Variable residence time–based model for solute transport in streams

Abstract

A variable residence time (VART) model is developed for longitudinal dispersion and transport of solutes in natural streams. The VART model is based on a “double‐layer” conceptual model of transient storage in streams. The upper layer is an advection‐dominated transient storage zone which includes instream and shallow hyporheic storage. The lower layer is an effective diffusion‐dominated storage zone that is deeper in the streambed and farther beneath the banks. The VART model is characterized by the following features. First, a varying residence time is used to simulate the transient storage process. Second, no user‐specified residence time distribution (RTD) functions are required. Third, there are only four parameters to estimate. Fourth, the VART model is also able to accommodate stream water/solute gains or losses in the reach caused by groundwater exchanges. Fifth, the simple VART model is able to produce various compound solute concentration breakthrough curves (BTCs) commonly observed in streams. The early portion of BTCs follows exponential RTDs. The late portion of BTCs can essentially be any distribution. Sixth, performance of the VART model in reproducing various RTDs is comparable to other widely used 1‐D solute transport models that commonly utilize five or more fitting parameters and are limited to the simulation of user‐specified RTDs or underlying RTDs within them. Applications in hydrologically and geomorphically varied streams have indicated that the VART model is a powerful and flexible numerical tool for simulating solute transport in natural streams.