The role of physical and biological processes in aquifers and their importance on groundwater vulnerability to nitrate pollution

Currently, diffuse pollution by nitrate is considered one of the main causes of groundwater quality deterioration. Many methodologies have been suggested to map aquifer vulnerability to nitrate pollution; however, no standard method has yet been established. Many proposed methods only take into account the aquifer's protection and do not consider the physical or chemical processes in groundwater. More specifically, recent studies indicate that the fate and concentration of nitrates in aquifers mainly depend on the following: (a) the efficiency of natural attenuation (e.g. biological denitrification), (b) the physical processes of attenuation (e.g. dilution). Both the processes can lower the nitrate levels in groundwater. The aim of this research is to describe the features and role of these processes in aquifers. According to previous studies and our results, dilution and denitrification occurring in aquifers have a key role in the attenuation of nitrate. More specifically, the occurrence and effectiveness of denitrification were evaluated in a plain area in NW Italy through nitrogen isotopic fractionation analysis; the volumetric flow rate per unit width perpendicular to the flow direction, which is a function of the hydraulic gradient, hydraulic conductivity and aquifer thickness, was instead used for the assessment of nitrate attenuation capacity by means of the dilution process. Knowledge and understanding of these processes is essential to the identification of areas where groundwater is more prone to contamination. Thus, the evaluation of groundwater vulnerability to nitrate contamination cannot be separated from the comprehension of these processes.