The overexploitation of water is most clearly seen where the population is concentrated in metropolitan areas, which increases the demand for extracting
Table 6.2. Characteristics of the main aquifers in Central America. (From Losilla et al.,
Guatemala Four very important aquifers are located in Guatemala: the upper and lower aquifers in the central highlands and in the valley of Guatemala. The upper aquifer in the high plateau is mainly formed by volcanic quaternary rocks, and the lower aquifer basically consists of lutitas, welded dacite and andesite tuffs, and basaltic andesite lava flows from the tertiary era, which has been fragmented locally. Although we have little information about the aquifers in the high plateaus, which have yet to be exploited, it is known that they are not confined, with a depth of 250 m and a production of 3-70 l/s. Land in the central plateau is now mainly for cultivating coffee and vegetables, as well as for pastures and brushland.
The aquifers in the upper and lower valley of Guatemala are hydrologically connected. The upper aquifer has no quaternary formations, and its depth varies from 5 to more than 50 m; the lower aquifer is of tertiary formation, with a depth of 200-250 m, and extends over 550 km2 with a water flow of up to 300 l/s. The recharge area of these aquifers is in the valley of Guatemala, with the exception of part of Lake Amatitlan, as well as other areas covered by urbanization.
Nicaragua The Managua aquifer is located in the western central area of the country and extends for approximately 600 km2; it has a saturated thickness that ranges from 200 to more than 450 m. There are approximately 160 excavated central wells and a total of 663 perforated wells with different uses, among which are domestic, municipal, industrial, agribusiness and irrigation. The depth of the perforated wells ranges between approximately 42 and 500 m. The average water flow from the wells is 3170 m3/day, with a production capacity that ranges between 470 and 8500 m3/day. In 1996, the production of water was 131.4 x 10 m3/year. Nearly 1.5 million people were supplied from the groundwater in this aquifer, via the Managua aqueducts. According to a 1993 JICA/INAA study, west of the Managua aquifer there is large-scale irrigation for the main crops of maize, sorghum and beans. Central-pivot irrigation covers an area of approximately 247 ha. During 1993, the irrigated area was about 170 ha, and there was a total of 1700 h of pumping. There is an annual discharge of approximately 1.24 million cubic metres. The quantity of water required is approximately equal to the water consumed; when the planted area is 150 ha, consumption is 1275 million m3, which is similar to the amount of water extracted annually.
Honduras In Tegucigalpa, the capital of Honduras, there are more than 500 perforated wells, which yield 1-3 l/s; there are zones in the volcanic ash that have low yields, although some have greater yields (2-20 l/s).
El Salvador In the department of San Salvador, a region of economic importance to the country, there is a group of aquifers that form a very complex water system due to the emergence of springs and of connections between surface and groundwater flows. To the west of the San Salvador aquifers, water for agricultural activity is extracted from wells located in the Zapotitan Valley, which is dedicated to agriculture. To the east is the San Salvador aquifer, which coincides with the metropolitan area of the capital city, and covers
Country Characteristics an area of approximately 185 km2, with an annual yield of 42 x 106 m3/year (1.35 m3/s). The rest of the aquifers of this zone are mainly in the coastal plains or along the coastline. In general, they are very limited aquifers because of their proximity to the coast and because of the influence of salt water.
Approximately 227 million cubic metres of groundwater is extracted annually and 80% of the potable water supply comes from groundwater. The reliable yield of the existing groundwater deposits in the country are estimated at 83 m3/s.
The largest basins at the highest elevation with potential groundwater, in order of importance, are the Lempa and Jalponga rivers combined, Grande de San Miguel and Paz. However, the potential is not uniformly distributed. The aquifers in El Salvador were formed according to structural zones: the northern Sierras, the central depression, and the mountains and plains of the Pacific coast are formed by impermeable rocks. There are coastal aquifer formations with depths of more than 150 m and with average water flows of 16 l/s. The Santa Ana aquifer in the western zone has a high potential for exploitation, with a flow of 3.5 x 106 m3/year/km. Costa Rica The characteristics of high permeability in the layers of fragmented and igneous lava, combined with high rainfall, favoured the formation of highly potential aquifers in the central and northern part of Costa Rica's Central Valley, where more than half of the population lives. These aquifers are called the Upper and Lower Colima and are separated by a low permeability layer that acts as an aquitard, which allows the descending and ascending vertical transfer of water.
It has been estimated that the Lower Colima extends for approximately 230 km2 and that the Upper Colima spreads over approximately 170 km2. The maximum thickness is about 300 m. The outcropping of this lava is limited to the river canyons in the lower part of the valley. According to SENARA/BGS (1989), the Upper Colima aquifer recharges from the Barva aquifer through the tuffs of the unit known as the Tiribi formation and from the La Libertad aquifer by vertical percolation. The Upper Colima also receives a large part of its recharge from rain infiltration in those areas where there are no overlying layers. The Lower Colima is recharged from the Upper Colima by vertical percolation through the tuffs and ignimbrites of Puente de Mulas, or from the surface where the Upper Colima is absent. The average recharge in the aquifer system has been calculated at 8200 l/s (TAHAL, 1990).
The flows extracted from the wells that collect from both aquifers are 50-120 l/s (SENARA/BGS, 1989). The depth of the water table level varies, depending on the surface topographical irregularities; but, in general terms, it ranges between 50 and 100 m. The direction of the underground flow is from north-east to south-west in both aquifers. Panama The hydrological characteristics of the geological formations in Panama are little known because of the lack of systematic studies. It is known that production from wells is generally acceptable. The majority of the aquifers that are exploited are of a type of fissure flow in volcanic rocks and of sedimentary and fissured conglomerates. The depth of the majority of the wells ranges from 20 to 110 m and production is 2-15 l/s.
groundwater at rates that exceed the capacity of the natural cycle to recharge the aquifers. Aquifers under virtually all of the metropolitan areas in Central America show signs of overexploitation. At the same time, urban expansion is covering the surfaces from which the aquifers would naturally be recharged. So while demands on the aquifers rise, their supply falls. The case of the aquifers in Guatemala, Managua in Nicaragua, and San Pedro Sula in Honduras exemplify this problem.
In Guatemala, a continuous decline of groundwater has been identified in the southern basin of the valley of Guatemala, as well as in the metropolitan area. In the case of El Salvador, the urbanized surface of the metropolitan area has increased almost exponentially, from 6.8 km2 in 1935 to the current 91.5 km2, and this has mainly taken place in the largest aquifer recharge areas. Because of this, the areas with the highest rate of infiltration have been reduced, whereas the areas with an infiltration rate of 0.05 (the rate assigned to areas of low impermeability) have increased by the same proportion. The same thing has happened in San Pedro Sula, Honduras.
The demographic projections of the United Nations Development Program (UNDP) indicate that by 2010 more than 60% of the population in all countries in the region with the exception of Guatemala will be concentrated in urban centres. All of these, with the exception of Honduras, are located within the Pacific region.
As mentioned above, groundwater in Central America is extremely vulnerable to pollution because the aquifers are relatively superficial and are covered by fractured or permeable materials. In areas of high precipitation, the infiltration of polluting agents potentially toxic to human health can be from 30% to 50% (Reynolds, 1992). The main sources of groundwater pollution are agricultural and industrial activities, along with domestic runoff.
A study conducted by the Food and Agricultural Organization (FAO) of the problems of pollution in 16 Latin American countries determined that in urban areas the main sources of polluting agents from agricultural activities are fertilizers, pesticides and food-processing industries, whereas in rural areas the contaminants are associated with pesticides and fertilizers of chemical origin. Intensive agriculture is one of the main sources of income in Central America. Great volumes of water are used as farmers seek the highest possible levels of performance, thus forcing irrational use in the dry season and uncontrolled use of pesticides and herbicides, which in turn lead to situations of risk.
A total of 10.1 million tonnes of chemical fertilizers were used in the region from 1980 to 2000. Interestingly, as shown in Fig. 6.2, the use of chemical fertilizers in Central America has revealed quite irregular tendencies, with use increasing by 20% in some years (1983 and 1997) and falling markedly in others (1982 and 1989). This use of agrochemicals and fertilizers in the region has contaminated some important aquifers.
Pollution caused from mercury and phosphates has been observed in Guatemala. In El Salvador, rivers and streams in the principal agricultural areas are highly polluted by pesticides, particularly by DDT in cotton cultivations in the south-eastern coastal plains. Concentrations of 3.15 mg of DDT per litre of water have been discovered in the Río Grande de San Miguel, which is triple the
lethal limit for fish. Toxaphene (non-biodegradable) pollution has been detected in Nicaragua in concentrations that exceed acceptable standards (Silvel et a/., 1997; CIEUA, 1998; Aquastat, 2001). In the case of Nicaragua, this problem is of great concern for the western aquifers (Leon-Chinandega) and the valley of Sebaco, as is the case in Guatemala for the sugarcane and banana plantations in the Pacific and Caribbean coastal regions (Choza, 2002).
Pesticides such as toxaphene were detected in the Siguatepeque aquifer in Honduras and in the Leon-Chinandega aquifer in Nicaragua, although in smaller concentrations to those permitted by the World Health Organization (WHO). There was intensive cultivation of cotton on top of these aquifers during the 1960s and 1970s. Likewise, insecticides such as clorado, carbofuran and 2,4-D, which are used for agricultural production, have been detected in rural areas of Honduras, although in concentrations that are lower than those permitted by the WHO.
In La Libertad spring in Costa Rica, on three occasions, concentrations of nitrate that exceed the norm of 45 mg/l NO- have been detected, while a slight increase in the nitrate concentration of 2 mg/l was observed between 1986 and 1997. This indicates that in less than one decade the maximum permissible concentration would have been reached.
Nitrates have been detected in El Salvador, Nicaragua and Costa Rica. In the north-eastern sector of Managua's aquifer, high concentrations of almost 45 mg/l NO- were reported, probably because of the use of nitrogenous fertilizers (Hetch, 1989).
The non-agricultural causes of contamination in urban areas are domestic runoff and industrial waste. For example, there is substantial data on faecal pollution in the aquifers located in urban zones. According to monitoring by the National Administration of Aqueducts and Sewers (ANDA) of the public wells in El Salvador, of 183 samples that were taken during a 10-year period from 31 wells, it was found that 10% of the wells showed high rates of coliform, which exceeded the permitted level. Likewise, there are zones in the metropolitan area where the percentage of wells contaminated by coliforms in excess of permitted levels is more than 50%; this is where there has been indiscriminate land use for both residential and industrial purposes.
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