Detecting small-scale variability of trace elements in a shallow aquifer

Groundwater samples collected from an unconfined shallow aquifer were analysed for major and trace element (TE) concentrations with the aim to investigate small-scale variations possibly linked to fertilizer residual products applied until 2004. The field site, located near Ferrara (Northern Italy), covers an area of 200 m2 and was a former agricultural field then converted into a park and equipped with a grid of 13 monitoring wells. Three monitoring campaigns were carried out in June 2007, March and June 2009 in order to detect spatial and temporal variations in water quality. Groundwater nitrate, chloride, bromide and sulphate concentrations decreased with time indicating that the fertilizer plume was slowly replaced by unpolluted groundwater. However, the groundwater composition showed values of TEs (Fe, Mn, Al, As and Hg) above the recommended international and national guideline values. Dissolved TE concentrations varied randomly in the three campaigns, while TEs in the solid matrix did not show particular enrichment factors induced by fertilizer use. The data indicated that the dominant factor involved in determining small-scale spatial variability of TE concentrations in this shallow aquifer was the sediment-water interaction, while the temporal variation of TEs was driven by the organic matter leaching from the topsoil and by water table oscillations, which in turn drove the groundwater redox status. This study emphasizes the need of small-scale TE spatial resolution to discriminate between anthropogenic non-point sources of pollution (like fertilizers) and background concentrations.