A Case of Hydrochemical Characterization of an Alluvial Aquifer Influenced by Human Activities

21 physico-chemical parameters have been determined in 47 groundwater samples collected in October 1994 from the alluvial aquifer of the Pisuerga river, located at the north-east of Valladolid (north-western Spain). Conventional hydrochemical methods and multivariate statistical analysis have been used to discriminate natural phenomena and pollution processes in the area. It has been demonstrated the existence of a heterogeneity in groundwater mineral contents related to anthropic actions such as irrigation, source of irrigation water and location of polluting sources. This study demonstrates the existence of two sub-units in the alluvial aquifer (left and right river banks) with different hydrochemical characteristics, and identifies three different singular areas in the left bank. Groundwater from this bank showed higher levels of dissolved ions caused by concentration processes derived from the predominant use of groundwater for crop irrigation, whilst in the right bank of the aquifer less saline water from canals is used. Degradation of groundwater quality has been observed in some areas of the left aquifer sub-unit (north, Santovenia and south-central sites) where anomalous high concentrations of sulphate, sodium, chloride and nitrate were measured, and can be related to the presence of potentially polluting sources such as leachates from industrial waste disposals, sewage effluents and use of organic and inorganic fertilizers.