Groundwater Quality Monitoring in a Coastal Mediterranean Aquifer Affected by Agricultural Contamination and Seawater Intrusion- Extrusion Processes (Vélez River, Andalusia, Spain)

Water quality conditions have been assessed in the Río Vélez coastal aquifer (Andalusia, South Spain: an area of important agricultural and touristic development). This shallow fluvio-deltaic aquifer consists of coarse detrital deposits, of Quaternary age, that extend over about 20 km, being subdivided in two sectors –fluvial and deltaicby a substratum elevation. The deltaic sector is highly vulnerable to seawater intrusion, but during periods of severe drought the substratum elevation prevents the advance of the salt water to the fluvial sector. Permeable and sandy deposits -that generally underlie the cropping areas of the inland sectorprovide favorable conditions for vertical leaching of nitrate to the aquifer. Nitrate concentrations exceed the maximum contaminant level (MCL) for drinking water of 50 mg/l established by the European Community in almost all the wells sampled in the aquifer, with an average value of 85 mg/l and maximum up to 400 mg/l. In addition, the construction in the 1990s of a dam upstream has resulted in a decrease in aquifer recharge and in a significant advance of salt water intrusion during the 1991-1995 period. The 1996 to 1998 abnormally humid period has led to an effective flushing of the deltaic aquifer. At present, the main objective of hydrological policy is to use the reservoir to supply water for both irrigation and human consumption, and to stop pumping from the aquifer to prevent new episodes of marine intrusion. In addition, a careful management of agricultural practices is needed in order to improve water quality in the aquifer, which is considered as a freshwater reserve. INTRODUCTION The Río Vélez fluvio-deltaic aquifer is situated in the Mediterranean coast of Andalusia, S. Spain, some 40 km east of Málaga, and extends over the lowest sector of a 610 km watershed (figure 1). The economy of this region is based on tourism and a profitable agriculture specialized in subtropical (mainly avocado trees) and outof-season crops. Two main consequences of the semi-arid Mediterranean climate of the area have to be emphasized due to their influence on groundwater quality: a) the seasonal scarcity of surface water resources when the water requirements are highest (normally from June to September), and b) the irregular year-to-year distribution of rainfall and runoff generation. Both circumstances –which are graphically shown in figure 2recommend the regulation of water resources. The first approach to fit the water demand with its availability was based on pumping the aquifer. This specially happened during the last 70s and 80s decades, and was made without appropriate control. At the same time, an alternative scheme based