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Evaluating the Potential Effects of Deepening the St. Johns River Shipping Channel on Saltwater Intrusion in the Surficial Aquifer, Duval County, Florida

Project Chief: Rick Spechler
Project Modeler: Jason Bellino
Cooperator: U.S. Army Corps of Engineers
Period of Project: October 2011 - September 2012

Problem Statement

Jacksonville’s main shipping channel is a 20-mile stretch of the St. Johns River extending from the Atlantic Ocean at Mayport, Florida to the Jacksonville Port Authority’s (JAXPORT’s) Talleyrand Marine Terminal just north of downtown Jacksonville (fig. 1). The United States Army Corps of Engineers (USACE) is in the process of evaluating the costs and benefits of a proposed plan to deepen the shipping channel in the St. Johns River to 48-53 feet below North American Vertical Datum 88 (NAVD 88) to permit new Panamax ships to dock at the JAXPORT ship terminals. Currently, the 20-mile stretch of the shipping channel from the mouth of the river to Talleyrand Marine Terminal is maintained by the U.S Army Corps of Engineers at a depth of about 41-43 feet below NAVD 88 to allow existing deep-draft ships to dock at JAXPORT terminals.

Prior to dredging of the St. Johns River to the current depth of 41-43 feet below NAVD 88, the USGS performed a study on the potential effects of dredging on the surficial aquifer adjacent to the river. This study incorporated a field investigation which included installation and monitoring of surficial aquifer water levels and chloride concentrations. The conclusion of the study was that dredging the St. Johns River to 40 feet below NAVD 88 probably would not have a significant effect on saltwater intrusion in the surficial aquifer.

In order for the new Panamax ships to dock at JAXPORT terminals, the channel may need to be deepened to as much as 53 feet below NAVD 88. Because other ports in the United States are expected to consider similar deepening plans, it is expected that the approach developed for this study could be applied to other areas to evaluate the effects of channel deepening and identify areas which should be monitored.

Objectives

The USGS used a variable density groundwater flow and transport model to quantify the range of water-quality changes expected to result from deepening of the St. Johns River shipping channel. The model was used to simulate saltwater intrusion in the surficial aquifer, and identify areas that could be susceptible to groundwater quality degradation and that would require water-level and/or water-quality monitoring.

Approach

In order to estimate the potential effects that deepening of the St. Johns River will have on saltwater intrusion in the surficial aquifer, a numerical model, SEAWAT, was used. The SEAWAT model is capable of simulating advective and dispersive transport in a three-dimensional, variable-density, groundwater flow system. Like most commonly used groundwater modeling programs, SEAWAT is based on Darcy’s Law and requires that model properties be assigned using the assumption of an equivalent porous medium.

The study included: 1) a data acquisition phase to compile and analyze available surficial aquifer water use and hydrogeologic data, and 2) a numerical model to evaluate areas likely to be affected by deepening of the St. Johns River channel.

Results

Not yet available.

Information Product

A USGS Scientific Investigation Report (SIR) will summarize the potential effects of deepening of the St. Johns River Shipping Channel on saltwater intrusion in the surficial aquifer in Duval County, Florida.