” waters that have been incorporated into the aquifer in an older and wetter climatic context, which no longer exists. In this setting, usually well cones grow progressively with time until they reach the technical limit of abstraction. During the time that groundwater reserves are being exploited, significant economic benefits may be realized and the negative effects of such abstractions are generally very limited. For these reasons, groundwater mining may not be defined strictly as overexploitation.

The mining of fossil waters may be a suitable option under four main management conditions (Llamas, 2001; Ribera, 2016): (1) The paleowater aquifer management plan must guarantee that, with planned volumes of abstraction, it is possible to obtain water for a long period of time (e.g., between 50 and 200 years); (2) the abstraction plan must verify the non-existence of any kind of environmental impact on other areas of the aquifer, especially for large aquifer systems and, if appropriate, its impact in others countries as well; (3) water users must be informed about the fossil nature of the water in order to make informed investments or consider future supply alternatives; and (4) the salinity of groundwater must be assessed according with its uses.

## Water Quality Effects Related to Intensive Pumping

Changes in groundwater quality resulting directly from or induced by abstraction can be classed as overexploitation if the changes have a negative effect upon the value of the resource (Morris et al., 2003) or affect related ecosystems. The aquifer’s water chemical composition is the result of a combination of natural mineralogical, physical, chemical, and biological factors where the mineralogy of the rock or sediment grains, the hydrochemistry of the different groundwater inputs, the REDOX state, the pH or temperature conditions, and the residence time of the water in the aquifer are usually the most affected. Some aquifers show a relative homogeneous and stable distribution of their water composition in extension and depth but in other cases, important variations in groundwater chemistry are found.

At the initial stages of groundwater exploitation, the effect on the chemistry of aquifer distributions may be small or negligible. In intensive pumping contexts, the progressive drawdown of an aquifer’s water levels and the depletion of shallower formations permit deeper geological levels to increase their volume of water contribution with respect to the shallower waters inside the well. In this situation, a typical effect is a change in the REDOX state due to the minor contribution of shallower and richer oxygenated groundwater levels as well as increases in salinity.

The induction of flow of low-quality water into the aquifer because of a new hydraulic gradient distribution is typical of the groundwater degradation process. This kind of pollution produced in some wells changes the quality of the water abstracted over time, affecting not only major solute water compounds, such as chloride, sulfates, and sodium, but also minor components such as flurorine, bromide, and arsenic.

In some cases, the origin of the pollutant is natural. For instance, some authors have described these effects in sedimentary multilayer aquifer systems in the center of Spain (Iglesias, 2001); in Argentina, pollutants were associated with “pampeano” sediments, where significant increases in arsenic in groundwater were related to increased pumping for human supply. Most likely the decline in the water level created a change in the REDOX state and mineralogical destabilization of arsenopyrite contained in some thin-grain strata interbedded in the aquifer system.

In other situations, the origin of the pollutants is clearly anthropic. Increases in agricultural production through groundwater irrigation is a well-acknowledged fact (Burke, 2001), but the effects, especially long-term ones, of such increases are, in most cases, not clearly studied.

Finally, in coastal areas, decreases of fresh groundwater discharges to the sea due to high augments of abstractions in most irrigation and touristic areas have caused modifications of natural density-driven and chemical equilibriums between marine and inland groundwater masses and the progressive salinization of most wells (Custodio, 1993; Fidelibus et al., 1993; Iribar & Custodio, 1993; Morris et al., 2003).

Groundwater salinization processes also may appear in closed continental basins affected by irrigation, especially if the geology of the basin is mainly composed of an alternation of clastic and carbonate rocks, with interbeded gypsum or salt layers which have a high capacity for dissolution, as in Monegro’s aquifer system at the Ebro’s tertiary basin in central Spain (Samper & García-Vera, 1993) or in the Inner Mongolia Yao Ba area (Morris et al., 2003).

In other regions with intensive agricultural activity, the main threat is the increase in concentration of nitrogen compounds in groundwater due to the absence of integrated fertilization–well abstraction plans and poor knowledge of the aquifer’s hydrogeological and physicochemical characteristics.

## Microbiological Contamination

Poor well design and construction or a bad sitting of the well, in addition to a bad pumping pattern, could initiate the introduction of pathogens such as viruses, bacteria, or protozoa species into the aquifers, according to their metabolism and resilience (i.e., coliform bacteria such as Escherichia coli or other organisms such as Legionella pneumophia, Clostridium perfringens, or Vibrio cholerae).

Widespread groundwater use in rural and many urban areas for drinking water, with a minimum grade of disinfestation in some cases, can cause severe health problems such as gastroenteritis, cholera, hepatitis, typhoid fever, or giardiasis (Bitton & Gerba, 1984).

## Geomechanical Effects: Terrain Subsidence

When significant water volumes are abstracted from aquifers, the equilibrium of pressures is changed between the fluid and the aquifer solid structure. In porous non-consolidated aquifers, the fluid and the solid grains support all the weight of the sediment column. If the water is extracted, these types of aquifers respond with a more compact geometrical distribution of the grains, reducing the volume of porosity. This reduction takes place mainly in the vertical axis of the aquifer (its thickness) and is important only in some geological contexts. The existence of silts, which are more plastic, or clay layers above or intercalated in the aquifer system is a factor that could provoke a significant topographical depression at the surface, especially if the exploitation persists over an extended period of time. Many examples of water abstraction subsidence exist around the world, mainly in aquifers under or near big cities that use groundwater resources for their supply (Iglesias, 2001) or in aquifers used for intensive agricultural production (i.e., San Joaquín Valley, CA; Bouwer, 1977). In some big cities or in their metropolitan areas, such as Mexico City, the variation of the topographic surface generated by abstractions forces frequent repairs to the city’s sewer network.

In the city of Bangkok, like other megacities in southwest Asia, most of the population’s homes are built over alluvial, estuarine, or deltaic sediments, with an alternation of fine gravels and sands (with moderate to high transmissivity rates and shallower water heads) and silt levels, which permit intensive groundwater exploitation. In Bangkok (Morris et al., 2003), such exploitation resulted in depletion of groundwater levels by up to 60 cm during the 1980s, producing land subsidence that reached values close to 80 cm in the center of the city.

However, in other aquifers where the existence of clay–silt levels is not so prominent, the subsidence process is significantly lower. This is the case of the lower Llobregat Valley aquifer system, a free-type coarse gravel and medium sand alluvial aquifer without significant silt levels, where the effect of more than 100 years of intense abstraction (between 30 to 73 hm3/year) seems not to have created land subsidence rates higher than 10–20 cm (Figure 8).

Figure 8. Local land subsidence around a 70-year-old well, related to more than 100 years of groundwater exploitation in the lower valley Llobregat aquifer (Barcelona). The terrain has been historically refilled and covered by asphalt, but the external structure of the well still reflects land level variation.

# Groundwater Sharing Approaches

Hydrological planning and management of aquifers has experienced dramatic changes since the late 1980s with the introduction of basic environmental factors such as evidence of the Earth’s global warming and the onset of the Internet revolution, which has allowed real-time transmission of information across the globe.

These changes can be seen, for instance, in the evolution of classic criteria for water governability toward more inclusive water governance or between the hydrostatic term of groundwater reserves toward more hydrodynamic and sustainable groundwater renewable resources.

At the same time, new management tools have developed such as vulnerability analysis (Vrba & Zaporozec, 1994) or methodologies to calculate groundwater protection areas (Bear & Jacob, 1965; Albinet, 1972; Wyssling, 1979). The goal is to extend parts of water management decisions to the general public and to end users while, at the same time, preserving water resources and related ecosystems as well as coping with specific contexts such as management of drought episodes or the establishment of rules of water access for small islands. These goals need to be accompanied by economic considerations of costs for their implementation.

## Concepts of Groundwater Reserves and Resources

Schwarz (1989) defined the classic main function of an aquifer as a component of a water resource system, highlighting its ability to be an important supply source (water reserve) and a suitable system for storing water and improving the quality of the water supplied.

In the late 2010s, the role of the aquifer as a guarantee of life and a preserver of the quality of dependent ecosystems has been joined to these classic functions. A new interdependent vision for the future global use of water (GWP, 2000) has been established through the principles and methodologies of the United Nations’ Integrated Management of Water Resources (IWRM) concept.

Aquifer water reserves represent the totality of available water that can be extracted at a given time. This static concept refers to the quantity of groundwater contained in a given moment, but groundwater is generally a renewable resource, and the volume of groundwater varies with time. For this reason, a groundwater resource is defined as that flow of water that can be obtained from an aquifer system during a period of time long enough to undergo permanent abstraction, regardless of seasonal fluctuations. The concept of a groundwater resource is associated with the dynamic concept of the water cycle and the introduction of the dimension of time, which can be defined using the aquifer’s water balance.

A groundwater balance can be defined as the difference between water inputs (I) and outputs (O) that take place in the aquifer during a defined period (i.e., flow units, usually in Hm3/year) and produce variations in the aquifer’s storage (S). The general equation is:

$Display mathematics$

The types of inputs and outputs are diverse, according to natural or human origins, and unique to each aquifer. Table 1 and Figure 9 describe some of the most common.

Table 1. More Common Components of Groundwater Balance

Inputs

Description

Outputs

Description

Rainfall recharge

Depends on the climatic context

Pumping abstractions

Depends on the intensity of the abstractions

River infiltration (influent river)

Depends on the relation between river and aquifer water levels and the vertical permeability of the riverbed

River drainage (effluent river)

Depends on the relation between river and aquifer water levels and the vertical permeability of the riverbed.

Contributions from other aquifers

Depends on the relation between water head aquifers

Contribution to other aquifers

Depends on the relation between water head aquifers

Artificial recharge disposition

Basins, pools, wells

Artificial drainage disposition

Ditches, wells, galleries, qanats

Urban recharge

Losses from city water networks

Springs

Depends on the relation between the piezometric and topographic level

Seawater intrusion

Related to intense pumping abstraction rates near the seaside

Contribution to wetlands

Depends on the relation between wetland water level and aquifer water head

Most aquifers are linked to the natural water cycle, unlike other natural resources such as ore deposits or fossil water aquifers (see “Long-Time Intensive Groundwater Exploitation: Over-Abstraction or Groundwater Mining”). In any case, a good aquifer balance is based in a good understanding of the conceptual model of an aquifer’s inputs and outputs (Figure 9).

Figure 9. Relationship between a conceptual aquifer model (A) and its associated hydric balance (B). The different inputs and outputs that the balance quantifies are used to obtain storage variation for a specified time. These must be previously identified in the conceptual model, and also the hydrodynamic relationship between different water bodies involved must be established (e.g., the water head relationship between the river and the unconfined aquifer). At the same time, the conceptual model must fix the geographic and hydrogeological boundaries of the balance.

This means that for a given time and in a normal climatic context (e.g., 1 year or an annual average of several years), if one pumps a groundwater volume only equal to (or slightly lower than) the sum of inputs, important effects related to the aquifer water level and to its associated ecosystems will not be generated. In contrast, when the abstraction regional rate is systematically higher than water inputs, the quality and quantity of groundwater lessens.

## Management of Aquifers

For centuries, many communities around the world have used groundwater as their main source of water, and many people are entirely dependent on it. Additionally, a large number of agricultural operations are partly or extremely dependent on groundwater, under different climatic contexts, especially in arid or semi-arid zones.

During the 20th century, the world’s population grew from 1.6 billion around 1920 to close to 7.5 billion in 2016, and agricultural production increased from 50 million hectares to more than 250 million hectares in 2017. These increases indicate that the demand for water has become seven times higher than a century ago. An important part of this water increment has been contributed by aquifers through the advantage of two technical and scientific advances: the invention of the turbine submergible pump and improvements in well drilling.

Unfortunately, the evaluation of these advances and the planning of an adequate monitoring network have not occurred. In Spain, for instance (Llamas, 2001), the different regional basin water agencies did not initiate their first quantitative evaluations of groundwater reserves until 1960–1970 when aquifer abstraction problems began to appear.

Historically, during the 20th century, many classic management practices were addressed to prevent hydrological affection between wells and consisted of fixing a minimal (prefixed) distance between catchments. In many cases, this solution was shown to be inefficient because these distances did not take into account the heterogeneity of hydrogeological aquifer parameters, which can cause significant decreases in the static and dynamic water levels of nearby wells.

A management groundwater plan must be adapted to the conceptual model of an aquifer’s behavior and its hydric balance in order to maintain the quality and quantity of its groundwater reserves, in accord with specific social, political, or institutional contexts, legal regulations, and technical and environmental requirements. One attempt to incorporate hydrogeological aspects into a general water management strategy is the Groundwater Directive (Comisión Europea, 2008), which was developed in response to the requirements of article 17 of the previous Water Framework Directive (European Commission, 2000).

According to Morris et al. (2003), an effective aquifer management plan requires awareness of the state of groundwater quality and quantity (with representative hydrogeological data that must be provided by monitoring), in compliance with the following: (1) a minimal understanding of the aquifer; (2) an understanding of widely accepted and clear existing water laws and rights; (3) surveillance for monitoring adherence to regulatory measures; and (4) government and user awareness of the importance of groundwater and the plans for its preservation.

From a methodological point of view, the design of a management groundwater plan follows the execution of several orderly steps or stages. The first stages are normally executed through the evaluation of preexisting information in order to determine the distribution and temporal evolution of groundwater levels and the localization and extension of the main abstraction cones for the purpose of generating piezometric maps. Additional hydrochemical information should be compared with piezometric information to check any link between the quality of the water and abstractions rates or the effect on other interdependent water bodies.

At the same time, a management plan requires an approximation of annual renewable resources (i.e., the relationship between aquifer inputs and outputs) under different climatic contexts (dry, normal, and wet scenarios). This range or average of flow values, associated with statistic deviation, can be obtained from an aquifer’s water balance through use of methodologies defined previously, thus presenting an accurate identification and quantification of the aquifer’s inputs and outputs (FCIHS, 2009).

The general stages or studies that are commonly included in a groundwater management plan are as follows:

1. 1. Locate and identify water wells or catchments and collect level and groundwater data. Collect and analyze previous drillers’ logs and preexisting measurements of water levels and water quality analyses that are available for each well. Translate and introduce this information into a Geographical Information System (GIS), with an associated database that is able to produce graphics and basic mathematical (analytical) operations for analysis in next stages, and for establishing the first monitoring program.

2. 2. Prepare piezometries and graphs showing the temporal evolution of groundwater levels, their related hydrochemical main parameter concentrations, and historical rates of aquifer extraction.

3. 3. Determine the amount of groundwater that is extracted by each well and other aquifer water outputs (e.g., evapotranspiration, exports to other groundwater bodies, or overflow of freshwater to the sea).

4. 4. Make a robust conceptual model of the aquifer and its related hydrogeological balance. The difference between the inputs and outputs (in volume units) from the balance determines a first value for aquifer water storage variation for a specified period (initially, at least 1 year).

5. 5. Establish a general estimate of groundwater reserves managed by the plan and fix the annual renewable resource with the water inputs and according with their respective storage variation.

The annual average aquifer storage variation can be fixed comparing the volume of groundwater between the beginning and the end of a representative number of hydrological years. These annual volumes can be approximate, for instance, in obtaining the resulting volume for crossing the piezometry at the beginning and the end of each year (Figure 10), taking account the porosity of the aquifer.

Figure 10. The water volume below the water head level (A) and above the aquifer bottom (B) under stationary conditions should be considered the reserves for a specific time (in the case of the aquifer 3D figure, 120 days after the start of pumping) and calculated if the effective porosity (me) is known. In many cases, the groundwater level is not static and transitory-hydrodynamic effects must be accounted for, such as abstraction cones (D). (C) Topographic surface. Graphic created using VMDOFLOW 3D explorer code (Waterloo Hydrogeologic, 2005).

The accuracy of the value depends fundamentally on the quality of the geological information available. The reserve volume obtained should be considered an aquifer reference, but it is not enough information for drawing up a good management plan. When extensive time series data are utilized of groundwater balance that have included different climatic contexts, one is then able to define for dry, wet, or normal groundwater recharge climatic contexts the different relationships between water inputs and outputs, and to define the proper abstraction rates for each management situation. The annual renewable groundwater resource (ARGR) is the average of the annual aquifer natural water inputs for a representative climatic inter-annual series.

1. 6. Redefine the original pumping rates in the aquifer, taking into account the value of ARGR and the relation between the variation of levels and the quality of groundwater. Quality aspects are especially important in aquifers affected by intensive exploitation (e.g., Sonora State, Mexico [Morris et al., 2003]), in coastal areas (e.g., many aquifers along the Mediterranean coasts [UNEP, 2015]), or in an urban context (e.g., Jakarta or Bangkok [Morris et al., 2003], or Barcelona’s metropolitan area [Iribar & Custodio,1993]).

2. 7. Determine if the extraction of groundwater can cause subsidence or the collapse of the terrain. If so, prepare a specific surveillance program.

3. 8. Develop (if necessary) a plan to control excessive groundwater extractions. This plan may include several options such as economic incentives, taxes according to the pumped flow, restrictions only in negative climatic contexts, and substitution of groundwater from desalted or regenerated water. Ideally, the reduction of abstraction could be voluntary and assumed by community groundwater users, but should also be established and implemented by a regional water agency. Usually, the more common ways are to (a) control the quantity of the water abstracted, (b) modify the timing of abstractions, (c) relocate the abstraction boreholes, and, more recently, (d) define and develop systems to increase recharges of the aquifers.

4. 9. During adverse climate periods, such as prolonged droughts, a management plan must consider more restrictive additional measures, normally of an exceptional and nonpermanent character.

5. 10. Perform periodical reviews. A management plan is a dynamic sum of norms that must be periodically adapted to socioeconomic and natural water balance changes (e.g., Earth’s global warming).

Finally, not all aquifer systems require the same management complexity. According to their hydrogeological setting, it is possible to detect five groups that pose special challenges (modified from Foster & van der Gun, 2016): (1) multilayered aquifers systems where the evolution of groundwater development usually results in a large number of shallow wells in free aquifer zones, which are susceptible to gradual depletion and contamination by downward leakage when a second series of large wells is drilled to exploit other deeper levels; (2) free-type aquifers with high connectivity with surface and related shallow water bodies; (3) non-renewable groundwater resources (fossil water or paleo-waters); (4) aquifer systems at risk of salinization; and (5) urban–peri-urban zone aquifers.

## Coupled Use of Spatial Analysis Tools and Numerical Simulation Software in Groundwater Management

Numerical aquifer models are one of the most powerful hydrogeological tools to solve flow and transport general equations in 2D or 3D and to provide spatial and temporal data distributions of water heads, temperatures, flow sense, or solute concentrations (Holzbeker & Sorek, 2005). Many different software tools are currently available (2019) to help users to set up numerical models.

The aim of mathematical models is to assist in the solution of practical groundwater situations, simulating water flow and solute transport (solving their respective numerical flow and transport equations), usually in porous media. In many cases, the modeling evaluations are useful for improving understanding of hydrogeological systems. Forecasting and studying responses in relation to different hydrological contexts is also one of the greatest benefits of modeling work. As of 2019, codes such as MODFLOW (Harbaugh et al., 2000) and FEFLOW. have become the most popular.2

Finally, it is necessary to state that numerical model results must represent an aquifer’s real behavior, and implementation of a model’s results must be tempered by strong knowledge of the conceptual aquifer model and the results of its related hydrogeological balance.

In parallel, GIS software allows storage and manage of an enormous amount of georeferenced data and offers many possibilities of presenting this information in maps at any scale. The combination of numerical models and GIS’s software is especially useful, for instance, for defining different aquifer management polygons (ACA, 2003) and for vulnerability mapping (Figure 9).

## Aquifer Vulnerability as a Methodology to Describe Pumping Effects

The concept of groundwater vulnerability is based on the assumption that the physical environment may provide some degree of protection for groundwater from natural or human impacts, especially concerning shallower contaminants entering the subsurface environment. The term aquifer vulnerability was introduced by Margat in the late 1960s (in Vrba & Zaporozec, 1994) to describe the degree of resilience of groundwater against the effect of contaminations as a function of their hydrogeological characteristics. The main consequence of groundwater vulnerability is that some land areas are more vulnerable to groundwater affection than others, and this spatial distribution can be represented in maps (Figure 11). Another consequence is that the value of natural vulnerability depends on several hydrogeological and independent factors such as permeability, recharge rates, groundwater depth, litology and granolumetry of the aquifer, soil type, and topography. These factors can be analyzed, quantified, and combined to obtain an integrated vulnerability index (VI) that is representative of a specific area or polygon.

Figure 11. Vulnerability map of an alluvial unconfined aquifer, created using DRASTIC methodology (Aller et al., 1987). The more than 500 polygons with different vulnerability indexes, obtained by the combining of seven hydrogeological factors, are grouped into five categories, from very low to very high vulnerability. Modified from ACA (2003).

The analysis of intrinsic or natural vulnerabilities is one of the most useful planning tools, especially for free-type aquifers (Vrba & Zaporozec, 1994), regarding many contaminant processes in unconfined aquifers (Fetter, 1992), and it is not complicated to calculate and plot these vulnerabilities using GIS software.

However, the most popular methodologies to calculate the vulnerability of aquifers, such as DRASTIC (Aller et al., 1987) and GOD (Foster & Hirata, 1988), have not directly incorporated the possibility of evaluating the vulnerability of pumping. One possibility is to define and add into the vulnerability analysis a new specific factor (i.e., P factor, representing pumping) which penalizes well abstraction. This P factor can be defined according to cone extension, their persistence in time (Figure 12), or their maximum depth.

Figure 12. Plot of the P factor in Tordera’s fluviodeltaic aquifer system (Catalonia). The light brown area defines polygons affected with permanent pumping cones, and the green color defines the area with nonpermanent pumping depressions.

Modified from ACA (2003).

## Specific Management Contexts

### Small Islands and Sharing Aquifers

Groundwater management methodologies that can be applied on islands have much in common with methodologies for coastal aquifers. Some coastal areas and islands in tropical or subtropical areas often have high population densities, with prominent tourist and agricultural activities that create a chronic hydric stress context, which is especially intense on small islands.

Freshwater stress may worsen with climate change and the evolution of seawater level increases. In this context, optimum management of freshwater resources becomes critical but is often difficult due to a lack of hydrological and geological data (Falkland & Custodio, 1991).

The main threats to the quality of groundwater resources on small islands are augmentations of aquifer salinity related to marine intrusion processes and the existence of contaminants related to the infiltration of urban or agricultural pollutants.

The relatively small dimensions of most islands around the world as well as their geological origin usually allow limited possibilities for obtaining large groundwater volumes from aquifers. Many of these small islands have volcanic origins (e.g., Hawaii, the Canary Islands, Iceland, the Azores, and the Galapagos Islands). Volcanic islands present peculiar hydrogeological characteristics inherited from their specific geological structure. In most cases, the bulk of volcanic edifices are composed of lava flows and tephra deposits, which are intruded by dykes in eruptive zones. Dykes can form subvertical barriers to groundwater flow, while tuff deposits may form impervious layers that are subparallel to the topography (Custodio, 2004). All of these factors control groundwater flow and limit the design of catchments, which usually have large diameter wells, wells with subhorizontal drains, or inclined drainage galleries. These different abstraction techniques generate non-conical, but extensively irregular areas of groundwater depletion.

### Transboundary or Shared Aquifers

International, transboundary, or shared aquifers are defined like groundwater bodies, with some of their hydrogeological boundaries shared by different countries. The number of these international aquifers is growing with the expansion of hydrogeological knowledge of the Earth. In 2009, the International Shared Aquifers Resources Management Program (ISARM) of UNESCO had detected more than 270 groundwater bodies within this category (UNESCO-IHP, 2009).3

In shared aquifers, typically national borders intersect regional groundwater flows. In this situation, it is not unusual for one of the two countries to contain the main extension of the recharge zone (with depth piezometric levels), and the other the principal discharge zone. The spatial distribution of the water head levels and the hydrogeological parameters will control or favor the presence and distribution of well fields or other groundwater-related surface water bodies (rivers, lakes, or wetlands).

Shared aquifers follow the same hydrodynamic or hydrochemical rules as intra-national ones, and for this reason, an international boundary is not, in many cases, a good region for the management of these aquifers. Shared aquifers require very specific and multidisciplinary development plans (Pury & El Naser, 2001) due to their political proximity.

In some cases, the existence of transboundary aquifers incites water conflicts. One of the most serious is along the mountain aquifer system between Israel and Palestine but according to United Nations data, there are signs of discord in many other locations such as the Slovak-Hungarian Gabcikovo–Nagymaros area (Eckstein, 1995), and conflict may occur in other world zones where resources are limited.

A solution to avoid such conflicts is the application of conflict prevention programs and the establishment of transnational cooperation in political, social, and scientific affairs. This methodology has been implemented with relative success in Guarani’s aquifer system (SAG), a subcontinental fresh and thermal groundwater body between Brazil, Argentina, Uruguay, and Paraguay.4 The first main objective was to establish a limited number of basic integrated rules between the four nations in order to ensure sustainability of the resource. A common institutional, legal, and technical framework was created for managing the aquifer and preserving it for future generations.

Due to its complexity, this kind of shared governability must establish a clear definition of common objectives between different states and the establishment of adapted strategies in addition to creating a management staff with transboundary responsibilities (AFD, 2011). Ideally, these common objectives should be taking into account the user’s point of view, according to the governance’s general rules (Foster et al., 2009; Foster & van der Gun, 2016).

## Global Climate Change Predictions in Future Aquifer Management Methodologies

According to UN evaluations, global climatic behavior in the second half of the 21st century will most probably be associated with a rise in global mean temperature and a significant increase in the ocean’s water level (IPCC, 2007). The main effects of this new climatic context for the coming decades will be an increase in global average precipitation complemented by changes in spatial distribution and intensity. In many parts of the world, such as central-northern Europe, Asia, North America, and vast areas of Argentina and southern Brazil, increases in average rainfall are predicted. Elsewhere in zones in southern Europe and the Mediterranean, South Africa, West Australia, some parts of the Amazon basin, and central areas of the United States, water availability may decrease by 5% to 40%. Nevertheless, local studies has to be increase to fix more adequately these regional variations. For instance, initial studies on aquifers by the Catalan Water Agency in 2009 predicted a groundwater resource decrease in their territory that could vary between 10% and 30%, related solely to the decline of main water inputs (recharges from rain and rivers) for the period 2070–2100 (GC-ACA, 2009).

The aquifer’s recharge rates are affected by changes in rain distribution in two ways. Due to decreases in precipitation ranges, which implicitly decrease natural recharge water availability, and because infiltration rates also depend on rain intensity, strong and short storms produce lower infiltration rates than prolonged rains of medium to short intensity.

Similar effects should be examined in aquifers’ recharges from hydraulically connected rivers, especially in arid, semi-arid, and Mediterranean climatic zones. An increase in high, but short river flow periods, combined with longer dry seasons, will diminish global values of river infiltrations and augment the number of periods of groundwater discharges to rivers.

The third great impact will be the intensifying of seawater intrusion into many coastal aquifers, especially if the catchments are close to the seaside and are on flat islands or low-lying countries (Oude-Essink et al., 1993). The reasons for this impact are the expected increases in the average sea level due to melting glaciers and polar ice caps as well as the repositioning of the natural saline–fresh groundwater interphase.

According to Taylor et al. (2013), it is possible to separate global warming effects into direct effects and induced or indirect effects. The first group is mainly related to changes in actual distribution of global precipitation, including the frequency of climate extremes (floods and droughts) and the fact that natural replenishment of groundwater occurs from both diffuse rain recharge and focused recharge from leakage from rivers, lakes, ephemeral streams, and wetlands. Indirect impacts are more difficult to evaluate and are related mainly to changes in human activities and in land use. The main effect will occur in agricultures areas, which do not respond to changes in rain precipitation in the same manner as natural ecosystem fields. For instance, in many croplands, a reduction of rain recharge induces an increase in pumping where a high volume of water returns to the aquifers. Unfortunately, these augmentations of water storage in irrigated areas, which can duplicate the storage rates of natural areas (Taylor et al., 2013), are associated in many cases with a degradation of groundwater quality in the form of an increase in salinity, nitrogen compounds, or diverse types of agrochemicals.

The evidence of global warming and the prospects of its effects could play an important role in future planning of groundwater resources. These processes will increase aquifer abstraction rates in many parts of the world and will heighten the need to develop more efficient methodologies, not only to prevent undesirable effects of overflow from rivers or increases in heavy rain episodes, but also to catch and store these water volumes. Another possibility in the future may be the augmenting of the use of salinized aquifers and their progressive introduction into the general scheme of integrated water management.

## Governance Concept and Its Application in Groundwater Management

The complications of planning and managing natural resources at a global scale have motivated political science circles to propose a new concept of water governability different from the classic one and based on a hierarchical number of state or supra-state bodies, with little or no interactions with water users. The UN has named this new concept “Governance” and defines it as the exercise of political, economic, and administrative authority in the management of nations’ affairs at all levels. Thus, this concept embodies a mechanism, processes, and institutions through which citizens of various nations may articulate their interests, mediate their differences, and fulfill their legal rights and obligations (Foster et al., 2009; Foster & van der Gun, 2016). It follows that water governance embodies a framework for effective water (and groundwater) resources management, including the delivery of water services in a socially responsible, environmentally sustainable, and economically efficient manner.

In turn, groundwater governance focuses on the exercise of appropriate authority and the promotion of responsible collective actions to ensure sustainable and efficient use of groundwater resources for the benefit of dependent ecosystems as well as humanity. Public participation and transparency will have a positive impact on quality of governance (De Stefano et al., 2013). Transparency is the first step in public participation, since it implies that people have access to essential information in order to make informed decisions, which is especially important in governing a not easily visible but a common resource (Harding, 1969; Foster et al., 2009) such as groundwater.

# Remarks and Suggestions

Groundwater management plans generally entail a better distribution of water resources and a progressive incorporation of quality parameters within the safe yield concept. However, much work is yet to be done in facing future challenges for controlling the undesirable effects of groundwater abstraction.

It is important that future groundwater management plans include specific guidelines to (1) ensure energy efficiency and quality construction of abstraction wells, (2) develop water agencies with specialized staffs for controlling regional or larger depression cones and theirs effects on groundwater quality, (3) implement strategies for enhancing aquifer water inputs, and (4) examine actual governability of hydrological systems, with the goal of implementing social, participative, and sustainable governance regimens.

## Ensuring Construction Quality of Pumping Wells

Few people doubt the influence of construction quality of groundwater wells regarding the intrusion of inorganic or bacteriological aquifer contaminants or the relationship of construction quality to well energy efficiency.

One of the most important activities in ensuring the quality of wells is the updating and enforcing of norms and technical standards for constructing water wells. The first standard was published in 1948 by the American Water Works Association (AWWA, 2006) and has been periodically revised.5 Many countries have created national norms or professional standard recommendations, taking into account the whole or parts of this first work.

In securing the suitable execution of standard recommendations, it is also necessary to create appropriate educational programs at different grade levels to provide adequate technical training in well construction and the management of groundwater resources. This is one of the main objectives of the Integrated Water Resources Management Programs (IWRM; UNEP, 2014).

## Incorporating Integrated Monitoring Networks

Forecasts of global increases in the use of groundwater, especially in countries with arid or semi-arid climates, will force more rigorous control of the spread of the most significant cones from pumping, as well as forcing control of progressive extractions to ensure future resource availability.

To this end, it is also essential for decision-making purposes to incorporate into many aquifer systems monitoring networks that measure the evolution of piezometric levels and track hydrochemical or isotopic indicators.

In parallel, flowmeters should be provided, at least for main pumping groups, to allow more rigorous control of well abstractions. These data must be compared to the aquifer’s annual water inputs in order to investigate whether legal water exploitation limits need to be revised by hydraulic administrators or user communities or by various administrators, in the case of shared aquifers, so that a realistic volume of abstraction and a safe yield can be established.

## Augmenting Groundwater Inputs: Expanding Use of Artificial and Induced Recharges of Aquifers

Artificial recharge of aquifers (AR) is of great importance for joint use of water resources and a real alternative in augmenting the storage and quality of groundwater (Todd, 1959; Aiken & Kuniansky, 2002; Gale et al., 2002). Recharging entails a planned replenishing of groundwater reservoirs by external and quality-controlled water.

AR facilities are classified according to surface methods using basins, ditches, ponds or artificial lakes, or riverbeds to incorporate water into underlying unconfined aquifers. Depth methods, mainly applied to confined or unconfined aquifers with deep water levels, are also used, generally with injection wells or drains.

The efficiency of an AR project depends on the clogging rate in the recharge areas. Clogging is produced by solid suspended particles incorporated in the water.

AR facilities can be designed for various purposes, such as the following:

1. 1. Rebalancing the volume of an aquifer’s water inputs and outputs: In aquifers where overexploitation has caused a decrease in the groundwater’s level and it is not possible to reduce abstraction rates, planned AR systems can help maintain admissible local or regional groundwater heads.

2. 2. Improving water quality in the aquifer: Artificial recharge systems that operate with fresh water help, through dilution processes, aquifers whose water has excessive concentrations of particular compounds or high salinity.

3. 3. Reducing concentrations of most pathogenic microorganisms and reducing recharge water turbidity using water seepage through unsaturated zones: The mechanical filtering of solid particles and, in some cases, biochemical levels, applied in the bottom of recharge basins, can also decrease concentrations and the persistence of certain organic undesirable trace compounds.

4. 4. Reducing the intensity of or stopping the risk of ground subsidence in geological formations affected by high pumping rates.

5. 5. Decreasing water evaporation losses (compared to storage in dams) and, especially in deep recharge systems in confined aquifers, increasing the safety of water resources from radioactive or biological accidents or attacks.

6. 6. Acting as a fundamental element of aquifer storage and recharge (ASR) management methodologies.

However, AR operations may create negative impacts in the aquifer if economic, technical, and environmental factors are ignored. Major concerns for surface dispositions are the reduction in the predictions of recharge water volume due to direct evaporation, the incorporation of undesirable compounds into the aquifer because of the elimination of the non-saturated zone, the infiltration of contaminants from buried landfills under or influenced by recharge water plumes, and the generation of artificial ecosystems in the basins (Figure 13), normally filled by opportunistic or undesirable vegetal and animal species. For deep AR systems, problems can arise by mixing the aquifer’s original water with recharge water, in many cases containing diverse chemical, biological, and thermometric characteristics.

Figure 13. Pre-filtration basin from an artificial recharge dispositive, affected by a “life explosion” that may affect the quality of the recharge water and future infiltration rates.

## Main Technical, Economical, and Hydrogeological AR Requirements

An area that may be selected to implement an AR facility requires three basic conditions: (1) The reservoir must be a geological formation with an appropriate transmissivity range and large enough to incorporate the water volumes fixed by the management plan; (2) the water used for recharge should be a suitable future source of water supply when it mixes with the original aquifer groundwater; and (3) the AR disposition must be technically efficient and economically competitive.

## Use of Underground Aquifer Storage With Recovery (ASR) Technologies, as a Variation of Conventional Artificial Recharge Solutions

Big cities and tourism, coupled with large increases in irrigation areas and the food industry’s water requirements, create enormous pressures on traditional water resources (surface and groundwater), which translate into competition between users. Traditional solutions, such as water transfers from external basins, are increasingly more complex and socially, environmentally, and economically costly; in many cases, such solutions are only temporary.

One feasible solution is to produce new water resources and integrate them into traditional joint use methodologies. These new resources are reclaimed waters produced in water treatment urban plants as well as fresh water from saline underground aquifers or from the sea. Both practices have some clear advantages compared to conventional water resources, but they also have disadvantages and challenges that are necessary to overcome (Table 2).

Table 2. Main Advantages and Challenges of Most Used Nonconventional Water Resources: Reclaimed and Sea-Desalted Water

Great augmentation of water availability for multiple uses, including environmental ones

The disposition has a high economic cost related to the price of required energy

Combined with conventional resources, can reduce the risk of scarcity and augment the supply guarantee

At the source: production of liquid effluents with contaminant capacity and high salinity

Opens the possibility of using significant volumes of produced waters in AR disposition, especially in areas where conventional waters do not achieve annual renewable levels, to reduce water deficits

At the source: production of slurries or muds with contaminant capacity (especially in the case of reclaimed waters)

In some contexts, the use of produced waters can improve the quality of conventional water sources by mixing and dilution processes, and assures the supply

Deficient use and management caused by bad social perceptions of these water resources; may also induce decreases in economic investments to protect conventional water resources

Quality changes of the previous water source

Changes in the quality of related ecosystems (rivers, lakes, wetlands, estuaries)

The production of reclaimed or desalted waters does not follow the rhythm of the natural water cycle. Thus, new methodologies need to be developed to coordinate use of natural waters and industrial waters. One of these methodologies is Aquifer Storage and Recovery (ASR).

ASR takes advantage of the aquifer’s ability to transfer and store large volumes of water during low demand periods in order to recover them in times of strong consumption. In ASR operational schemes, the replenishing of the aquifer is through AR techniques (usually using vertical wells (see the section “Pumping Wells: Main Characteristics and Analysis of Their Effects”) and the classic dependency on availability of suitable water for recharge, controlled by the natural water cycle, is avoided through the use of desalted or reclaimed water.

ASR has been relatively successfully developed in some parts of the United States (e.g., Florida [USACE, 2014]) and in other countries such as Australia, the Netherlands (Rambags et al., 2013), and the United Kingdom, where several types of recharge waters have been tested, including reclaimed ones.

In Florida, ASR methodologies has become an important tool for providing water. In 2004, the Peace River well field had 21 ASR wells (Eckman et al., 2004), with a total capacity of 730 L/s. The water source came from the Peace River and was pumping off-stream to a raw treatment plant. Following treatment, the water was pumped to a clear-water storage tank. From this reservoir, water was delivered to meet supply contracts and the surplus was injected into the aquifer using ASR reversible wells. In the periods when the company was unable to divert enough water from the river to reach supply contracts, injected water was recovered from the aquifer using the same wells.

In the Netherlands, apart from larger storage projects in the Zuid-Beverland areas, a small-scale ASR disposition for irrigation was installed in the Nootdorp site (Rambags et al., 2013). Roof water from greenhouses was collected and stored in a brackish aquifer. The roof water was first treated by slow and rapid sand filtration and after that, the treated water was pumped into a tank that provided the minimum pressure required to inject it into the aquifer. The injection well flow was between 2.7 and 4.2 L/s. The recovery of injected water began when the tank water level fell beneath a predefined minimum. The wells were designed with different depth and screen positions to minimize the augmentation of salinity during the recovery period. With this disposition, major injection rates occurred in the bottom of the aquifer using larger wells, and recovery occurred at the aquifer top using shallower wells.

However, some issues have emerged which may affect the potential use of ASR in aquifers. The main issues are as follows: (1) a poor recovery efficiency, resulting from mixing of native aquifer water and injected water; (2) the existence of aquifer minerals with a high capacity of dissolution, which augment the salinity of the injected waters and, in general, the hydrogeochemical impact of the injection (Gaus et al., 2000), especially in the case of reclaimed waters; and (3) a lack of hydrogeological knowledge, especially for deep confined aquifers, and their spatial distribution of permeability

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## Notes:

(1.) This linear behavior is accepted in confined or unconfined aquifers with a small dynamic drawdown compared with their saturated thickness (<10%).

(2.) For FEFLOW.