Temporal evolution of groundwater composition in an alluvial aquifer (Pisuerga River, Spain) by principal component analysis

A set of quantitative analytical data from the alluvial aquifer of the Pisuerga river, located at the north-east of Valladolid (Spain), has been processed by multivariate statistical techniques in order to investigate the evolution of the groundwater composition between two surveys. The original matrix consisted of 16 physico-chemical variables, determined in 32 groundwater samples collected in 32 sampling sites of the aquifer. The surveys were carried out in October 1994, at the end of the irrigation and low water period just before autumn rains, and in April–May 1995 at the beginning of the irrigation and high water period and after the recharge of aquifer. The experimental 64×16 matrix was analysed by Principal Component Analysis (PCA), and the resulting Principal Components (PCs) and Varimax rotated PCs (VFs) analysed by means of box and bivariate plots. The exploration of the correlation matrix allowed to uncover strong associations between some variables (sulphate, chloride, hydrogencarbonate, sodium, potassium, magnesium, calcium and electrical conductivity) as well as a lack of association between the others (pH, nitrate, and trace metals). PCA showed the existence of up to five significant PCs which account for 71.39% of the variance. Two of them can be initially assigned to ‘mineralization’ whereas the other PCs are built from variables indicative of pollution. Box and bivariate plots of the two first PCs showed that ‘mineralization’ is maintained through surveys and that trace elements increase from the first to the second survey in all zones, as a possible consequence of soil leaching caused by the rise of the groundwater level. Varimax rotation allowed to ‘break up’ the ‘mineralization’ PC into two VFs, assigned respectively to ‘natural’ mineralization and to ‘saline’ man-made contamination (sodium and chloride). Box and bivariate plots of VFs confirmed the existence of a differential ‘natural’ mineralization pattern, now ascribed exclusively to dissolution of geological materials, as well as the presence of a zone with unusually high levels of ‘saline’ contamination attributed to leakages from sanitary systems. The plot of VFs also allowed to visualise seasonal variations, identifying a process of dilution of the ‘natural’ mineralization during the recharge of the aquifer, and an increase of the ‘saline’ contamination, assigned to a variety of non-natural causes.