Exploring Regional Groundwater Supply Alternatives to Manage Saltwater Intrusion in the Hilton Head Island Area

As part of the Coastal Georgia Sound Science Initiative (CSSI), the Georgia Environmental Protection Division engaged CDM to develop a groundwater flow and saltwater transport model to study saltwater migration in the Upper Floridan aquifer beneath Hilton Head Island, and to evaluate how combinations of potential pumping reductions and engineered systems could be used to manage saltwater intrusion.  Groundwater chloride measurements at monitoring wells on Hilton Head Island and nearby areas have indicated the presence of saltwater since the late 1970s.  Historically groundwater withdrawals in the area of Savannah, Georgia and the Hilton Head Island vicinity have contributed to saltwater intrusion beneath Hilton Head Island.

The CSSI included extensive field investigations of the area, including offshore boring installation, geophysical studies, and downhole specific conductivity profile measurements. Data from these field studies were incorporated into a variable density coupled flow-transport groundwater model developed using a coupled version of the DYNFLOW and DYNTRACK modeling codes (www.dynsystem.com). A transient simulation of the period from 1915-2007 was conducted to study the historical migration of saltwater from the surficial aquifer into the Upper Floridan aquifer and the advance of saltwater beneath Hilton Head Island. The model was calibrated to historical heads, observed tidal fluctuations, a local pumping test, and specific conductivity data.

Once calibrated, the model was used to study the relative impacts of Savannah and Hilton Head pumping on historical saltwater migration, and to evaluate the impact of potential pumping reductions on the projected saltwater transport beneath Hilton Head.  Model simulation results are being used by an interstate committee tasked with developing options for managing salt water intrusion to ensure groundwater supply to local industries and municipalities.