Simulation of groundwater contamination in the Adra river delta and eutrophication of two lagoons (Almeria, Spain)
The Adra delta is located on the eastern limit of Almeria in South-East Spain. Entirely devoted to agriculture for many years, greenhouse crops now cover the entire delta. The intensive use of fertilizers and the recycling of irrigation water have resulted in an alarming increase in nitrate concentrations in the groundwater of the eastern part of the delta. Two wetlands located in this sector are protected as natural reserves and are included in the Ramsar Convention. These wetlands are discharge zones for the aquifer and have shown high nitrate concentrations for some years. The objective of this work was to study the effect of irrigation water recycling on nitrate contamination of groundwater and to highlight the effect of this contamination on the wetlands. A model simulating nitrogen transfer in the unsaturated soil was used to simulate nitrate concentrations in groundwater recharge. Considering the dilution of nitrate into the aquifer and the presence of denitrification processes in the eastern sector, measured nitrate concentrations in the aquifer were adequately reproduced, thus confirming the contribution of groundwater recycling through irrigation in aquifer contamination. Simulation of a hypothetical stop of irrigation water recycling showed that a period of ten years would be necessary for nitrate concentrations to reach levels measured before the start of greenhouse agriculture. Wetland efficiency in attenuating groundwater nitrate contamination has been stable at 92 to 94% from 1991 to 1997. Nitrate concentrations are nevertheless very high in the wetlands, threatening them with eutrophication.