Nitrate attenuation in agricultural catchments: Shifting balances between transport and reaction

Simultaneous observations, across multiple spatial and temporal scales, are needed to understand the complex interactions between hydrological and biogeochemical processes in catchments and the primary controls on NO3(-) availability and mobility. This paper addresses these issues by using data collected from a detailed field experiment, carried out on two topographically different hillslopes (one steep and the other flat) located within an agricultural catchment in Western Australia. Continuous hydrometric data and measurements of chemical tracers, geochemical parameters, and NO3(-) concentrations taken from the shallow perched aquifer across riparian, midslope, and upland locations were analyzed and interpreted through a simple process-based numerical model of transport and reaction. The NO3(-) concentration data indicated that the temporal and spatial patterns of NO3(-) concentrations within the hillslopes are linked to the state of hydrological connectivity of the three landscape units as the shallow perched aquifer developed during the winter. Significant NO3(-) attenuation occurs within the riparian zones after the transport of NO3(-) from midslope sources begins. Application of a mixing model, which partitions the riparian zones into three water source components, and the numerical model of NO3(-) transport and reaction indicates that different mechanisms, dilution in the steep hillslope and denitrification in the flat hillslope, are responsible for much of the observed NO3(-) attenuation in the riparian zones. In this way, this work highlights the importance of hillslope topography in determining the relative roles of transport and reaction in NO3(-) attenuation and export from riparian zones. The experimental results also supported the use of the Damkohler number, a simple dimensionless number that is a measure of the competition between transport and reaction processes, which allowed a favorable comparison of our findings with previous results published in the literature for different geographical settings.