Contaminant exchange rates in estuaries – New formulae accounting for advection and dispersion

Andutta, Fernando [James Cook University] Ridd, Peter [James Cook University] Deleersnijder, Eric [UCL] Prandle, David [National Oceanography Center (UK)]

The transport timescales of water in an estuary are important measurements for monitoring pollution threats to the estuarine ecosystem. In this study we re-evaluated the application of simple analytical solutions to estimate these timescales and found that the Land Ocean Interaction Costal Zone model (LOICZ) uses similar equation as from the fresh water fraction model, and thus often resulting in shortened transport timescales. Therefore, the LOICZ model is neither based upon the well-known Knudsen relation nor Fischer formulation. Three transport timescales, namely water renewal, residence time and exposure time were calculated using analytical solutions for a range of estuaries worldwide. The analytical results were compared with available estimates of residence times from numerical models. The theoretical formulation from the LOICZ, the fresh water fraction model, and a newly proposed modified LOICZ model were used to calculate water renewal. Residence times and exposure times were calculated using the Constituent-oriented Age and Residence time Theory (CART). The modified LOICZ model was found to be the most comparable to residence times from numerical models, with r2 ∼ 0.7. In addition to the proposed modified LOICZ model (which uses Fischer formulation), we have developed an advection–dispersion timescale diagram. This graphic conceptual model provides a visual representation of the relative contribution of advective and dispersive processes to water renewal for different estuaries. Estuaries can be categorized as either dominated by dispersion, dominated by advection, or having dispersion and advection of similar magnitude.