Groundwater nitrogen speciation in intensively cultivated lowland areas

Abstract

The study was conducted in Ferrara Province (Italy), a lowland area covering 2636 km², located in the southern part of the Po River Delta. It is an intensively cultivated area, with more than 50% of land cultivated with winter cereals (32.11%) and maize (22.63%). The main nitrogen fertilizer used in this area is synthetic urea which is suspected to cause nitrate leaching towards shallow groundwater. A network of 56 piezometers, homogeneously distributed throughout the whole area, was installed in order to monitor both water table fluctuations and nitrogen species distributions in the shallow aquifer, over time. Data collected at the end of November 2010 were used to obtain maps of water table, urea (CO(NH₂)₂), ammonium (NH₄ ⁺), nitrate (NO₃ ^-) and nitrite (NO₂ ^-) distributions. Maps show an accumulation of NH₄ ⁺ overlapping a stagnant zone, where drained peaty soils are present. The peaty soils are characterized by a pH ranging between moderately acid and slightly acid, and by high values of organic matter content. Along the drainage line induced by peaty soils dewatering, the flow velocity is very low or almost motionless, determining anaerobic conditions. Instead, the largest accumulation NO₃ ^- is observed in the Eastern part of the province, where the groundwater head gradient is higher and soils are characterized by values of pH that range between 8.1 and 8.3, providing the best conditions for nitrification processes.