Why would pumice, a material with high porosity, not be a good aquifier?

Answer 1

Groundwater constitutes one of the main sources of water for human consumption. Underground reservoirs contain much more water than is available as fresh surface water, and it usually does not require treatment before it can be used. Groundwater is also often close to where it is needed, making long pipelines and storage structures unnecessary. Capital investment can be incremental and, therefore, groundwater is better suited to developing countries.

In many urban areas, groundwater can provide the most appropriate solution to the population’s water needs, because it can be used inexpensively to supply a large volume of good quality water. However, the suitability of groundwater for urban use depends not only on need, but also on the hydrogeological characteristics of the aquifer under consideration. In Latin America, these characteristics can vary widely.

Suitability of groundwater reservoirs
The hydrogeological systems of Latin America and the Caribbean (LAC) are as varied as its geography. Aquifers have a wide range of hydrogeological properties: highly permeable or relatively impermeable; porous or fractured; containing only a few thousand litres of water or many billions; their waters can be fresh or briny; and they may be pristine or highly contaminated. However, when groundwater is considered as a potential source of water for urban or suburban communities with high consumption requirements, the spectrum of possibilities is considerably reduced.

Generally, the main limitations in tapping groundwater sources for urban use are economic, because of the cost of exploration, extraction, conduction, treatment, storage, and distribution. In deciding on the type of supply to be used, groundwater costs should be compared to the cost of other available water resources. Only when groundwater is the only source of water that is readily available, is cost not an important factor.

Some of the geological and geographic conditions that favour the use of groundwater for urban water supply are:

Proximity to the consumption area;
Large-volume reservoir;
Shallow depth and low pressure; High water yield;
High rate of renewal; Acceptable water quality; and
Low risk of unwanted effects due to intense pumping.
Proximity to the consumption area
One of the highest costs of supplying water to cities is associated with its conduction from production to consumption sites, especially when this route is uphill or through some geographic obstacle, such as mountains or canyons. Consequently, the closer an aquifer is to a city, the more attractive it is as a water source.

The ideal situation occurs where an aquifer underlies the consumption area, particularly when it is artesian because no pumping or extensive conduction would be required and risk of contamination would be low. These conditions are (or were) relatively common in many cities of the world. However, urban use of artesian aquifers often lowers the water level below that of the well head; natural pressure is lost and pumping may be required. Even with this added cost, the economic convenience of having a groundwater reservoir under a city often easily outweighs other considerations.

Cities in which aquifers lie under the consumption areas include Mexico City, Buenos Aires-La Plata, Bangkok, and Lima. Other cities using aquifers must transport water over a considerable distance, e.g., Guadalajara and Monterrey in Mexico, Havana in Cuba, and Jakarta in Indonesia. Distance from the wells, springs, or galleries to the consumption area is always a major factor in the cost of water and a consideration in selection and development of well fields.

Large-volume reservoir
A suitable aquifer must also contain a sufficient volume of water for use over a relatively long period, i.e., 10 years or more. For example, consider the requirements of a city with 100 000 inhabitants, consuming water at the rate of 500 L/day per person. This city would use over 18 million m3 of water per year. Assuming an annual average recharge rate of 10% of the stored volume, at least ten times as much stored water (180 million m3) is needed to satisfy requirements without undermining existing reserves. To contain that volume of water, a geological formation must have a total volume several-fold bigger, e.g., 10 times as much for an effective porosity of 10% or about 1.8 billion m3. That volume would be contained, for instance, in a formation that is an average of 10 m thick and extends over 180 km2.

If we look at the requirements of a large metropolis, such as Mexico City, with a daily consumption rate of about 7 million m3 (2 500 million m3 annually), the usable volume (not considering the normal variation in hydraulic parameters) must be about 150 times larger than the hypothetical city above to accommodate Mexico’s medium- and long-term needs, i.e., a 2 700-km2 formation with an average productive thickness of 100 m. As discussed in Chapter 8, the aquifer in the valley of Mexico meets these requirements, with some limitations.

These figures are arbitrary, and the actual calculation of available water volumes is not, unfortunately, so simple, but the examples give a general idea of the size of aquifer that may be required to satisfy the water requirements of a large city.

Shallow depth and low pressure
Water for urban consumption must be easily or economically (or both) available. Depth is especially important, as drilling costs increase considerably if aquifers are more than a few hundred metres below the surface. Drilling costs are also high if piezometric levels (static levels) and pumping levels (dynamic levels) are far from the ground surface. In this latter case, operational costs can be radically increased by pumping costs.

With increasing depth, there is a tendency toward compaction and consolidation of sediments and an associated decrease in storage capability and hydraulic conductivity, with greater mineralization of the water. For this reason, and due to increased costs, most deep aquifers are unsuitable for water supply. However, deep aquifers occasionally contain good drinking water and may yield larger volumes. An excellent aquifer, meeting these conditions, is located in the Botucatu sandstone of Argentina, Brazil, Paraguay, and Uruguay; it is well over 0.8 million km2 in area and several hundred metres thick (Montano and Pessi 1988; Kimmelman et al. 1989).

The Botucatu sandstone contains one of the largest aquifers in the world with high permeability and low mineralization of the water. In spite of its depth, which frequently exceeds 1 000 m, and the presence of overlying, hard basaltic rock, which is expensive to drill, its high piezometric levels (frequently giving rise to artesian conditions) keep extraction costs low. However, this aquifer has been used only in a limited way in areas where access is difficult and expensive. Only recently have a significant number of wells been drilled into it.

High water yield
A key element in the use of aquifers for urban supply is sustainable well yield. Yield limits the number of wells that can be drilled. For example, to supply our hypothetical city of 100 000 (yearly consumption, 18 million m3; daily consumption, about 50 000 m3), 200 wells producing 1 750 L/minute or 1 000 wells producing 350 L/minute would be needed. Mexico City draws 55 000 L/second from 5 000 wells located throughout the city: 10–12 L/second or 600–720 L/minute per well.

The principal intrinsic property that determines the yield of wells is the hydraulic conductivity or permeability of the aquifer. Highly permeable formations provide the conditions for construction of high-yielding wells.

High rate of renewal
One of the most important features of an aquifer allowing its intense, long-term exploitation is its renewability. Renewability can be defined as the capacity of an aquifer to sustain its volume against a given level of extraction. It is related to the balance between the water recharged from and discharged to the surface and the inflow and outflow from and to contiguous water-bearing hydrogeological units.

In most cases, the key element for renewability of an aquifer used for urban water supply is the recharge volume from the surface (i.e., streams, lakes, rainfall, and melting snow), which, in turn, depends on precipitation in the recharge area or at the basin’s headwaters. Recharge rate is also a function of the permeability and state of the ground surface, slope, development of the hydrographic network, vegetation, artificial structures, and depth of the water table.

Some aquifers have a high rate of renewability, e.g., due to high rainfall, large recharge area, or poorly developed outflowing drainage, and they can be used heavily with little harm. Others have a limited rate and are, therefore, sensitive to overpumping. Assessing the renewability of an aquifer is essential in appraising its potential for urban use.

Acceptable water quality
Aquifer water must be of appropriate quality for human consumption. It must have a low level of dissolved solids, meet required standards for microbiological content, and be free of other impurities (miscellaneous organic or inorganic gases, liquids, or suspended solids), excessive radioactivity, or other health hazards. Poor quality water can often be treated to bring it up to standards. However, the high costs associated with treatment of heavily contaminated waters may make their use prohibitive.

The location of the recharge area of an aquifer, underlying a densely populated area, often makes the aquifer vulnerable to contamination from anthropogenic causes. This problem must be addressed when an aquifer is, or will be, used for drinking water.

In some cases, degradation of water quality may be related to hydraulic connection with aquifers of lesser quality or with surface-water bodies, e.g., seawater and salty lakes. Heavy pumping might promote the invasion of the unsuitable water from below or next to the aquifer. This phenomenon (called “saline intrusion”) is the main cause of aquifer degradation in coastal areas.

Low risk of unwanted effects due to intense pumping
Intense pumping, as is usually required to supply cities, may produce unwanted effects, such as subsidence or intrusion of unsuitable recharge waters or contiguous underground waters of poor quality or other fluids. Although assessment of these and other potential problems is not always carried out in advance, in many instances, overpumping has caused degradation, not only of the aquifer, but also of the overlying land on which the city is located.

The difficulties caused by pumping are related to dewatering or decreasing water pressure in the aquifer. The serious problem of subsidence in Bangkok, Mexico City, Shanghai, and Venice is a result of consolidation of dewatered sediments after intense pumping that exceeded the renewability of their aquifers. This phenomenon has illustrated the extent of the damage possible when hydrogeological conditions are not appropriate to the pumping rates and volumes being extracted.

Aquifers suitable for groundwater use
Only a few hydrogeological environments provide the volumes, well yields, renewability, accessibility, and water quality necessary to meet the requirements for urban water supply. For this reason, the list of water-bearing formations of interest to city hydrogeologists is much shorter than is shown in general hydrogeological surveys. The main high-production aquifers whose water is suitable for urban use are (Fig. 3):

Volcanic aquifers,
Alluvial aquifers,
Fig. 3. Cross-section of South America showing the location of various types of aquifers. 1. Lima, Iquique, Mar del Plata, Natal, Salvador, Maceió, Fortaleza; 2. Lima, Villavicencio, San Juan, Mendoza, Maracaibo, Santa Cruz; 3. Cochabamba, Valencia, Maracay, Querétaro, San Luis Potosí, Santiago; 4. Mexico City, Guatemala City, Managua, Quito; 5. Buenos Aires, San Nicolás; 6. Ribeirao Preto; 7. São Paulo, Santa Lucia (Montevideo) 8. Georgetown, Mar del Plata, several cities on the Brazilian coast.

Carbonate aquifers,
Pre-Tertiary sandstones and conglomerates, and
Coastal aquifers.

Volcanic aquifers
Volcanic regions contain some of the most productive aquifers. Several large LAC cities obtain a significant portion of their water from volcanic aquifers (Fig. 3).

Mexico City’s main sources of water (about 80%) are the aquifers contained in the complex pyroclastic formations filling the valley of Mexico basin. Guatemala City depends to a large degree (about 70%) on the groundwater reservoirs found in the Tertiary and Quaternary pyroclastic and lava formations of the Guatemala valley in the country’s volcanic highlands. Managua, the capital of Nicaragua, extracts all of its water for urban consumption from the pyroclastic Las Sierras formation, both directly from wells and indirectly from the crater lake, Asososca. Quito, Ecuador, obtains nearly 40% of its water from the colluvial and alluvial deposits of pyroclastic origin and tuffs of the Callejon interandino valley. Other cities using volcanic or related aquifers to some degree include Guadalajara, Puebla, and Toluca in Mexico; Quetzaltenango in Guatemala; Riobamba in Ecuador; La Paz in Bolivia; and San José in Costa Rica.

The volcanic complexes of Latin America are of three main types: the Mesozoic basaltic regions of Serra Geral; the island arch volcanoes of the Caribbean region; and the volcanic formations in the mountain ranges.

The basaltic areas of southern Brazil, northern Uruguay, and northeastern Argentina are composed of relatively compact rock, several hundred metres thick, which contains only minor volumes of water in fractures, in intercalated sandstones, in interflow porous zones, and in the weathered material of the surface. These waterbearing zones are not only difficult to find (particularly the first three types), but they usually do not provide sufficient water to sustain long-term high extraction (Da Cunha Rebouças and Fraga 1988).

The volcanic areas of the Caribbean islands occupy a relatively reduced region above sea level in several of the smaller Caribbean islands (Dominica, Martinique, Montserrat, and Saint Lucia). As a result of their limited size and the presence of better aquifers in most volcanic islands, these formations are used for water supply only by small towns and farms.

The volcanic zones of the mountain ranges, on the other hand, are extremely important water sources, because of both high population densities nearby and the frequent presence of excellent and abundant aquifers. These zones extend along the whole eastern Pacific “arch of fire” near the coast, from southern Chile through Bolivia, Peru, Ecuador, Colombia, Central America, and Mexico (and continuing further north through the western United States, western Canada, and Alaska) (Bedinger et al. 1989). They consist of a wide array of rocks of varied composition, petrography, and structure, including acid, neutral, and basic magmatic compositions, unlike the trap basalts of Serra Geral, which are systematically basaltic. Rock types found in the mountainous volcanic districts include rhyolites, trachytes, dacites, andesites, basaltic lavas, widespread pyroclastic formations, and associated alluvial and lacustrine deposits. From a geological perspective, these volcanic regions can be extremely heterogeneous, mainly because of the complexity of the petrogenesis associated with volcanic processes.

Depending mainly on its silica content, an ascending magma may solidify before reaching ground level (this is normally the case with rhyolitic and trachytic magmas) or it can reach the surface and flow downhill until it cools and hardens. The effusion of lava is normally accompanied by degasification, with ejection of various magmatic products into the atmosphere. In acid magmas, gas pressure builds up behind the solidified rocks and explosions may occur, causing widespread ejection of solid fragments and fluid material, which may fall as large fragments (bombs or scoria), medium-sized fragments (lapilli), or ashes. These materials can also flow downhill embedded in hot fluids, i.e., various hot gases (of which steam is by far the most common), water (often from phreatic eruptions or sometimes from melted snow), and liquefied soils (usually composed of volcanic materials from previous eruptions).

Volcanic formations are frequently altered by water erosion and transported downstream where alluvial and lacustrine deposits may accumulate. On less-exposed or gentle slopes, weathering processes may develop rapidly, and the volcanic glasses and proto crystals may be transformed into clay with the liberation of various chemicals, among which may be important plant nutrients. Soil formation in loose volcanic deposits can take place quickly (a few years). Where volcanic rocks are more compact, soil formation proceeds much more slowly (tens or hundreds of years).

Soil formation is important from a hydrological point of view because of the increased impermeability caused by the argillization of glasses. One of the main obstacles to the recharge of aquifers in volcanic areas (and to vertical flow in general) is related to the presence of a succession of buried palaeosoils, produced by the weathering of pyroclasts.

The main volcanic rock formations found in the mountainous volcanic areas of Latin America are:

Agglomerates and breccias;
Ash-flow tuffs;
Ash-fall tuffs;
Mud-flow and laharic tuffs;
Alluvial pyroclasts and reworked tuffs;
Lacustrine reworked tuffs; and
Lavas.
Agglomerates and breccias
Agglomerates and breccias form near the foothills of volcanoes as a result of local landslides, rolling of large and medium-sized rock fragments (blocks and lapilli), and the fall of various types of pyroclasts near the volcano (including bombs, miscellaneous scoria, pumice, blocks, lapilli, and ashes of various types and grain size). Hydrologically, they can be very productive, but their limited area reduces their use as water sources.

Ash-flow tuffs
Ash-flow tuffs result from the flow of pyroclasts, liquefied by water or volcanic gases, which often gives rise to thick accumulations in valleys and depressions. Pyroclastic materials formed in this way can be composed of solidified “live lava,” fragments of

“dead lava” from previous eruptions, pyroclasts from previous eruptions, or fragments of rocks tom from the walls of the substratum as the volcanic fluids rise to the surface.

These tuffs can be welded or nonwelded depending on their degree of consolidation. Nonwelded tuffs frequently contain water in usable volumes, because of their thickness, area, and effective porosity, which may average 35% (Bedinger et al. 1989), but their yields are somewhat smaller than those of welded tuffs, where porosity is secondary (related to fracturing) and much lower (about 3%).

Mean hydraulic conductivity (K) for welded and fractured tuffs is about 1 m/day; for nonwelded and friable tuffs, K is 4 × 10–5 m/day (Bedinger et al. 1989). However, the average values do not always reveal the high hydrologic potential of tuff deposits that are suitable for urban water supply. Bedinger and colleagues (1989) found K values of about 5 to 5 × 10–3 m/day for the 83.5 percentile, with effective porosity ranging from 4% for welded tuffs (few fractures) to 33% for nonwelded tuffs (friable).

Tuffs have good hydrogeological potential (particularly when fractured), with high yield. When their available volume is sufficient, they can be used as a source for urban water supply.

Ash-fall tuffs
Wind-driven tuffs can extend over large areas, accumulating as a: thin blanket over existing topography. Usually, they are composed of fine pyroclastic particles, with grain size decreasing with distance from the volcanic source. In spite of their areal extension, their potential as a water source is limited because they are seldom more that 10 m thick.

Mud-flow and laharic tuffs
Mud flows and lahars are catastrophic phenomena that may occur regularly at some locations, giving rise to important accumulations of poorly sorted sediments below volcanic slopes. These formations can extend many dozen kilometres down valleys to flatter land below.

When weathered, mud-flow and lahar sediments produce highly fertile soils. They may also contain abundant groundwater suitable both for urban supply and irrigation, when their area and thickness are sufficient. Because of these characteristics, mud-flow and lahar zones are often densely populated, although they may present a permanent threat to local populations. In 1985, Armero in central Colombia was buried by a lahar caused by sudden snowmelt on the slopes of the Nevado del Ruiz volcano during an eruption; 20 000 people were killed. Several other cities are located in similar high-risk areas, e.g., Ibague, not far from Armero, with a population of 400 000 people.

Alluvial pyroclasts and reworked tuffs
Because the surface of recently deposited tuffs is devoid of vegetation, they are exposed to erosion, and their particles may be transported by local streams toward lower-lying areas, where they accumulate to varying thicknesses. Alluvial pyroclasts of this nature are frequently found intercalated and interdigitated with other volcanic formations. They can have high values of hydraulic conductivity and contain suitable aquifers. When their volumes and rates of renewal are sufficient, they may also provide sufficient water to serve urban areas.

Lacustrine reworked tuffs
lakes are common features in volcanic areas because existing waterways are frequently obstructed by volcanic accumulations of various types. In most volcanic areas, lakes of various sizes and stages of evolution can be found. The last stage of evolution is their transformation into a lacustrine plain. lacustrine sediments are normally finer than alluvial ones. For this reason, lacustrine formations tend to act as aquitards (or quasi-aquicludes) rather than aquifers, and their yield of groundwater is usually small to nil.

Lavas
lavas are primary volcanic rocks. They form by solidification of magmas, under atmospheric or quasi-atmospheric conditions. lava rock formation can take place inside the volcano, in the case of silica-rich, high-viscosity magmas, or outside, when the magma has lower viscosity and is silica-poor; the latter consolidate as lava-flow rocks. Magmas of intermediate composition may produce rocks of either type. Rhyolitic, trachytic, and dacitic magmas frequently produce explosive eruptions, whereas basaltic and andesitic magmas give rise to more peaceful volcanic episodes.

lava-flow rock formations are heterogeneous; a vitreous crust of solid rock forms on their rapidly cooling external surfaces and a more crystalline core of materials cooling at a slower rate forms toward the centre and base of the mass. Gas bubbles are often trapped beneath the solid crust, producing highly porous material containing vacuoles and vesicles. In some cases, these spaces may be interconnected, giving rise to high effective porosity in which water circulation is facilitated (i.e., high hydraulic conductivity). In other cases, the vacuoles and vesicles remain isolated and water circulation is more difficult. The base of the flow normally includes fragments of rock that create many anfractuosities and empty spaces, producing a highly permeable solidified material.

Because of these processes, lavas can develop a network of open fractures, usually interconnected, allowing the flow of significant volumes of water. In brief, lavas can form hydrological units of high productivity if they possess large volumes of gas, experience intense contraction phenomena, or flow over loose rocky surfaces. On the other hand, when gas content is low (as in most basaltic flows), contraction fissures are few and small, or foreign rock fragments are not picked up from the substrate, the hydrological potential of the solidified rock can be limited (e.g., the plateau lavas of Serra Geral in southeastern South America).

Lava flows in various volcanic areas of North America have effective porosities averaging 15% for cavernous and fractured lavas and 1% for moderately dense to dense ones (Bedinger et al. 1989). Hydraulic conductivity values average 0.5 m/day and 4×10–4 m/day, respectively.

Groundwater renewability
An important element that facilitates groundwater exploitation in volcanic areas is the normally high rate of renewability, due to the continued “youth” of the hydrographic systems in these areas. Fluvial valleys do not have time to form, because of continuing obstruction of their courses, as a result of the various volcanic accumulations (e.g., lava flows, landslides, mud flows, lahars, and ash flows).

The various lakes and depressions formed in volcanic regions frequently become recharge areas, i.e., surface “sinks,” for existing aquifers. In many volcanic aquifers, infiltration can amount to more than half of the precipitation. From this point of view,

volcanic regions share characteristics with karstic areas: poorly developed hydrographic drainage, presence of “recharge” lakes or depressions, and fracture flow through open fractures (as in lavas and welded tuffs). The main difference from karstic areas is the less-important role of dissolution along the fractures.

Alluvial aquifers
Alluvial aquifers are those contained in sediments of alluvial origin. In this section, . only the less-consolidated Cenozoic alluvial formations will be considered: older alluvial formations are included in the section on pre-Tertiary sandstones and conglomerates later.

Alluvial formations are found throughout Latin America, from Punta Arenas in southern Chile to Tijuana in northwestern Mexico and from Trujillo in Peru to Fortaleza in Brazil. They are the most abundant type of aquifer, both in number and area. Alluvial aquifers can be found at high altitudes, sometimes over 4 000 m above sea level (as in the Peruvian-Bolivian altiplano) or several hundred metres below sea level (as in a number of regions in the continental margins).

These aquifers have a wide range of characteristics: they can vary in area from a few square kilometres, with low productive volume, to tens of thousands of square kilometres, with huge production capacity. Yields can also vary. In some cases, well productivity may be as low as a few litres per minute; in others, it can reach hundreds or even thousands of litres per minute. Not all alluvial aquifers can provide the sustainable output of good quality water required for widespread urban use. In fact, most cannot.

However, there are still a large number of alluvial formations that do contain enough renewable groundwater to meet water demand in many cities of the continent. Some cities obtaining a significant amount of their water from alluvial aquifers include Buenos Aires-La Plata, Junin, Río Cuarto, and San Nicolás in Argentina; Cochabamba and Santa Cruz in Bolivia; lea, Lima, Piura, and Trujillo in Peru; Santa Marta, Sincelejo, and Villavicencio in Colombia; Maracay and Valencia in Venezuela; Aguascalientes, La Paz, Mexicali, Querétaro, and San Luis Potosí in Mexico; and Olinda, Natal, and Pelotas in Brazil. A large number of other cities that obtain most of their water from surface sources, draw a portion from alluvial aquifers, often for industrial use (e.g., Santiago, Chile; Asunción, Paraguay; and Montevideo, Uruguay).

In addition, many aquifers not classified here as alluvial were deposited as a result of alluvial action. They are included elsewhere, because of other characteristics that are more significant in defining their properties and dynamics, e.g., many alluvial carbonate sediments, alluvial deposits of pyroclastic origin, some coastal alluvial sediments interfingered with marine littoral deposits, and pre-Tertiary consolidated conglomerates and sandstones of alluvial origin.

Alluvial formations are composed mainly of detritic sediments of varying grain size and composition. Their granulometric fractions can include gravel, sand, silt, clay, or any intermediate or composite grain size. In general, the larger the predominant grain size, the higher the porosity of the material. Also, well-sorted alluvial sediments have a higher porosity than poorly sorted sediments of comparable mean grain size.

Only the coarser sediments (fine sand to gravel) may contain enough water and have a high enough permeability to give rise to aquifers suitable for urban use. Effective porosity and hydraulic conductivity values of coarse sediments in alluvial aquifers range

from 12 to 25% and 1 to 0.7 m/day, respectively (Bedinger et al. 1989). Silty and clayey formations are not usually good aquifers, but behave as aquitards or aquicludes, i.e., they have low hydraulic conductivity.

These characteristics apply not only to alluvial sediments, but also to any sediment with similar granulometric properties. However, the vast majority of detritic sediments containing aquifers in Latin America are of alluvial origin.

Classification of alluvial aquifers
Detritic sediments undergo considerable changes with age. The older a sediment, the more likely it is to have experienced consolidation and diagenetic processes. These processes are often, but not necessarily, associated with the depth to which the sediment has been buried at some time in its geological history. The older a sediment, the greater its chances of being affected by secondary processes that modify its properties.

Consolidation processes include general compaction, hydrolysis of ferromagnesium minerals and feldspars, formation of clays and other secondary minerals, and cementing with silica, iron hydroxides or oxides, or carbonates. Deeper in the earth, where temperature and lithostatic pressure increase beyond a certain degree, other petrogenetic phenomena of a more diagenetic nature occur: neoformation of clays and micas, anhydritization of calcium sulfates, formation of some sulfides (as pyrites), crystallization of graphites and magnetites, and hematitization and goethitization of limonites.

All these processes tend to decrease porosity, hydraulic conductivity, and storage capacity. Because of slower flow and longer contact with mineral surfaces, groundwater becomes increasingly mineralized, i.e., total dissolved solids increase. Higher temperatures at greater depths promote this mineralization. Therefore, as a rule, relatively recent detritic formations contain better aquifers than older formations of . the same type.

With consolidation, however, porosity and flow may increase. Consolidated sedimentary rocks have a greatly diminished intergranular porosity (and flow), but fracturing can reverse this trend causing them to behave like a fractured aquifer, e.g., in crystalline rocks.

In this book, I have arbitrarily divided detritic alluvial formations into three categories according to their age:

Young alluvia, still associated with present fluvial valleys or basins;
Older alluvia that have suffered some consolidation, usually of Tertiary origin and only infrequently related to the largest present orographic and hydrographic features; and
Older sedimentary rocks of alluvial origin, generally pre-Tertiary, without any direct association with existing reliefs and having often experienced a relatively high degree of consolidation and some diagenesis.
Another key criterion for classifying alluvial sediments is general geomorphology. Alluvial sediments from the foothills of the Andes are completely different from the sediments of the large central plains; alluvial deposits from the undulating shield plateaus are different from the sediments of the intra montane Andean or sierra basins. I have subdivided “modem” sediments (late Pliocene to Present), in which present relief is still a determinant factor in the geological and hydrogeological characteristics of the formation, according to their geomorphic location. Alluvial basins of Tertiary origin are included

here as a separate subtype of alluvial aquifer. Pre-Tertiary sedimentary rocks are dealt with in a separate section along with other rocks of similar age, but different origin.

Alluvial formations containing aquifers that are suitable sources of urban water supply include:

Tertiary alluvial and molassic basins;
Intramontane alluvial basins;
Foothills alluvia;
Plains alluvia;
Shield and platform alluvia; and
Coastal alluvia.
Tertiary alluvial and molassic basins
Throughout the Latin American continent, there are a large number of older alluvial deposits, more or less consolidated, that contain usable aquifers. Aguascalientes in Mexico and São Paulo in Brazil draw their water from this type of aquifer. These formations are often thick, frequently exceeding 200 or 300 m, and contain water with a relatively high degree of mineralization, particularly in their deeper zones. Because of their age, they are frequently cemented with various types of matrix (clay, silica, carbonates, iron oxides and hydroxides; sulfates, etc.), which considerably reduces their actual and effective porosity and, therefore, their permeability.

However, because of the great volume of these deposits, even a small proportion of the formation can provide sufficient water for urban use or irrigation.

Intramontane alluvial basins
Intramontane alluvial basins are alluvial valleys located throughout the mountainous regions. They can be relatively narrow with steep longitudinal profiles, steep slopes, and narrow alluvial plains on their floors or wide valleys with more gently sloping thalwegs and moderately inclined lateral slopes. Narrow, deep valleys are usually the result of strong river-bed erosion, as opposed to slope erosion which tends to produce wider valleys and more abundant accumulations on the valley floor.

The largest alluvial deposits are built in fluvial valleys downstream from semi-arid or arid (or periglacial) upper basins, where slopes are devoid of vegetation and are, therefore, subject to severe erosion. In humid areas, valleys are narrow and deep and alluvial deposits are less important. However, even in areas that are now humid, large alluvial deposits, which have been inherited from more arid geological periods, may be found.

When large, broad accumulations of alluvia are found in humid or subhumid regions, they are frequently of tectonic origin. Important alluvial aquifers of this type are found in the Lake Valencia graben in north-central Venezuela (Peeters 1968), the Cauca graben in Colombia, and the longitudinal valley graben south of Santiago, Chile.

Valleys of an intermediate type are the Cochabamba valley in Bolivia (Von Bomes 1988) and the upper Magdalena valley near Neiva, Colombia, which are relatively narrow and steep (although they appear to be also tectonically generated). The valleys of Querétaro

and San Luis Potosí in Mexico, on the other hand, are wide, with relatively gentle longitudinal and lateral slopes.

Intramontane valley aquifers usually provide good quality groundwater, but the volumes are not always large enough to supply cities or irrigate farms. Cochabamba and Cali withdraw large amounts of water — for urban use and irrigation in Cochabamba; mainly for irrigation of sugarcane crops in Cali. Often, the presence of a significant volume of surface water has limited the development of groundwater resources, as in Cali and Neiva in Colombia where most water comes from the Cauca and Magdalena rivers, respectively. However, irregularity of river flow (Cochabamba) and degradation of the surface water (Lake Valencia near Valencia and Maracay, Venezuela) have caused these cities to use groundwater almost exclusively for human use and sometimes for irrigation as well.

Foothills alluvia
Aquifers in foothills alluvia occur on both sides of the Andean and Sierra ranges. Similar aquifers can be found in the foothills of the mountainous areas of the shields (Brazilian and Guyanan), of the coastal escarpments in these regions, and of the basaltic plateaus of southeastern South America (i.e., the foothills of the Serra do Mar escarpment in the states of São Paulo, Paraná, Santa Catarina, and Rio Grande do SuI in Brazil).

The characteristics of the alluvial formations containing these aquifers are similar throughout the region. They usually consist of coarse deposits (agglomerates or conglomerates, gravelly sands or gravelly sandstones, and various other types of sandy deposits with varying degrees of consolidation and silt and clay content). These formations arise from the merging of a number of alluvial fans at the outlet of mountain valleys, as they spread out over the level plain.

Locally, the presence of less subsident (in relation to the plain) or less elevated (in relation to the mountains) blocks of rock next to the foothills, frequently covered with older sedimentary formations, may prevent the accumulation of foothills formations to any appreciable thicknesses. In other cases, active faulting and strong subsidence processes may allow the accumulation of very thick layers of foothills alluvial deposits.

The thickness of these sedimentary units is variable, but it normally increases gradually from lowlands toward the mountains. Maximum thickness of the coarse alluvial formations (which usually contain the water) is found a few kilometres or a dozen kilo metres from the foot of the escarpment, where they may be several hundred metres thick. Further away, although the actual thickness of the whole sedimentary sequence increases, the alluvial formations become shallower and finer, with lower hydraulic conductivity and poorer well yield.

Among the many cities that obtain their water from foothills alluvial aquifers, Villavicencio in Colombia and Santa Cruz in Bolivia are perhaps the most representative examples.

Plains alluvia
A significant area of the South American continent (as much as one-third) is occupied by the vast flatlands extending from the llanos in the Orinoco delta ‘to the lower pampas south of Buenos Aires. In large measure, the upper parts of these basins were filled during the last geological epochs (Pliocene to Holocene) by alluvial (but also lacustrine

and eolian) deposits, transported from neighbouring highlands: the Andes in the west and north, Brazilian and Guyana shields and basaltic planalto in the east.

Toward the south, these deposits are affected by the irregular flow of streams from the semi-arid foothills of the Andes through the dry pampas and, therefore, contain intercalated layers of coarser and finer fluvial deposits, often including sediments of fine grain size and salts (halite, gypsum, and anhydrite) or lenses of lacustrine or eolian sediments. In the” north, the climate is more humid and, therefore, the intercalated deposits include more sandy and finer lenses and fewer very coarse layers, except where they are close to mountains or related to channel deposits in the larger rivers. This type of sedimentation varies with successive local environments and climatic changes, sometimes providing a record of fluvial activity during the Quaternary period.

Similar plain accumulations are found in the rivers draining plan alto and shield basins (e.g., the Paraná River and its tributaries), where alluvial deposits are also a succession of sandy formations with frequent lenses of finer elements and carbonate-cemented or silicified lenses or layers.

Many Argentinean cities draw part or all of their municipal water from these aquifers, e.g., Junín, San Nicolás, and several municipalities of the greater Buenos Aires-La Plata area (La Plata and Quilmes). Cities obtaining part of their water from the Paraná “sands” or correlative formations (called the Arenas Puelches in the southern Paraná region) are Asunción in Paraguay and Rosario, Santa Fe, and other smaller urban centres in Argentina (Fili 1983; Herrero 1983; Auge et al. 1988).

Similar alluvial deposits, although devoid of silica and carbonates, occur near the Orinoco flood plains, where sandy (mainly quartzic) accumulations have resulted from both sediment transport by the main river and lateral supply provided by the tributaries descending from the Guyana shield and the mountain regions of the north and west. Two of the largest cities of the llanos (Ciudad Bolívar and Ciudad Guayanain Venezuela) use alluvial aquifers of this type to complement the water drawn directly from the river (Menendez and Araujo 1972).

Shield and platform alluvia
Quaternary alluvial formations are found throughout the shield regions, both in the valleys within the shields and at their periphery at the outlets of alluvial streams, existing and ancient, where they encounter the lower-lying inland flatlands or the narrow coastal plains. These deposits vary in thickness, but are somewhat thinner than analogous deposits in the intramontane valleys of the mountain-sierra region and its foothills. They are particularly well developed in the semi-arid Brazilian northeast, along the Brazilian coastal plains (where they frequently occur as foothills deposits of the Serra do Mar escarpment and its northern extension), and in the fluvial valleys of the Uruguayan-Río Grandense crystalline island.

These formations, which are frequently formed of quartzic or arkosic sandy or gravelly material, may contain significant volumes of groundwater and can deliver relatively high yields because of their porosity and hydraulic conductivity. Several cities of northeast Brazil and the Atlantic southern coastal plains use groundwater from this type of aquifer. In the northeast, groundwater is widely used because of the lack of surface water. Along the southern coast, there are few large rivers because the divides are not far enough from the ocean to allow development of extensive fluvial systems (Geyh et al. 1983). Coastal rivers in Brazil tend to be short, with small basins, and average flows are rather limited in

spite of high local levels of precipitation. This has promoted the use of groundwater in these areas, sometimes resulting in the intrusion or upwelling of saline water in aquifers.

Some of the larger alluvial valleys in the states of São Paulo and Minas Gerais also draw water from alluvial aquifers, but to a lesser degree because of their access to permanently flowing streams, which are a consequence of higher precipitation volumes.

Aquifers in the alluvial deposits of the Guaíba, Maranhão, San Francisco, and Tietê rivers in Brazil and of the Demerara and Essequibo rivers in Guyana are tapped for urban use. In Uruguay and Rio Grande do SuI state (Brazil), alluvial deposits are relatively thin. However, several cities use them. In Uruguay, several small cities surrounding the metropolitan area of Montevideo draw water from the Pliocene Pleistocenic (sandy-gravelly) Raigón aquifer and there is potential for additional use by the city of Montevideo itself. In southern Brazil, Pelotas gets water from the Graxahim formation underlying the São Gonzalo waterway and Uruguaiana uses an aquifer on the margins of the Uruguay River.

Coastal alluvia
Coastal alluvia formations occur in all coastal areas of the LAC: from northwestern Mexico (La Paz, Mexicali, and Tijuana) to the Pacific coastal plains of South America (Lima, Trujillo, and Valparaíso) and the southern pampas (Mar del Plata, next to the crystalline islands of Tandil and La Ventana), and from northeastern Brazil and Guyana (Fortaleza, Georgetown, Maceió, and São Luis island) to the coastal regions of the Caribbean and the Gulf-of Mexico (Santa Marta in Colombia, Maracaibo in Venezuela, and Veracruz in Mexico among others).

These alluvial deposits are frequently interdigitated with sediments of littoral origin that can also act as good aquifers if they consist of coarse material (beaches, bars, and eolian ridges) often without a break in hydraulic continuity. These complex groundwater reservoirs are usually easily accessible; water is abundant and not far below the surface.

However, coastal aquifers are susceptible to saline intrusion (or upwelling) when water is extracted too rapidly. Some cities experiencing salinization of wells include Lima in Peru, Santa Marta in Colombia, Coro in Venezuela, Rio Grande and Natal in Brazil, and Mar del Plata in Argentina. Buenos Aires-La Plata also has a salinization problem, but, in this case, the salinity is coming from salts contained in a coastal formation.

Conclusion
Alluvial aquifers are the most common aquifers in the LAC region. Their dimensions, grain size, and petrographic composition vary widely, as does porosity and hydraulic conductivity. However, on average, these units are hydrologically highly productive, with frequent potential for urban water supply. Problems are mainly associated with their shallowness, which, although it is an advantage economically, may lead to contamination from surface sources. The use of alluvial aquifers requires special care, but their potential as a water source for urban use can be high.

Carbonate aquifers
Carbonate rocks are abundant throughout the world. Some occur on sea bottoms and near shores at various depths (oceanic organic muds, reefs, tidal plains, and calcareous beaches), some in lacustrine, palustrine, or even alluvial environments. They can be of

igneous origin (carbonatites) or they may have undergone metamorphic transformation (marble).

Carbonate aquifers can contain material with high primary porosity and relatively less-important fracture porosity (e.g., reef and lumachelle formations, calcarenites, carbonate tuffs, and miscellaneous detritic sedimentary materials). However, porosity may be secondary, having developed through fracture and chemical dissolution (e.g., most aquifers occurring in compact limestones and dolomites).

Carbonate rocks are often hydrogeologically dynamic. With time, diagenetic processes tend to reduce primary porosity, through local dissolution and recrystallization of the carbonate minerals contained in the formations. On the other hand, circulation of water through fractures tends to dissolve minerals of the walls, “eroding” them and forming underground waterways that grow gradually. Because these processes often take place simultaneously, some carbonate aquifers have relatively high primary porosity (still not completely affected by diagenetic processes) and developing secondary porosity (in fractures).

These rocks can contain considerable volumes of water in intergranular spaces and in fractures. Water action can enlarge the fractures and” therefore, facilitate water circulation. These mechanisms are called karstic processes and the aquifers contained in these formations are often called karstic aquifers. When wells (or springs) connect with the main karstic waterways, these aquifers can be extremely productive and highly suitable as water sources for large cities and agricultural irrigation.

However, there are a number of limitations to use of this type of groundwater resource. First, because carbonate aquifers frequently are highly discontinuous, not all boreholes are productive; many can be dry if they do not contact the main fracture system. Second, although yields can be impressive, they may not sustain extraction of large quantities of water. In many cases, the reservoirs contain less (sometimes much less) water than other types of formations with smaller yields. An additional element of concern, relates to the fast flow of the groundwater through the open fractures. The rapid movement does not allow for degradation of contaminants that may be carried from the surface into the groundwater system. However, in spite of these problems, karstic aquifers remain among the best and most reliable for urban water supply.

Karstic aquifers of Latin America
Although carbonate formations are widespread in Latin America, highly productive carbonate aquifers are most frequently found in the north, in the Caribbean and Gulf of Mexico. Important carbonate aquifers are located in Barbados, Cuba, Jamaica, Puerto Rico, and several islands of the Bahamas archipelago, in the neighbouring Yucatán and Florida peninsulas, in the Mexican hinterland (Nuevo León, Tamaulipas, and Coahuila states), and in the coastal areas of northern South America.

Bridgetown (Barbados), Havana (Cuba), Montego Bay Oamaica), Mérida (Mexico), and Miami (USA) depend exclusively on groundwater obtained from carbonate aquifers. Other cities that depend to a large extent on this type of aquifer include Nassau (Bahamas), which also uses desalinized seawater, Kingston Oamaica), and several of the largest cities of Puerto Rico (San Juan, Ponce, and Arecibo).

Carbonate formations in Latin America are heterogeneous in composition and genesis, with varied porosity and degree of fracture and consolidation. Their hydrogeological properties are similarly diverse. Some are very compact, nonfractured limestones or dolomites with low porosity and almost no useful water content. On the other hand, a number of high porosity, densely fractured carbonate formations provide huge volumes of water and have excellent potential for urban water supply. Highly porous carbonate aquifers can be found in the molassic basins of the Sierra Madre del Sur, Mexico (e.g., in the Huacapa River basin near Chilpancingo), in the foothills of the Jamaican highlands toward the northern side of the island, in southern Puerto Rico, and along the coast of Venezuela. Typical karstic aquifers (with fracture flow) are found in Havana South (Cuba), Montego Bay Oamaica), the Yucatán peninsula in southeastern Mexico, Nuevo León in northeastern Mexico, and the Morelos formation of south central Mexico.

Karstic aquifers are especially vulnerable to contamination. They are located near cities (even beneath urban areas in some cases) and, therefore, domestic and industrial wastes can easily find their way into the groundwater reservoirs. Second, agriculture is more intensive in areas surrounding the cities; hence fertilizers and pesticides are abundant. Third, rapid water circulation within the karstic aquifer system does not allow for adequate filtration and purification of the recharged water.

These problems are encountered in all karstic regions of the continent. Industrial and domestic wastes contaminate the urban aquifer in Kingston Oamaica) and Mérida (Mexico). In Havana, it is suspected that the intensive agricultural activity in the recharge area of Havana South is polluting the karstic aquifer that is the main source of water for the city of Havana and surrounding areas. Carbonate aquifers are very sensitive to anthropogenic interference and careful management is necessary for their continued use.

Pre-Tertiary sandstones and conglomerates
Although other formations, e.g., carbonate rocks and lavas, are found in the large pre-Cenozoic sedimentary basins of South America, the main groundwater reservoirs are contained in the sandstones and conglomerates of these regions and are of alluvial, coastal-marine, or eolian origin. The characteristics of the older sandstones and conglomerates that make their aquifers suitable for urban use are:

Sufficient thickness, at least several hundred metres;
Sufficient lateral extension, several thousand to tens of thousands of square kilometres;
Not overly affected by macro faulting and folding, which may disrupt hydraulic continuity;
Primary porosity at least in the “medium” range, in general over 5%; in some cases, fracture flow may compensate for reduced primary porosity;
Hydraulic conductivity at least 0.1–1 m/day;
High well yields of at least 100 L/minute (depending on investment);
Depth not more than 1 000–2 000 m;
Relatively shallow static and dynamic water levels (sufficient pressure to give rise to artesian wells is desirable, but is often lost with heavy extraction);
Sustainable rate of renewal, normally related to recharge volume from the ground surface; and
Low level of mineralization of the water, i.e., concentration of total dissolved solids less than 0.05%.
The main sedimentary basins of the continent, whose hydraulic continuity has been relatively unaffected by tectonic events, are located around the cratonic or tectonic regions of South America and in the central plains. An example is the huge Amazon sedimentary basin, which is composed of pre-Cenozoic sedimentary fill covered by a large Cenozoic sequence. It is virtually untapped, because of its depth, the low population density in the region, and the abundant surface water available.

Another large sedimentary basin, the Paraná basin, underlies the Paraná River and its tributaries. It is very deep (6000–7000 m along its central axis under the Paraná River in Argentina) and is composed of an impressive sequence of Paleozoic to Cenozoic sedimentary rocks. It contains a large number of conglomerates and sandstones that contain regionally or locally usable volumes of water. The Devonian deposits consist of older formations of arkoses and coarse sandstones and a younger unit of sandstones. Because they are normally found at great depths, their use is impractical.

The Permo-Triassic layers also possess coarse detritic formations at their base. They are the conglomerates (tillites) of glacial origin (Itararé-San Gregorio) and the sandstones formed in a fluvial-glacial environment (Rio Bonito-Tres Islas). These units contain water, but their use is limited due to their depth over large areas and poor water quality. The upper part of the neo-Gondwanan sequence is also composed of sandstones (Estrada Nova) that are locally used as aquifers in southern Brazil and Uruguay.

The upper filling of the Paraná basin is neo-Gondwanan and is composed mainly of eolian sandstones (paleodesert of Botucatu-Tacuarembó) and a thick accumulation of basaltic flows. Botucatu is a medium- to high-porosity sandstone, poorly consolidated, and contains one of the largest aquifers of the continent, extending from Mato Grosso to Uruguay, with an estimated storage capacity of 10 000–20 000 km3. The Botucatu aquifer contains good-quality potable waters, produces high yields (often 500 L/minute), and is artesian over a large portion of its area. Despite these advantages, the aquifer is used only near its outcropping, because the formation is covered, over most of its area, by a basaltic mantle several hundred metres thick (locally over 1 000 m) that is not only largely unproductive hydrogeologically, but is also difficult and expensive to drill through (Da Cunha Rebouças and Fraga 1988; Montaño and Pessi 1988; Kimmelman et al. 1989).

The upper part of the Paraná basin sequence comprises relatively thin deposits of late Cretaceous and Cenozoic origin. Some of these contain usable groundwater, e.g., the Bauru formation in Brazil and the Mercedes-Asencio formations in Uruguay, but the most commonly used aquifers are in the Pliocene-Pleistocene alluvial sediments described earlier.

Coastal aquifers
These aquifers are defined simply by their location near the coast. There are many possible types depending on the historical geology of a specific area. Many are the result of the geological interactions of continental and littoral or marine formations. In some cases, they are composed exclusively of coarse marine or coastal detritic deposits, e.g., beach or dune sands, or miscellaneous shallow-water sandy deposits. Others are made up of marine or littoral carbonate rocks. A considerable number of coastal aquifers are alluvial, with or without intercalation of coastal or marine formations, and a smaller number can be volcanic, composed of older coarse detritic sedimentary rocks or crystalline rocks.

In spite of the variati9ns in genesis and sedimentological characteristics, their location next to the sea puts these aquifers into close contact with the highly saline groundwater usually contained in suboceanic geological environments. They are, therefore, especially sensitive to overpumping. These hydrogeological units are low-lying, frequently below sea level or only slightly above, and occur at the mouths of current and ancient fluvial basins, in close association with existing waterways, at their point of maximum flow near the ocean.

The main problems involved in using these aquifers relate to salinization of their waters. Because of its lower density, fresh water floats on more saline water. However, the difference in density is only 2.5%, and the relatively thin layer of fresh water that frequently overlies saltier groundwater can be many metres below sea level. However, when careless pumping removes the fresh water too quickly, salty water will tend to replace it from below. This upwelling of the salty water may not occur for several years; thus, the effects of overpumping may not be felt until it is too late to remedy the problem.

A large number of Latin American cities are located along the Atlantic, Caribbean, and Pacific coasts, and 30–40 of them rely on groundwater drawn from various types of coastal aquifers, Among them are Mar del Plata, Argentina (taking water from an alluvial aquifer on the shores of the Atlantic); Natal and Recife, Brazil; Santa Marta, Colombia; Havana, Cuba (which gets all its water from a karstic aquifer on the southern coastal plain); and Lima, Peru (which obtains about 40% of its water supply from a coastal alluvial aquifer).

Answer 2

It comes down to the difference between porosity and effective porosity. While pumice does have large pores, they are poorly interconnected, meaning that water can not travel from pore space to pore space easily.

Answer 3

Porosity does not equal permeability. There may be a lot of holes, but if those holes aren't connected in some way to let the water flow then it makes for a poor aquifer.

Answer 4

Pumice is formed by gases trapped in silicic solution with lessened pressure of confinement but not total freedom to the atmosphere where gasses would escape, creating a uniform flow toward the outside hence promoting continuity of its pores.