The role of permeability distribution on nitrate fate and transport, in different scale experiments under saturated conditions

A range of laboratory and field experiments has been performed to quantify NO3- clean up times in sandy loamy sediments representative of the Po Plain lowlands (northern Italy), an intensively cultivated area affected by serious nitrate pollution for decades. Column and tank experiments were performed to elucidate the relevant transport and redox processes affecting nitrogen dissolved species, in order to study the same processes in controlled 1-D and 3-D environments. In addition, a grid of piezometers were installed and monitored in a former agricultural field, to quantify the presence and degradation of nitrogen dissolved species in the shallow unconfined aquifer. NO3- was found to be persistent in column, tank and field experiments, due to limited availability of organic substrates needed for denitrification processes. The comparison between laboratory transport experiments and the field site monitoring highlighted the role of permeability distribution in defining clean up times, with silt and clay lenses that hamper groundwater flux and provide a continuous source of nitrogen via diffusion. This study confirms that laboratory experiments can give a good representation of the redox processes undergone by nitrogen species in saturated conditions, but despite of this, laboratory experiments should not only account for 1-D flow and transport processes, but also for 3-D flow field.