A study published in the latest issue of Environmental Geology now confirms that the major source of contamination in the upper part of the aquifer is of agricultural origin. Scientists forecast that, over time, the most mobile pollutants will reach the deepest parts of the aquifer, affecting one of the most important wetland areas in Europe.
The Doñana National Park, which covers 760 km2 and has been awarded UNESCO World Heritage status, could be at risk from progressive ecological damage over coming decades. Manuel Olías Álvarez, from the Department of Geodynamics and Palaeontology at the University of Huelva, and lead author of a new study on agricultural pollution, told SINC that although the movement of subsurface water is very slow, “pollutants such as nitrates will reach the park area” and that, over time, these will affect the deepest parts of the aquifer.
A team of scientists from the universities of Huelva and Cadiz reached this conclusion after studying variations in the quality of subsurface water in the Almonte-Marismas aquifer in Doñana. The hydrogeochemical features of the water (temperature, pH, electrical conductivity, dissolved oxygen and oxidation / reduction potential) show high nitrate and sulphate levels at the aquifer’s surface.
In seeking possible anthropogenical (caused by human action) factors influencing the quality of subsurface water, the scientists found that the areas of the aquifer where intensive agriculture is carried out contained the highest concentrations of pollutants. “The water table is very close to the surface and the soil is very sandy and permeable, with very little capacity to retain pollutants, which makes the aquifer very vulnerable,” Olías told SINC.
All the heavy metals found in the subsurface water of the aquifer at above-average concentrations were linked to agricultural pollution. Metals present, such as aluminium, cobalt, chrome and nickel, for example, which had a similar distribution pattern to the sulphates and nitrates, came from fertilisers or pesticides used in traditional agriculture.
The pollutants have been found in the upper part of the aquifer in agricultural areas, but “the subsurface water flows towards deeper areas and the interior of the Doñana National Park (except in the coastal area, where the water flows towards the sea),” the researcher pointed out. The quality of the water in the Almontes-Marismas aquifer is affected by lithological factors, the amount of time it has been present in the aquifer, and agricultural and urban pollution. The sulphates and nitrates, and other trace elements such as bromides, are stratified within the aquifer.
In order to avoid these pollutants increasing still further, the researcher suggests the “only solution” being a change to an organic agricultural model that does not use fertilisers or pesticides, since it is “almost impossible for intensive agriculture not to pollute the aquifer”. The study says that sustainable agriculture is increasingly being carried out in order to limit the concentration of metals, but that “greater efforts are needed to control subsurface water pollution”.
Subsurface water started to be exploited “intensively” for agriculture in the areas surrounding Doñana and to supply the tourist resort of Matalascañas from the 1970s onwards. The research now published highlights numerous other studies that show the alarming impact on the park of the withdrawal of subsurface water. “We are already seeing a large range of changes taking place in some ecosystems near the areas where subsurface water is being most intensively removed,” say the researchers.
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