A Three-Dimensional Water Quality Model for Estimating TMDLs in a Blackwater River Estuary, the Lower St. Johns River, FL

Abstract

The determination of a Total Maximum Daily Load (TMDL) for an impaired waterbody is inherently a quantitative analysis; typically requiring the application of mathematical models to estimate unknown loads, relate loads to target concentrations, and to evaluate implementation strategies to achieve water quality targets. The determination of TMDLs for the Lower St. Johns River is complicated by the hydraulics of the estuary and multiple point and non-point loading sources. The relationship between anthropogenic enrichments and processes impacting nutrient, algal and dissolved oxygen dynamics is further complicated by the blackwater nature of this estuary, as characterized by natural occurrences of low oxygen, low transparency and high primary production. To aid in the evaluation of water quality processes in the estuary and the development of TMDLs, a mathematical model was developed for the Lower St. Johns River estuary using the CE-QUAL-ICM water quality model. The model computes and reports concentrations, mass transport, kinetics transformations, and mass balances for 32 water quality state variables. The three-dimensional hydrodynamics for the application were developed by linking CE-QUAL-ICM with the predictions from the Environmental Fluid Dynamics Code (EFDC), applied to the estuary by the St. Johns River Water Management District (SJRWMD). The SJRWMD developed point and non-point source loadings to the water quality model. Specific modifications were made to ICM to increase its applicability to the Lower St. Johns River. The modifications included the addition of three state variables to address the colored dissolved organic matter of this estuary. The model was applied using data for two years of simulation, from December 1996 to November 1998. Model development and model modifications are discussed. l Department of Civil Engineering, Mississippi State University, Box 9546, Mississippi State, MS 39762; 2 US Army Engineer Research and Development Center, Waterways Experiment Station, 3909 Halls Ferry Road, Vicksburg, MS 39180; 3 St. Johns River Water Management District, P.O. Box 1429 Palatka, FL 32178-1429. James Martin is the primary contact and can be reached at 662-3237194 or imarti n @ en gr.msstate.edu.