Human activities affecting continental aquatic ecosystems include intensive agriculture, specially developed throughout the second half of the 20th century. The excess of nitrogen fertilizers applied to agricultural fields is leached into the aquifers causing pollution by nitrates, which is transported in turn towards the discharge areas, affecting surface waters at rivers, lakes and oceans. In addition, nitrogen undergoes various transformation processes, being denitrification one of the most important as it is the main natural process to remove nitrogen in lakes, rivers and wetlands to the atmosphere. High concentrations of nitrates produce different types of stress in the waters, generating different environmental issues and risks to human health. To improve the qualitative status of these surface and groundwater bodies, it is necessary to develop a detailed study of the source, fate and transformation processes nitrogen is undergoing.
This study and characterization of the nitrogen cycle has been developed in the Campo de Montiel aquifer (Central Spain). It was determined that between 11-34% of the fertilizer annually applied is leached into the aquifer, being considered the responsible mechanism for the high concentrations of nitrate in groundwater, higher than the maximum allowed by the legislation. Nitrate dissolved in groundwater is slowly transported to discharge zones and affects the Ruidera lakes, a protected Natural Park, having been estimated exports of 23% of the applied nitrogen. The source of this contamination by nitrates in water can be analysed from the study of δ15N and δ18O isotopes of the nitrate, since this relationship varies depending on the pollutant source. It was determined that the main source of nitrate in the aquifer of the Campo de Montiel is the use of nitric fertilizers in agricultural areas. In addition, the isotopic values of δ18O are closely related to isotopic fractionation processes by denitrification, contributing to the natural attenuation of nitrate in groundwater.
Key words: isotopes: nitrates; groundwater; nitrogen biogeochemistry cycle; contamination