Aspectos hidrogeológicos de los humedales Ramsar de alta montaña en España

Abstract

En la última década España ha aumentado la Lista Ramsar de humedales con algo más de una veintena de nuevas zonas, tres de las cuales constituyen las primeras lagunas de origen glaciar en España: El Parque Nacional de Aigüestortes i Estany de Sant Maurici, los Humedales de la Sierra de Urbión y los Humedales del Macizo de Peñalara. En el año 2012 un equipo del IGME inició el estudio de estos tres humedales con el objetivo de caracterizarlos hidrogeológicamente. Para ello se procedió a analizar las características físicas, geológicas, hidrogeológicas e hidroquímicas de cada uno de estos humedales, primero a partir de referencias existentes en la literatura científica y después, mediante una campaña de campo realizada en julio de 2012. Los resultados ponen de manifiesto que, en contra de lo que se pensaba, existen indicios de una posible dependencia de los tres humedales de las aguas subterráneas = The number of wetlands declared to be of international importance has increased by 24 in Spain in the last decade. Spain currently has 74 wetlands included in the Ramsar list. Among these are the first examples of high altitude wetlands (that is to say, higher than 1500 meters) and of glacial origin: Aigüestortes i Estany de Sant Maurici, Humedales de la Sierra de Urbión and Humedales del Macizo de Peñalara. The first two are to be found in the Ebro Hydrographic Basin and the third in that of the Tajo. The paper analyses the physical, geological, hydrogeological and hydrochemical characteristics of the three selected wetlands in each of the said sites: Estany de Sant Maurici, Laguna de Urbión and Laguna Grande de Peñalara. A literature study has been carried out supplemented by a water sample campaign in July 2012. The main contribution of this study is the examination of various hydrogeological aspects of these wetlands, which have not been examined in the past, possibly because it was thought that they had no relationship with groundwater. The three wetlands have clearly differentiated hydrogeological features. Only one of the three, Estany de Sant Maurici, is located within the area of a groundwater body, known as the Macizo Axial Pirenaico, made up of a fractured and compartmented carbonated system on which rest moraine deposits from the bottom of the valley which makes up the lake’s basin. However, this does not mean that there may not exist small local aquifers in areas classified as impermeable or of low permeability. This may be the case of the Laguna de Urbión, which is over a glacial moraine which rests on Cretaceous sandstones and marls, as well as the Laguna Grande de Peñalara in which the fractures and cracks of the orthogneisses on which it rests, along with the development of the processes of meteorization on their surfaces, may encourage the development of local aquifers responsible for the storing of melt water in the summer months and the maintenance of a base flow. These facts, coupled with the low fluctuation of the water level in wetlands throughout the year, and the existence of permanent outfalls from them, are good reasons to suppose that there may be a certain share of groundwater in the functioning of these wetlands. To confirm this hypothesis some detailed studies would need to be made to record the continuous fluctuation of the water level and the evolution of the physic-chemical characteristics of the water column of each of them across the seasons. Piezometers would also have to be installed in appropriate locations in each wetland to identify vertical flows and establish the corresponding hydrological balances.