Georgia Estuary (GaEst) was developed for Georgia Environmental Protection Division (EPD) to compute the dissolved oxygen sag curve in the vicinity of waste discharge points in Georgia estuaries. GaEst is a management tool used to predict water quality under varying present and future conditions. It is one of the tools that EPD uses in conducting estuary analyses in order to determine the available assimilative capacity and maximum daily load which can be placed on the estuary's resources by wastewater dischargers and non-point sources. The model results then form a basis for the issuance of permits limiting wastewater discharges into the surface waters of Georgia.

GaEst is a modified steady-state, branching, one-dimensional, tidally-averaged model for coastal waters for DO standards review and wasteload allocations.

In 1963, Dr. Robert V. Thomann created the basic model framework and solution technique used in GaEst. Thomann's method numerically solves the steady state form of the one-dimensional advection-dispersion equation for estuarine mass transport. He employed a finite segment approximation to the finite difference method for the ordinary differential equation. For years, Manhattan College used a condensed version of this model (called ESTUARY) for teaching and demonstration in their Summer Institute courses on Water Quality Modeling.

In 1971, HydroScience, Inc. applied this method in a three-dimensional model of Boston Harbor. Later, Chapra and Nassa re-worked the Boston Harbor model and produced a general purpose Fortran batch program (called HAR03) that is fully documented and suitable for one-, two-, and three-dimensional systems.

In 1981, Bill Leo of HydroQual, Inc., converted ESTUARY into a main-line batch program in HP BASIC and transferred this model technology to the Georgia Environmental Protection Division (GaEPD). Leo's HP batch model was renamed Georgia Estuary.

In 1982, Dr. Roy Burke III of GaEPD developed a user-friendly interface for this program and converted it from batch mode to fully interactive (for the HP9845/A micro computer). The program became GaEst Version 1.0 and found successful application in three major projects in Georgia's coastal waters: Brunswick, North/St. Marys, and North Newport Rivers.

In 1989, Dr. Burke transported GaEst V1.0 from the HP to the PC-DOS operating environment using Microsoft Quick Basic, extended model capabilities, enhanced analytical opportunities, and improved the user interface. With these capabilities GaEst, now called Version 2.12, represented the accumulation of field experience with this technology along with an expression of the practical needs of modeling for TMDL and NPDES Permit development by state regulatory agencies.

In 2004, Dr. Chris Wilson and Dr. Burke developed GaEst Version 3.0 for the Microsoft Windows operating systems using Microsoft Visual Basic .Net software and included significant enhancements and new features:

         Unlimited branching is allowed, however the branches must be dendritic (braided or looped branches are not allowed); in other words, each branch must have an open

         upstream boundary not connected back to another branch.

         The number of segments, waste discharges, and oxygen injection sites is now unlimited.

         Freshwater flows can be directly input, computed using a basin yield factor (cfs/sq mi), or computed by the Rational Method (Q = CiA) using up to four land use types.

         Additional state variables have been added: CBOD2, NH3, and an arbitrary nonconservative substance.

         Computes ammonia toxicity limits based on EPA guidance documents

         Chloride concentrations can be computed as a conservative substance.

         Multiple windows capabilities for viewing and organizing model inputs and outputs.

         New table and graphical features to assist model calibration.

         Powerful editing capabilities for model structure - example, deleting or inserting branches.

         Friendly and intuitive "build new model" procedures.

         Capable of joining two separate models with simple operation.

         Persistent integrity checks to ensure valid model structures at all times.

         Multiple models can be included in a single (*.est) file for convenient comparison of alternative modeling scenarios and enhanced partitioning analysis.

         New routines to refine the model grid for convergence testing.

         New options for residence time or freshwater washout calculations.

         New "Calculator" to help determine maximum allowable segment lengths, minimum distance to boundaries, dispersion coefficients, BOD decay rates, reaeration rates, net

         P/R rates, and DO saturation.

         Provides errors/warnings for input values and results that are out of bounds.

         Flexible spreadsheet-style entry of required model inputs.

         Interactive graphical displays of results suitable for analysis and reporting.

         Modernized file/data input and output capabilities plus report-quality graphs and tables.

         Ability to load and save DOS Version 2.1 legacy files.

         Improved documentation with model activities directly connected to manuals.