Compartmental model of nitrate retention in streams

[1] A compartmental modeling approach is presented to route nitrate retention along a cascade of stream reach sections. A process transfer function is used for transient storage equations with first-order reaction terms to represent nitrate uptake in the free stream and denitrification in the storage regions. In the context of a short-term nitrate injection we define nitrate assimilative capacity as 1 A, where the attenuation factor, A, is the fraction of injected nitrate mass that is flushed past the outlet of stream. Net exchange with groundwater is modeled by allowing free stream discharge to vary from one reach section to the next. A Monte Carlo simulation was used to compare results of the compartmental model with the OTIS numerical model. Out of 350 Monte Carlo simulations of a stream consisting of five reach sections the highest relative percent difference was 15%, most being well below 10%, as determined using moment analysis on breakthrough curves. Moment analysis on published experimental breakthrough curves showed assimilative capacities did not differ from those determined with the compartmental model by more than about 0.035 and were well within the uncertainty due to possible errors in measured stream metrics and net exchange with groundwater. The results show that the compartmental modeling approach, commonly used in analysis of groundwater data, can also be useful in evaluating nitrate retention in streams.