Application of isotopic and geochemical tools for the evaluation of nitrogen cycling in an agricultural basin, the Fucino Plain, Central Italy

Abstract

Geochemical and isotope data collected in the agricultural area of Fucino Plain, Central Italy provided information about the relationship between the nitrate cycle and agricultural practices and seasonal changes in hydrology, in particular concerning the interaction between groundwater and surface water. The nitrate cycle of the alluvial aquifer of the Plain has been summarized in a seasonal-dependent conceptual model, where the shallow alluvial aquifer plays a fundamental role contributing to contamination of irrigation channels during periods of no manure application. Based on isotopic fingerprinting, chemical characterization and agricultural practices, the main nitrate source involved in the nitrate cycle in the study area is manure that is applied during the fall and winter periods. A nitrogen contribution from the deep alluvial–lacustrine aquifer was also documented in this study. The isotope data also showed the occurrence of denitrification occurring in nitrate pools in the shallow aquifer, which is characterized by low-permeability layers. The nitrate pool in the shallow aquifer is related to infiltration after the application of manure. The isotope and concentration data showed that the increase of nitrate in the irrigation channels under non-runoff conditions is controlled by seepage from the shallow aquifer. The cycle ends with a new application of manure, generating considerable increases of “fresh” nitrate concentration in the channels. The historical trend of decreasing water table conditions and increasing nitrate content in the shallow aquifer threatens the deep aquifer, used for drinking purposes, since heavy pumping can induce the transport of nitrate from the shallow aquifer toward the capture zone of the deep wells.