Modeling Nitrogen Fate and Transport at the Sediment-Water Interface

Abstract

Diffusion is an important fate and transport mechanism which regulates the distribution of dissolved and airborne pollutants in the environment. Of particular interest is diffusive mass transfer of pollutants across sediment- and soil-water interfaces, a problem which is relevant to a wide spectrum of environmental fate and transport problems, such as lake eutrophication and nitrogen cycling in wetlands. In this chapter, we present analytical and numerical models describing nitrogen cycling in two distinct freshwater ecosystems, bed sediments in eutrophic lakes and wetlands. The primary focus is on the formation of a dynamic, relatively thin aerobic layer at the sediment/soil interface, and diffusive transfer and geochemical transformation in sediment/soil beds. Applicability of the two models is demonstrated by comparison against observational data obtained from the Chesapeake Bay bed sediments and a restored treatment wetland. The generalized sensitivity analysis (GSA) method is implemented to identify most sensitive parameters in each model, and the Monte Carlo method (MC) is applied to quantify model predictive uncertainty. Despite the complexity of processes and uncertainty underlying the responses of the two aquatic ecosystems, comparison with observations shows both models performed well using the MC method. In both the cases, diffusive mass transfer controls pore-water nitrogen concentration and sediment/soil flux, and affects nitrogen mass retained or lost by denitrification.