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Trifinio Aquifer

Trifinio Aquifer

The technical activities of this pilot study were executed by a local tri-national team coordinated by the UICN (Unión Internacional para la Conservación de la Naturaleza) that was the local implementing partner of UNESCO. A close cooperation was set with the Plan Trifinio1. Apart from compiling and analysing existing information (mainly limited to climate, geology and hydrogeology), the focal components of the pilot study were a geophysical survey (subcontracted to the company Geofísica Aplicada), a study of gender issues (in cooperation with municipalities) and the development of a geo-referenced database (supported by IGRAC). In addition, initial advocacies for a multi-actor consultation platform for sustainable groundwater management were made at the occasion of meetings with local authorities and personnel of the Plan Trifinio.

Location, delineation and aquifer type

What originally was called “Trifinio Transboundary Aquifer” – named after the zone where the three countries Guatemala, Honduras and El Salvador meet – is in fact composed of two relatively productive and laterally disjunct aquifer units located in the valley floor of the sub-basin of the Upper Lempa river (Río Lempa). As will be shown below, their delineation was enabled by GGRETA’s geological and geophysical surveys. The aquifer system is considered to be representative for the groundwater setting in small alluvial valleys in mountainous areas around the globe, characterised by strong links 1 Plan Trifinio refers to the tri-national development plan for the area and the institution ensuring its execution between groundwater and surface water, in this specific case the transboundary Río Lempa.

General features of the Upper Lempa area

Topographic map of the Upper Lempa sub-basin Topographic map of the Upper Lempa sub-basin

As the topographic map shows, the Upper Lempa sub-basin is a mountainous area and includes territories of the three countries mentioned before. The sub-basin covers an area of 966 km2 and surface elevations vary between 720 and 2,720 metres above mean sea level. The relatively flat and wide river valleys are called Valle de Esquipulas (Guatemala), Valle de Nueva Ocotepeque (Honduras) and Valle de Citalá (El Salvador). Geology is characterised by volcanic rocks – mostly of Tertiary age – outcropping over most of the area and covered by Quaternary alluvial sediments (gravel, sand, clay) in the main river valleys. Climate varies from temperate at the higher elevations to semi-arid tropical in the river valley zone, with mean annual temperatures of 23 to 25 °C and average annual rainfall of 1,200 to 2,000 mm depending on the location in the sub-basin. Most of the rainfall is concentrated during the period May-October. Recent demographic data have not been collected, but the total population estimated in 2007 was around 100,000.

Water and water use in the area

The amounts of rainfall and their irregular distribution in time imply a considerable rainfall surplus during wet periods. Water balance calculations carried out by the project team suggest that in an average year this surplus adds up to values in the order of 300 mm (Valle de Nueva Ocotepeque), 700 mm (Valle de Esquipulas) and even more than 1,200 mm in elevated mountainous zones (Montañas del Macizo de Montecristo). This very significant surplus feeds the rivers in the area – Río Lempa and its tributaries – and recharges the groundwater reservoirs (aquifers). On the other hand, rainfall deficits occur during the dry part of the year and accumulate beyond the moisture storage capacity of the soils, resulting in net irrigation water requirements of 635 to 835 mm/year in the Valles de Esquipulas, Nueva Ocotepeque and Citalá. The pilot study did not assess the regime of Río Lempa, nor did it define how much surface water and groundwater is supplied for domestic, agricultural and industrial uses in the area.

The aquifers of the Trifinio area

Map highlighting alluvial aquifer units (blue) Map highlighting alluvial aquifer units (blue)

In this area only the Quaternary alluvial deposits form aquifers of more than local significance and potentially capable to produce transboundary impacts. Analysis of records of boreholes and wells, in combination with the geophysical survey, have improved the knowledge on the geometry and other properties of the aquifers. Field data have revealed, among others, that phreatic conditions prevail (free water-table), with relatively shallow water levels, and that groundwater flows generally towards the river, in downstream direction. In a hydraulic sense, the aquifers are only moderately productive, but they are abundantly recharged due to the considerable rainfall surplus during the rainy season and their favourable location as it comes to intercepting runoff from the surrounding mountainous zones. An important finding of the geophysical survey is that the Quaternary alluvial deposits form two separate spatial units, one in the Valle de Esquipulas and the other one in the Valle de Ocotepeque–Citalá. Consequently, the only transboundary aquifer in the Trifinio area is the latter unit, shared by Honduras and El Salvador only, not by Guatemala. It should not be overlooked, however, that all alluvial aquifer units in the subbasin, also those in Guatemala, are hydraulically connected by Río Lempa.

Gender issues

A proposal has been developed and widely discussed for a strategy of integrating gender issues in planning in the Trifinio region, under the general objectives of recognizing the capacities of women and strengthening the position of women (as regards their roles, rights and involvement in decision-making). Components under this specific project are training, networking with other entities involved in gender and in Integrated Water Resources Management (IWRM) and monitoring. Specific activities carried out are workshops, validation of tools, as well as dissemination of validated tools and training regarding their implementation.

Main groundwater management challenges in the area

Rio Lempa in low-flow conditions Rio Lempa in low-flow conditions

Pollution forms a major water resources management challenge in the area. A diversity of pollution sources is observed, such as domestic solid waste and waste water, chemicals used in agriculture (fertilizers, herbicides and pesticides), wastewater and solid waste from agro-industrial processing, and buried storage tanks of hydrocarbon products. Adequate sanitation is absent in many communities; wastewater and solid waste tend to be dumped untreated and uncontrolled to the environment, in absence of treatment facilities and controlled waste dumps; and many storage tanks are probably leaking. The alluvial aquifers – including the Ocotepeque–Citalá transboundary aquifer – are directly exposed and very vulnerable to pollution. Without effective pollution control, their groundwater is bound to become more and more polluted over time. At first sight, the alluvial aquifer systems do not seem to be threatened by groundwater quantity problems (such as declining water levels and exhaustion), but careful monitoring over a longer period is needed to validate this impression and to anticipate potential groundwater quantity problems in the future. Other major challenges may become apparent after improving groundwater governance in the area. Their early identification may prevent problems from escalating beyond the levels where they still can be controlled.