A multidisciplinary study was carried out, within the "Estudio Para La Proteccion Ambiental Del Acuifero Del Valle De Sula Y De Las Fuentes De Agua De La Reserva Forestal Del Merendon'' plan, focused on the appraisal and management of the water resources in San Pedro Sula area (Honduras). This paper deals with the hydrochemical study which is of great significance for the comprehension of the hydrogeological system. The aim is to highlight the relationship between the rock composition of the ground water recharge basin and the quality of the alluvial terrain aquifer. The aquifer is contained between alluvial sediments of the San Pedro Sula plain; these sediments, in the areas in contact with the Merendon mountain chain, comprise mainly alluvial fans generated by material transported by the rivers; the apical area of these fans is characterised by high permeability and is the site of a free monostrate aquifer. Moving towards the more distal fan areas, depressed areas of fine sedimentation (silt and clay) appear; the permeability of the sediments is very low and the aquifer divides into a more superficial one and a deeper one; the latter was found to be under slight pressure. Regarding the ground water recharge basin, represented by the Merendon mountain chain, this includes rocks in various degrees of metamorphism, intrusive rocks, anatectic products and hydrothermal products characterised by permeability through fracturing. The rocks of the ground water recharge basin and the scour of the pedogenetic layer have determined the present water quality and the soils and palaeosoils composition of the later alluvial sediment deposition. Bicarbonate-alkaline waters characterised by a specific electric conductivity <100 μS cm-1 are linked to intrusive rocks and anatectic products of the Zapot al river. These waters have a short underground watercourse. Calcium bicarbonate waters with specific electric conductivity >1000 μS cm-1, observed in Aldea El Carmen area, are connected to the dissolution of secondary calcite. This calcite is considered being the product of precipitation from carbonatic schists involved in granitic pluton intrusions. Deep alkaline-bicarbonate waters, warm in some cases, have a pH of 10. Sulphate-chlorinealkaline waters with pH around 4, occurs in the Manchaguala river basin, and are linked to sulphates produced by oxidation of metallic intrusulphurs formed at the margin of the pluton by hydrothermal processes.
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