[MQP] Salt Intrusion in Gatun Lake

Executive Summary

The Panama Canal runs from the Pacific Ocean in the southeast to the Atlantic Ocean in the northwest over a watershed area containing the freshwater lake, Gatun Lake. The canal facilitates the transit of 36 ships daily using three sets of locks, which displace large volumes of water into and out of Gatun Lake. The displacement of water has the potential to cause salt intrusion into the freshwater Gatun Lake. The ACP is currently expanding the Panama Canal by constructing a new series of locks, Post-Panamax locks, which will accommodate the transit of larger ships through the channel. The ACP does not expect that new locks to change the salinity levels of Gatun Lake (Jongeling, 2008). However, since the lake acts as a fresh drinking water source for Colon and Panama City, predicting future salinity levels in the lake is important.

The primary goal of this project was to model future salinity levels in Gatun Lake. In order to meet this goal, three objectives were completed: (1) collection of current water quality conditions in the lake, (2) modeling of current salinity levels based on historical data and comparison of those modeled levels to measured values and (3) modeling of future salinity levels after the expansion project is complete. First, current lake conditions were measured during a two-day water quality campaign in which water quality was determined at 13 stations within Gatun Lake. In-situ data in real-time were collected using a SBE 19plus SEACAT Profiler (SeaBird Electronics, Inc., Bellevue, Washington), which measures conductivity, temperature, and pressure in marine or fresh-water environments at depths of up to 7,000 meters. Then, water samples at various depths were collected in bottles and transported to the laboratory for measurement of salinity. Results showed salinity levels were below 0.5 ppt at all stations, salinity levels tended to increase below an elevation of 5 meters, and of all stations, Buoy D showed to have the highest salinity levels at the bottom layers of the lake due to its location in front of the Gatun Locks.

The second objective was to model current salinity levels. Hydrometeorological data, including air temperature, wind velocity, runoff, precipitation, and evaporation measurements, were obtained from ACP for the years 2003 – 2005. Saltwater intrusion and dispersion simulations were run by inputting these data into hydrodynamic software, Delft3D, which considers the hydrology within natural as well as artificial environments such as the Panama Canal lock system. In a contract with ACP, Deltares, the developer of the Delft3D software, created multiple scenarios to analyze the salt intrusion within the particular environment of Gatun Lake. Year 2011 salinity levels were predicted using the 2003 – 2005 data and the current shipping schedules (prior to expansion). Results showed that these salinity levels were consistent with the in-situ salinity measurements gathered during the water quality campaign, verifying that the software was representative of the real situation.

The third objective was to model future salinity levels considering the large PostPanamax locks and increased ship transits. Predictions were made using hydrometeorological data from 2003 to 2005 and also for data from 2008 to 2009. For all locations in the lake that were simulated, future salt levels were within the freshwater range (0 ppt – 0.5 ppt). These results are consistent with ACP expectations that the expanded lock system will not negatively impact the water quality in Gatun Lake with regard to salinity.

Delft3D modeling results showed that salt levels within Gatun Lake have remained consistent and are predicted to stay within the freshwater range, with salt levels less than 0.5 ppt. However, it is still possible that modeling efforts have not fully captured future water quality scenarios, or that changes in shipping schedules or operation of the locks could result in increased salt levels in Gatun Lake after the expansion project is completed. Therefore, a drinking water treatment plant was designed to produce potable water for the city of Colon and the Panama City metro area considering a brackish water range of 0.5 ppt to 15 ppt conditions in Gatun Lake. Panamanian law requires some form of flocculation, coagulation, sedimentation, or filtration in addition to disinfection. The design complies with Panamanian law, and includes cartridge filters for pretreatment, an electrodialysis reversal (EDR) system for salt removal and ultraviolet (UV) light for disinfection.