Monitoring and Modeling Nitrate Persistence in a Shallow Aquifer

A modeling study on fertilizer by-products fate and transport was performed in an unconfined shallow aquifer equipped with a grid of 13 piezometers. The field site was located in a former agricultural field overlying a river paleochannel near Ferrara (Northern Italy), cultivated with cereals rotation until 2004 and then converted to park. Piezometers were installed in June 2007 and were monitored until June 2009 via pressure transducer data loggers to evaluate the temporal and spatial variation of groundwater heads, while an onsite meteorological station provided data for recharge rate calculations via unsaturated zone modeling. The groundwater composition in June 2007 exhibited elevated nitrate (NO3 −) and chloride (Cl−) concentrations due to fertilizer leaching from the top soil. The spatial distribution of NO3 − and Cl− was heterogeneous and the concentration decreased during the monitoring period, with NO3 − attenuation (below 10 mg/l) after 650 days. A transient groundwater flow and contaminant transport model was calibrated versus observed heads and NO3 − and Cl− concentrations. Cl− was used as environmental tracer to quantify groundwater flow velocity and it was simulated as a conservative species. NO3 − was treated as a reactive species and denitrification was simulated with a first order degradation rate constant. Model calibration gave a low denitrification rate (2.5e−3 mg-NO3 −/l/d) likely because of prevailing oxic conditions and low concentration of dissolved organic carbon. Scenario modeling was implemented with steady state and variable flow time discretization to identify the mechanism of NO3 − attenuation. It was shown that transient piezometric conditions did not exert a strong control on NO3 − clean up time, while transient recharge rate did, because it is the main source of unpolluted water in the domain.