Document(s) 8 of 14

Several cities in Latin America and the Caribbean depend heavily on groundwater, generally because surface water is scarce. These cities are in volcanic regions, where river valleys are obstructed by lava flows or other volcanic features (e.g., Guatemala City, Mexico City, and Managua) or in karstic regions (e.g., Mérida and Havana).

The valley of Mexico⁴

Environment and history

There are probably few places in the world, where the physical environment has been as completely transformed by urban development as in Mexico City. The valley of Mexico is a 9 600-km² closed basin in the heart of a neovolcanic belt, that has been raised by geological forces to more than 2200 m above sea level (Fig. 15).

About 700 000 years ago, obstruction of drainage by volcanic material caused several lakes to form on the floor of the valley. Their total area was about 2 000 km² and they were connected during periods of high water. Three of the lakes (lakes Mexico, Chalco, and Xochimilco) contained fresh water; the other three (lakes Texcoco, the largest at 800 km², Zumpango, and Ecatepec) were brackish (Fig. 16).

The area was (and to a certain extent still is) sub humid. Annual rainfall was probably slightly higher than the current 600 mm at the bottom of the valley to 1 200 mm in the surrounding mountains. Average temperatures were relatively cool for the subtropical latitude, ranging from 8 to 15°C depending on altitude. The deep soils were highly fertile and easy to work. The land was completely covered by thick forests, particularly on the slopes of the mountains and in highland areas. The valley plains, which were also covered by forests, were soon cleared for agriculture. A number of springs, around the lakes and on the foothills of adjacent mountains, provided considerable volumes of good-quality water.

⁴ Material in this section is drawn from Herrera et al. (1982), Castillo Benhier (1983), Granados Velazco (1988), Ortega (1988), Cortes et al. (1989), Gonzalez Moran and Rodriguez Castillo (1989), Herrera (1989), Ryan (1989), Ward (1990), and Yepes (1990).

Fig. 15. Aerial view of Mexico City and the surrounding area.

Fig. 16. Ancient lakes in the valley of Mexico.

These features made the valley attractive to early indigenous people, who, over several thousand years, developed an economy based on locally domesticated crops and farm animals (e.g., corn, tomatoes, chilies, cacao, turkeys, escuincle dogs, and honey bees) and fishing. Because these civilizations did not have domesticated draft animals and did not use the wheel, most commerce depended on boat traffic.

Several groups successively inhabited and controlled the lacustrine area during the last few centuries before the arrival of Europeans. The last of these was the Aztecs. This group had arrived from the legendary land of Aztlán (probably located to the north in more arid territories) during the 14th century and, because the lake shores were already occupied, they were forced to settle on swampy land in the lake and make their living by fishing and trade with neighbouring communities.

Gradually, the Aztecs built an island in the southern part of the lakes, in the middle of Lake Mexico and a town developed (Tenochtitlán), which was later to become the main city of the valley. Through alliances and wars, the Aztecs built an empire, and Tenochtitlán grew to become a thriving city of several hundred thousand people. A bridge was built to join the island with the mainland, and large boats were used to transport people and merchandise throughout the area.

The Aztecs built earth dikes to control flooding and to separate brackish lakes from those containing fresh water. In addition, aqueducts were built to bring fresh water from springs, through the lake, and along the dikes to the city.

From Tenochtitlán to Mexico City

It is difficult to comprehend the extent of the change that has taken place in the few centuries since the Spanish conquest that began in 1521. Today, proud Tenochtitlán has disappeared, and excavations reveal only scattered archaeological remnants. In its place stands the highly urbanized downtown part of Mexico City.

Lake Mexico is gone. On its site are several hundred square kilometres of urban neighbourhoods. Lakes Chalco and Xochimilco are also gone. Only a few canals and small lakes remain. The rest of the area is covered by streets and buildings.

The three northern lakes have also been drained. The bottom of the former Lake Texcoco is a vast, flat plain, on which little vegetation grows because of the high alkalinity (pH over 10). Today, an intricate maze of wells and pipes pumps the brine contained in the lacustrine sediments out of the ground for extraction of sodium carbonate and sodium chloride.

The springs that provided water to the riverine populations are also gone. Now more than 5 000 wells extract over 50 m³ of water per second from an average depth of 100 m, gradually lowering the level of water in the aquifer by as much as 1 m/year. Overpumping and compression of the upper layers of sediment have caused widespread subsidence. Several areas have sunk more than 6 m. Because subsidence rates vary, many structures have been weakened and, in some cases, lean dangerously, e.g., the cathedral, the older Basilica de Guadalupe, and the Palacio de Bellas Anes. This phenomenon has been exacerbated by frequent seismic activity, of which the most recent destructive example was the earthquake in September 1985.

The forests that covered the adjacent hilly slopes have almost disappeared and widespread soil erosion occurs. Most agricultural land has been covered by pavement, houses, and other urban structures. Quarries have been excavated throughout the region to obtain construction materials. Some became garbage dumps, into which a portion of the annual 10 million t of garbage is thrown.

A significant portion of the garbage is disposed of on the “shores” of the former Lake Texcoco, especially in the southern section. Ciudad Netzahuatcóyotl, which is located in that part of the city, is a neighbourhood of 3 million people, built on the bottom of the lake. The environment in this recently created urban area has become highly degraded and developed areas alternate with garbage dumps and slums.

The valley’s drainage system, which used to enter the lake, is now channeled out of the basin, together with urban wastewaters, through a system of canals and tunnels into the Gulf of Mexico. A number of pumping wells that supply the city are located next to the Chalco canal, which carries away both wastewater and excess storm water. The risk of

contamination is obvious and, in fact, some wells had to be closed because of nitrates in the water.

The atmosphere of the valley has also changed with growth of the city. Emissions from 4 million vehicles and 25000 industrial establishments in a poorly oxygenated environment (because of the altitude) have created a serious health hazard. The air in the downtown core is particularly noxious.

More than 19 million people live in Mexico City, making it one of the largest urban centres in the world. Every year, the birth rate and migration from the rest of the country increase the population by nearly 0.5 million. By the year 2000, the city may have 25 million people and, by 2010, over 30 million. If corrective measures are not taken, the current problems will become worse, and the ancient valley may become one of the worst environmental nightmares of the 21st century.

The aquifer and the urban water supply

The aquifer underlying Mexico City provides the bulk of the city’s water. Some water is brought from outside the basin (from the Lerma-Cutzamala basin), but this amounts to less than one-fifth of the total volume. Bringing water into the valley from elsewhere is becoming impractical and too expensive. Although the resources of the Lerma-Cutzamala basin are almost exhausted, tapping the Balsas or the Amacuzac basins would mean pumping water 1200–1500m upwards and constructing long pipelines, storage reservoirs, and other costly engineering works. Using water from outside the valley would also affect communities that are dependent on that water for irrigation and their own supply.

Geology and hydrogeology of the valley

Mexico’s aquifer is contained in a number of Tertiary and Quaternary units with a thickness ranging from a few hundred metres to nearly 2 000 m. These units comprise a wide range of sedimentary materials, including various pyroclastic and alluvially reworked pyroclastic sediments, breccias, conglomerates and agglomerates, several types of sandy volcanic formations, volcanic ashes, lacustrine lenses, and intercalated lava flows (Fig. 17). These deposits are closely related to the volcanic activity that took place during the formation of the trans-Mexican neovolcanic belt and synchronous epeirogenesis.

The base of the sequence lies over the Cretaceous limestones of the Morelos formation, a 1 000-m-thick, heavily karstified unit that constitutes the “floor” of the volcanic sequence. The base is composed of conglomerates and sandstones of the Balsas group (Eocene-Oligocene epoch). This molassic group filled the grabens that developed during the post-Laramidic orogenetic period. It includes up to 500 m of conglomerates, covered by poorly sorted finer deposits (sandy, but also silty and clayey), that is up to 2 000 m thick.

Overlying the Balsas group is a complex volcanic sequence of early Miocene age, composed of various types of pyroclasts (tuffs, breccias, and agglomerates) and intercalated alluvial clastic sediments and lava flows. Its thickness varies from 390 to 1 750 m. Overlying the Miocene sequence is a 300- to 800-m-thick volcanic sequence that includes andesitic lavas, volcanic breccias, and tuffs, which in turn is covered by andesitic and dacitic volcanic material of early Pliocene age, including lava and associated unconsolidated pyroclastics (300–600 m thick). On top of these extrusive rocks, the pyroclastic flows of the Otomi formation are found, with ash-flow and ash-fall tuffs, breccias, and associated andesitic lavas.

The Otomi formation is covered by a complex sequence of volcanic units including Las Cruces, Zempoala, and Navaja formations and undifferentiated Pliocene pyroclastics and the Llano Grande, El Pino, Tlaloc, Popocatépetl, Chichihuanitsin, and Iztaccihuatl formations of Quaternary age. Finally, the plateau depressions are filled with a 500-m- thick sequence of alluvial and pyroclastic accumulations, called the Tarango formation in Mexico’s valley, over which a few dozen metres (locally up to 400 m) of lacustrine deposits are found.

The Mexican valley, therefore, has formed as a result of continuing volcanic accumulation, in which the molassic detritic formations of the early Tertiary era were covered by a long and complex succession of volcanic extrusions that include a large amount of pyroclastic material (reworked to some extent by fluvial action) and intercalated lava flows. During volcanic eruptions, tuffs, breccias, ash, and lava were formed; between these episodes, alluvial and lacustrine action was more important.

The main water-bearing rocks are those of the Tarango formation and associated alluvia and the Cenozoic sequence of fractured pyroclastic and lava flows. These are covered by younger lacustrine sediments, confining the main aquifer. This whole sequence can be up to 2 000 m thick, but the lower 1 500 m is more consolidated; the lower effective porosity (associated with fracturing) lowers the production potential of this section. Because the upper few dozen metres of the aquifer are too close to the upper lacustrine clays, continued pumping may produce dewatering and consolidation of these clays, unleashing subsidence processes. The usable portion of the aquifer is between 100 and 500 m below ground level.

The aquifer is recharged in the mountains (Sierra Chichinautzin in the south, Sierra las Cruces in the west, and Sierra Nevada to the east). The total recharge volume is estimated to be 25–50% of the precipitation (25% in Sierra Las Cruces, 35% in Sierra Nevada, and 50% in Sierra Chichinautzin). Of this volume, about half flows toward the valley of Mexico, the rest moves outward to other basins.

Accurate figures for inflow to the valley aquifer itself are difficult to obtain. However, recharge volume is certainly below 50 m³/second, which is the rate of extraction, because the aquifer is becoming depleted. An estimated inflow of 30–40 m³/second is probably fairly accurate. Additional lowering of the water level in the aquifer will cause an increase in flow from the sierras, because of the increased gradient. However, it is not enough to make up the deficit, especially if the extraction rate increases.

Precise forecasting of aquifer reaction to prolonged pumping requires accurate modeling. Only recently has adequate information on the geometry and hydraulic properties of the reservoir become available. A model of the aquifer has been developed at the Instituto de Geofísica of the Universidad Nacional Autónoma de México (Herrera et al. 1982). Use of the model in conjunction with newly acquired information is expected to allow assessment of the groundwater resources of the valley.

However, Mexico’s problems can only be solved if a different development approach is adopted. The valley’s environment has reached its limit, and the megacity is no longer sustainable. A much less concentrated and centralized countrywide plan must be implemented to encourage a better distribution of the population. Only a drastic change of course will allow the survival of the valley.

Fig. 17. Cross-section of the aquifer underlying Mexico City and surrounding geological structures. Note: arrows show groundwater flow.

The valley of Guatemala

Environment and history

The city of Guatemala, the national capital of the Republic of Guatemala, is by far the most prominent city in the country. It is located in a high valley on the Guatemalan volcanic plateau, at an elevation of 1 800 m above sea level.

Before Europeans conquered the country, the Guatemalan highlands were already densely populated by a civilization, whose economy, like that of the Mexican indigenous peoples, was based on the cultivation of com, chili peppers, tomatoes, and other local crops. When the Spanish arrived, the highlands region was inhabited by the Cakchikel nation of the Mayan group, with their capital in Iximche. As they had done in Mexico, the Spanish conquerors established their capital (in 1523) on the same site as the existing one. Iximche was about 100 km west of and at a higher elevation (about 2 000 m) than the present Guatemala City.

In 1527, after a destructive earthquake, the city was moved to a site about 80 km to the east at a lower elevation (1 530 m). The new city, Santiago de los Caballeros, was established at what is now Antigua. This site is at the foot of a high volcano (the Volcan de Agua) whose main crater contains a lake. In 1533, the Volcan de Agua erupted. The lake caused a flood and a mudflow destroyed and buried large sections of the city. Several hundred people died and most buildings were demolished.

In 1543, the city was rebuilt in the same valley, but was again destroyed by an earthquake in 1773. In 1776, it was established at its present location and named Nueva Guatemala de La Asunción. The old city was gradually rebuilt and today it is a medium-sized city of about 50 000 people (Antigua).

Guatemala City, which remained the capital of the colonial Capitania General de Guatemala, became the capital of the independent Republic of Guatemala in 1837. The city has grown from tens of thousands of inhabitants at the beginning of the 20th century to the current 1.5 million people, with all the requirements and strains on the environment that accompany accelerated urban growth.

The Guatemalan highlands are in an area of volcanic and seismic activity (Fig. 18). Twenty large volcanic cones, with elevations ranging from 2 000 to 4 220 m, and several hundred smaller ones dot the southern mountainous areas of the country. Volcanic eruptions and related seismic activity are common, and recent geology and geomorphologic development are closely tied to these phenomena.

The climate of the highlands is subhumid to humid, with annual precipitation ranging from about 1 000 mm in the drier north-facing slopes to more than 2 000 mm in the south. In the city of Guatemala itself, annual rainfall averages 1300–1500 mm, concentrated in the summer (June to September). Natural vegetation is deciduous forest up to 2 200 m and coniferous forest above this altitude. This forest was partly cleared (even during pre-Hispanic times) to make room for agriculture. This process has continued and has accelerated during the last few years. Today, the forests of the highlands have been reduced to small pockets on the steeper slopes that are not suitable for farming.

Fig. 18. Aerial view of the Guatemalan highlands.

Geology and geomorphology of the valley

The geology of the Guatemala valley is relatively complex (Fig. 19). Over a base of Cretaceous limestones and plutonic rocks, intense and prolonged volcanic activity gave rise to large accumulations of various volcanic rocks and associated deposits.

The valley is an elongated depression oriented in a north-northeast to south-southwest direction. Water flows both northward and southward from a portion of the continental divide that lies more or less perpendicular to the main axis of the valley.

A large lake (Amatitlán) has formed in the southern part of the valley, because of volcanic obstruction of run-off. It is drained by the Michatoya River, which flows to the Pacific Ocean. The northward flowing rivers (Las Vacas and its tributaries El Zapote and Tzalja) drain the smaller northern part of the valley toward the Caribbean sea.

Volcanic cones surround the city, the largest ones being the Volcan de Agua, about 20 km southwest of the city, whose summit is over 3 000 m above sea level; and the Pataya, about the same distance to the south, with an elevation slightly above 2 000 m. The valley is a large graben with locally intercalated horsts. Both the elevated and depressed areas are composed of volcanic rocks, although older limestones are found in the Las Vacas River in the northern part of the valley.

The oldest formation identified in the Guatemala valley is a calcareous unit of fractured limestones in the Las Vacas basin, with limited outcropping. Overlying the Las Vacas limestones, a thick sequence of lava flows and associated deposits of Tertiary age are encountered. These lavas are heavily fractured, resulting in high secondary porosity. The Tertiary lavas are covered by two sequences of fluviolacustrine and volcanic deposits of Quaternary age:

• A sequence of fluvio-lacustrine deposits, composed of volcanic materials alluvially reworked and deposited in river beds and plains or in lakes. Their thickness does not exceed 100 m. The alluvial sediments have a relatively high permeability, but the lacustrine deposits can act as aquitards or aquicludes.

Fig. 19. Cross-section of the Guatemala valley, showing geological formations of the region.

• A sequence of volcanic deposits, formed by a large accumulation of pyroclastic products including ash-flow tuffs. These materials have a wide range of consolidation levels, from loose to well-compressed. The maximum thickness of this formation exceeds 200 m. It is moderately to highly porous and contains an excellent aquifer, which is heavily used.

Water supply

Guatemala City gets 80% of its water from groundwater sources; this amount probably represents the maximum obtainable from these aquifers. Of the 5 m³/second required by the city (somewhat higher during peak periods), up to 4 m³/second comes from 200 wells distributed throughout the valley. The remaining 1 m³/second is obtained from surface sources outside the local valley.

A pipeline was built to bring river water into the city. Its capacity is 2.5 m³/second, although current flow is less than half of this. There are plans to build a battery of wells in the volcanic aquifer at Antigua to augment the volume transported by the pipeline. The only other surface water available is in Lake Amatithin, which is heavily polluted.

Because recharge of the aquifers takes place mainly in urban and suburban areas, strict controls are required to avoid degradation of the groundwater. Currently, controls are virtually nonexistent. However, decision-makers will have to remedy this situation, because Guatemala cannot afford to lose this key resource through lack of adequate protection or planning.

The Managua basin

History

The city of Managua is located in the large volcanic valley of central Nicaragua. It developed as a small Indian farming and fishing village by taking advantage of fertile volcanic soils, the abundance of water, and the proximity of the large Lake Managua (or Xolotlan).

When the Europeans arrived in 1522, the population of the territory was moderately high. The indigenous people south of the lakes, the Nicaraos, were of Mexican origin. They cultivated corn, cacao, chili peppers, and tomatoes and raised turkeys and dogs. At that time, Managua was a prosperous community of 40 000 people located on the shores of Lake Managua.

When the Spanish arrived, they settled on the narrow strip of the Pacific highlands and the shores of the large lakes Managua and Nicaragua (or Cocibolca). Managua was strongly affected by the Spanish conquest. Its population decreased to,a mere few thousand people, and it took three centuries to return to the pre-Hispanic level.

With the Spanish conquest, a relatively large colonial urban centre developed northwest of Lake Managua: the city of León. This city was destroyed by an earthquake in the 17th century, and a new city was founded on the western shore of Lake Nicaragua (Granada). Colonial history, and later that of the Republic of Nicaragua, was heavily influenced by the rivalry between the old city of León and the newer Granada.

Nicaragua became independent upon disintegration of the Union of Central America in 1838. After the formation of the newly declared republic, there was a period of conflict between the liberal factions based in León and the conservative ones in Granada. In 1852, as a compromise, a new capital was chosen on the site of the small village of Managua, which grew to become the largest city in the country.

In 1973, the city suffered a major earthquake that destroyed almost the whole downtown area. The city centre was not rebuilt, in spite of assistance that poured into the country from all over the world. The new government, which took power in 1979 after a long civil war, decided to conserve the area as a “green” core in which public parks and squares have now been established.

From 1979 to 1989, the country was in an almost continuous state of war. It was also under commercial boycott by the United States until the 1989 elections, and this seriously harmed the economy of the country. As a result, the city suffered enormously: basic goods were scarce, public services were unable to function properly, and capital investments were not available. The war also meant heavy casualties and a flow of war refugees from the countryside to the city. Managua’s population is nearly 1 million, or almost one-third of the country, and the city covers an area of more than 60 km².

The environment of Managua

Managua is located on the southern shores of Lake Managua, on land sloping from the Cordillera del Pacifico (locally called Las Sierras de Managua). The Cordillera del Pacifico reaches elevations of 900 m above sea level, whereas the lake is at an altitude of only 40 m. The depression extends southeastward through the Tipitapa plains to Lake Nicaragua, which is some 9 m below Lake Managua (Fig. 20).

Excess water from the Managua basin flows into the Tipitapa River and as groundwater toward Lake Nicaragua. The outflow of the Nicaragua basin reaches the Caribbean via the San Juan River. These two lakes cover 9000 km² (Lake Nicaragua, 8 000 km²; Lake Managua, 1 000 km²).

Several other smaller lakes of volcanic origin complete the lacustrine panorama of the Nicaraguan valley. The largest are lakes Masaya, Apoyo, Apoyeque, and Jiloa. Near Managua, there are a number of crater lakes, including Asososca, Nejapa, and Tiscapa.

The climate of Managua is subhumid-tropical with warm temperatures all year around. The lowest monthly average is 23°C in January and the highest is 31°C in April. Mean annual precipitation is about 1 200 mm, mainly falling from May to October, at the time of the arrival of the intertropical convergence. The dry season is from November to April.

Almost no permanent streamflow occurs on the slopes in the region; sporadic flow can be observed immediately after high rainfall in the largest drainage channels (e.g., the Rio Borbollon, which flows near the Las Mercedes airport). The main reasons for this limited stream flow are the high permeability of soils and surface formations, the concentration of rainfall, and the poor development of the hydrographic network, which is a common feature in volcanic landscapes because of frequent accretions of volcanic materials.

Geologically, the area is composed of various types of volcanic rocks and deposits. A volcanic range forms the western edge of the depression, extending in a northeast-southwest direction. It includes symmetric volcanic cones, explosive craters, and calderas.

Fig. 20. Aerial view of the Managua-Nicaragua valley.

Fig. 21. Cross-section of the geological structures near Managua.

All Cenozoic geological formations in the Managua area are, directly or indirectly, of volcanic origin. The oldest stratigraphic unit is a pyroclastic sequence called the Las Sierras group (Fig. 21). This group is composed mainly of relatively uniform, massive agglomeritic tuffs and breccias that outcrop in several areas within and near the city. The thickness of this unit is about 680 m and its total volume is 450 km³. Although the precise age of Las Sierras is not known, existing information indicates Quaternary age (or at least late Pliocene) for the group. Younger deposits have been shown to be older than 100 000 years. The relation between these deposits and the volcanoes surrounding the Masaya caldera has not been confirmed.

Overlying the Las Sierras group in the Managua region is a thin (10–20 m), layered volcanic sequence called the Managua group that includes airfall tuffs, ash, and lapilli beds. The source of these deposits seems to be related to recent volcanic activity in the Masaya caldera and the Nejapa-Miraflores area. Above the Managua group, a formation of ash, lapilli cinders, and lapillis is found (Motastepe formation). The source of this unit seems to be the volcanic structures of Cerro Motastepe, and the volcanic chimney found in the Caldera of Asososca.

The youngest deposits in the area are of lacustrine origin, related to sedimentation in the bottom of the large lakes and the smaller “lagunas,” and alluvially reworked volcanic material. The lacustrine layers are relatively thin, not more than a few dozen metres, although maximum thickness is not accurately known; the alluvial-volcanic sediments range from 0 to 90 m thick.

Hydrogeology⁵

Because of its permeability, thickness, and area, the Las Sierras group offers the greatest potential for groundwater extraction. It currently provides most of the water used in the Managua area. The Las Sierras aquifer is uniform in shape. In the area of the Pacific range, where the saturated zone is 200–300 m below the surface, most of the recharge takes place.

Groundwater flow seems to follow surface topography: either toward the Pacific Ocean, giving rise to several streams at the bottom of the deepest valleys, or toward Lake Managua. Some of the groundwater flowing toward the lake is discharged into small streams. near the Managua plain in several places where the water table intersects the

⁵ Material in this section is drawn from INETER (1988) and Krasny (1987).

surface. Some groundwater recharges lakes Asososca, Tiscapa, and Nejapa; the rest flows into Lake Managua.

The city of Managua has been using water from the Las Sierras aquifer, directly or indirectly, for some time. The main source of supply had been Lake Asososca, but as the city grew, its water requirements began to exceed the amount of groundwater feeding the lake. From 1914 to 1975, extraction rates increased to slightly more than 80 000 m³/day, causing the level of water in the lake to drop from 40.57 m above sea level in 1960 to 35.38 m in 1975. Since then, pumping rates have been reduced to 50 000–75 000 m³/day or less, and the water level has stabilized at around 36 m above sea level.

The Carlos Fonseca Amador well field is located in the area of the aquifer where conductivity is high (near Sabana Grande and the A.C. Sandino International Airport). At present, 12 wells are producing an average of 50 000–60 000 m³/day. Recently, two more wells were drilled. The amount of water pumped from wells in the Managua area is now slightly more than the amount drawn from Lake Asososca. The total amount supplied to the city, by the Instituto Nicaragüense de Acueductos y Alcantarillados, in July 1988 was 165 000 m³/day. This still does not meet current demand in the Managua region, which is now over 200 000 m³/day and is growing quickly. Interruption in water service can last up to 15–20 h/day; sometimes neighbourhoods do not receive service for days at a time.

Proposed new sources of water supply

Potential sources of water for the city are: Lake Managua, Lake Nicaragua, new wells in the Las Sierras aquifer, and the lagunas.

Lake Managua

Excess water from Lake Managua, which is slightly brackish, flows to Lake Nicaragua via the Tipitapa River. Over the last few years, however, the lake has behaved as a closed basin and its salt concentration has increased due to evaporation.

Intense agricultural activity, including the use of pesticides, and continuing disposal of wastewater from Managua into the lake have affected the quality of its water to the point where it is unsafe for drinking or bathing. The dumping of toxaphene (a pesticide related to DDT (dichlorodiphenyltrichloroethane)) and other waste substances, including mercury, into the lake by the Pennwalt and Hercules chemical plants during the 1970s was especially damaging. According to one United Nations environmental study, the water contains 60 t of mercury. According to another study, 17% of all deaths in Managua are related to mercury poisoning and pollution in the lake.

Lake Nicaragua

Lake Nicaragua contains good-quality fresh water with only minor sources of contamination from Granada and other smaller towns. There is a positive balance between inflow and evaporation; average flow in the San Juan River, which flows from the lake to the Caribbean, is high.

This lake probably represents the best option for water supply, not only for Managua, but also for other towns and for irrigation. However, because of its distance from Managua (40 km) and lower elevation, water extraction from Lake Nicaragua could be beyond the economic means of the country at this time.

New wells in the las Sierras aquifer

The aquifer provides a good alternative for increasing the water supply, although the volume of water available from it can be somewhat limited. Aquifer conductivity is high, and its uniformity makes well location simple. There is, however, a need to ensure that long-term extraction does not upset the balance of the aquifer. It is important to consider the water taken from the lakes in this balance, because, hydrogeologically, these surface bodies act as open wells.

The lagunas

The small lakes (lagunas) in the area are, in effect, “groundwater outcrops.” Their use, therefore, must be limited by the same constraints affecting long-term operation of well fields. They also need special protection because of their exposure to potential sources of contamination.

Of the eight largest lagunas near Managua, Asososca is the only one used as a source of water for the city. Three others are situated in or close to the Managua metropolitan area: Acahualinca, Nejapa, and Tiscapa. The first two are inadequate sources, because of insufficient volume and poor water quality. Only Lake Tiscapa, which is similar to Lake Asososca, might be appropriate. It has the added advantage of being in the centre of the city, but the consequent disadvantage of higher vulnerability to contamination.

The northern lagunas (Apoyeque and Jiloá) located in the Chiltepe peninsula near Lake Managua are not suitable (chemically) for urban water supply.

The remaining two lagunas (Apoyo and Masaya) are east of Managua, outside the city, but close enough to be accessible. Lake Apoyo’s water is of poor quality, but the water in Lake Masaya is acceptable and its volume is eight times that of Lake Asososca. However, the rate of renewal of water in this lake is low and communities near it are already consuming a significant amount. It is unlikely that bringing water from this lake to Managua would be economically feasible.

Conclusions

The environment in and around Managua has been damaged, particularly as a result of poor management of its abundant water resources. Unfortunately, correcting the problems that have accumulated over years of environmental degradation and mismanagement will require considerable effort and investment that is not readily available, given the country’s difficult economic and political situation. Although a first step toward the necessary social awareness and understanding of the problems is being taken, much work and international assistance will be required to turn the Managua basin into the harmonious environment of the past.

The capital of the Mayan nation: Mérida of Yucatán

With 700 000 inhabitants, the city of Mérida is the largest urban area of the Yucatán peninsula in Mexico. It is situated in a fragile environment: the flat limestone platform of northwest Yucatán. Mérida was founded by the Spanish conquistadors in 1542 on the site of the Mayan city, Tho. It became the principal city of colonial Yucatán and later the capital of the state of Yucatán.

Geology

The city is located on a limestone plateau. The oldest outcropping sedimentary rocks are Paleocene limestones, overlaid by the Eocene microcrystalline, white, carbonate rocks of the Chichen-Itza formation. The Miocene epoch is represented by the Bacalar formation of white marly limestones, over which lie the Pliocene carbonate formations Carrillo Puerto and Estero Franco. Overlying the latter are less-pure limestones. They gradually become more clayey, and yellowish and reddish, forming lateritic soils. The younger (upper) levels are hard white limestones, which are covered by Quaternary deposits (more commonly found to the north and west of the peninsula).

The Yucatán peninsula has been affected by the northeast-southwest oriented tectonic activity (which is the cause of the faults associated with the Hondo River, Lake Bacalar, Chetumal Bay, and Ascensión alignments and the northwest-southeast topographic divide of the Sierrita de Ticul).

Geomorphology

The peninsula is part of the coastal plains of the Gulf of Mexico, with average elevation less than 30 m above sea level. The Sierrita de Ticul, which is the highest point on the peninsula at 275 m, has a typical karstic geomorphology with almost no surface hydrographic network. Most rain water filters into the underground systems through sinks or cenotes.

Climate

The climate of Mérida is tropical-subhumid, with an average temperature of 26°C and annual precipitation of about 1 050 mm (potential evapotranspiration is 2 000 mm). In the rest of the peninsula, the level of annual precipitation varies from a minimum of 500 mm near the northwestern shore to more than 1 300 mm in the northeast.

Hydrogeology

Yucatán’s limestone formations contain a huge groundwater reservoir, which in the northern half of the peninsula (62 240 km²) receives as much as 9 350 million m³ of water per year (Lesser Illades 1976). This is a typical karstic aquifer, with the water contained in a network of open fractures that has developed through dissolution of carbonate minerals, such as calcite, aragonite, and dolomite. The depth of the groundwater varies from 100 m in the south to less than 10 m in the north. The maximum thickness of the aquifer is not much more than 160 m; the average is much. less. The weak gradient of the water table (about 4 m from the middle of the peninsula to the sea) is certainly the result of high conductivity in the aquifer.

Environmental problems in Mérida

Mérida is completely dependent on groundwater for its supply, and karstic aquifers are sensitive to contamination. Water moves quickly through the open fractures and its quality is not improved significantly as it is in other types of aquifers. Polluted recharge areas can, therefore, quickly become public health hazards if they are upstream of the pumping wells. Overpumping, on the other hand, brings sea water into the system and wells must be closed, reducing the amount of water available and making parts of the distribution, conduction, and storage system useless. In addition, the city has also been disposing of its liquid wastes into the aquifer system for some time.

The result of these practices has been deterioration of the only available water source in the Mérida region. For Mérida to continue to develop in a sustainable manner, water management methods must change, taking into account the vulnerability of the karstic aquifers.

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