A study on solute NO3N transport in the hydrologic response by an MRF model

Abstract

Abstract The results of field studies on solute transport over basin-scale distances are analyzed using the Mass-Response Function (MRF) model and its generalization proposed in this paper. The generalization proposed concerns reversible production/removal processes of solute. It is assumed that equilibrium concentrations in the mobile phase are proportional to the instantaneous fraction of solute mass sorbed in the fixed phase. Such assumption, which is thought of as representative of large-scale transport volumes, is derived from the theory of two-component convection-dispersion in soils and is aimed at endowing the model with predictive characters. It is seen that the use of generalized MRF's poses serious computational problems, which are partly related to the conditional behavior of the instantaneous response of solutes on the impulse time. The conditionality of the transfer functions on the injection time is related in a rational manner to the combined effects of convection-dispersion during the hydrological cycle and of sorption processes that occur between fixed and mobile phases. The relevance of the problem addressed lies in the ability of solute generation and movement over large scales to predict the dominant modes of the phenomena on the basis of the knowledge of parameters with a clear physical significance. In this paper, this ability is tested with reference to suitable field experiments. The results show that a number of characteristic residence times are the most important properties for basin-scale transport processes and for the evolution of resident and flux solute concentrations. An MRF model of solute NO 3 N concentrations in river waters is constructed, the architecture of which is tailored to solute generation and transport processes occurring in a gauged watershed. The theoretical results are compared with the experimental observations and are found to agree with them well. It is argued that MRF's are rational models of the complex chain of events occurring in large-sclae solute sorption and transport, and may be validly employed for quantitative studies of environmental impacts due to the release of chemical species in surface or subsurface waters.