An analysis of performance models for free water surface wetlands. 
Although treatment wetlands are intended to attenuate pollutants, reliably predicting their performance remains a challenge because removal processes are often complex, spatially heterogeneous, and incompletely understood.  Although initially popular for characterizing wetland performance, plug flow reactor models are problematic because their parameters exhibit correlation with hydraulic loading.  One-dimensional advective-dispersive-reactive models may also be inadequate when longitudinal dispersion is non-Fickian as a result of pronounced transverse gradients in velocity (preferential flow).  Models that make use of residence time distributions have shown promise in improving wetland performance characterization, however their applicability may be limited by certain inherent assumptions, e.g. that transverse mixing is nil.  A recently-developed bicontinuum (mobile-mobile) model that addresses some of these weaknesses may hold promise for improving wetland performance modeling, however this model has yet to be tested against real-world wetland data.  This paper examines the state of the science of free water surface wetland hydrodynamics and transport modeling, discusses the strengths and weaknesses of various steady state models, and compares them to each other in terms of each model's ability to represent data sets from monitored wetlands.