Hydrogeochemical controls on shallow alluvial groundwater under agricultural land: case study in central Portugal

A hydrogeochemical study employing graphical, multivariate statistical, and modeling tools was conducted in an area of alluvial deposits in the Mondego river basin, to determine the factors and processes controlling the shallow groundwater chemistry. Groundwater was collected from 29 observation wells in six sampling campaigns, between March 2001 and September 2002. Samples were analyzed for basic physicochemical parameters, major ions and some minor ions. Hierarchical cluster analysis was applied to the data set, including both the spatial and temporal monitoring data and resulted in the definition of eight distinct water types. With the aid of hydrochemical (and physicochemical) scatter plots, it was possible to identify the main processes controlling the groundwater chemistry: (1) evapotranspiration and recharge; (2) calcium and magnesium carbonate and CO₂ dissolution; (3) nitrate leaching from agriculture; (4) oxidation and reduction; and (5) cation exchange. These processes are frequently common to more than one water type but unique in combination and/or extent, in space and/or time. Geochemical modeling of the water types (using PHREEQC) allowed the validation and, to a certain extent, quantification of the processes that affect the shallow groundwater evolution. These tools can provide an essential support for the assessment of groundwater vulnerability to contamination and for the elaboration of groundwater resource management strategies.