Evaluation of the Estuarine Retention Time in a Mississippi Estuary: The Bay of St. Louis

Author(s): Camacho, R.; Martin, J.

The Estuarine Residence Time (ERT) is an important hydrodynamic-water quality parameter that evaluates the amount of time a substance remains in an estuary. The water quality of estuarine systems is often closely linked to the ERT. For example, estuarine systems with a low ERT are less vulnerable to algae blooms than estuarine systems with a higher ERT. Also, this parameter can be used to assess the characteristics of the transport of contaminants within estuaries as well as to evaluate the auto-depurative (self-cleansing) capacity of these systems.

This paper presents the results of an initial analysis of the ERT of the St Louis Bay Estuary, MS, using the linked hydrodynamic and water quality models EFDC and WASP. The computation of the ERT is based on the method described by Miller and McPherson as computed by the time required to reduce the concentration of a conservative constituent to some percent of its original concentration. For the analysis, an initial dye concentration was set to 100 units within the system, with all boundary conditions set to zero, and the models run until the dye concentration was less than 1 percent of the original value (a value of 10 percent remaining is usually used to estimate the ERT). The ERT was also evaluated using water age, a state variable in EFDC. Low and high hydrologic conditions were estimated for the inflows of the system using the information developed by previous studies to evaluate the response of the system. Results suggest that the estuary is characterized by a relatively low ERT and demonstrated that the use of a hydrodynamic model as EFDC is an effective means to evaluate the ERT in an estuary.

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