Large tank experiment on nitrate fate and transport: the role of permeability distribution

A long-term elution experiment to study the saturated transport of pre-accumulated fertilizers by-products was conducted within a large tank (4 x 8 x 1.4 m) equipped with 26 standard piezometers. Sandy sediments (35 m3), used to fill the tank, were excavated from an unconfined alluvial aquifer near Ferrara (Northern Italy); the 14 field site was connected to a pit lake located in a former agricultural field. To evaluate spatial heterogeneity, the tank’s filling material was characterized via slug tests and grain-size distribution analysis. The investigated sediments were characterized by a large spectrum of textures and a heterogeneous hydraulic conductivity (k) field. Initial tank pore water composition exhibited high concentration of nitrate (NO3-) sulfate (SO42-) calcium (Ca2+, and magnesium (Mg2+), due to fertilizer leaching from the top soil in the field site. The initial spatial distribution of NO3- and SO42- was heterogeneous and not related to the finer grain size content (<63 µm). The tank’s material was flushed with purified tap water for 800 days in steady-state conditions; outflowing water was regularly sampled to monitor the migration rate of fertilizer by-products. Complete removal of NO3- and SO42- took 500 and 600 days, respectively. Results emphasized organic substrate availability and spatial heterogeneities as the most important constraints to denitrification and nitrogen removal, which increase the time required to achieve remediation targets. Finally, the obtained clean-up time was compared with a previous column experiment filled with the same sediments.