During World War II, the North Carolina Shipbuilding Company built a large structure encompassing nearly six acres to support the fabrication of ship components for the U.S. Navy. Known as Building C1, the structure is now part of the North Carolina State Ports Authority terminal located on the Cape Fear River south of Wilmington, NC. For the past two decades Building C1 has been used for the bulk storage of fertilizer products. Over time, components of the fertilizer products have leached into the soil and groundwater underlying and adjacent to the building. The primary contaminant of concern is ammonia, which is not only toxic but which can also be explosive in high enough concentrations.
Determined to act as a good citizen as well as operate a safe and environmentally compliant facility, the North Carolina State Ports Authority turned to WithersRavenel to provide a solution that did not involve demolishing the existing facility to access the contaminated soil and groundwater.
Since ammonia is a gas at standard temperature and pressure, WithersRavenel evaluated various alternatives that could be used to capture and remove the gas using either passive or active soil venting or vacuum extraction techniques. Before proceeding with the full-scale design, WithersRavenel conducted a pilot test that involved installation of a 200-foot-long slotted pipe using a directional drilling machine in the space between the building floor and the underlying water table below one of Building C1’s storage bays, where high concentrations of ammonia were known to be present. A variable speed fan was attached to the pipe to create vacuum pressure in the unsaturated soils beneath building floor. Results of the pilot test demonstrated that the technique was suitable for larger scale implementation.
WithersRavenel subsequently designed and oversaw the construction of a full-scale system, which consists of eight approximately 400-foot-long horizontal slotted pipes, each attached to a dedicated and weatherproof variable speed fan. In its first year of operation the system removed more than 3,000 pounds of ammonia from the subsurface.
To address contaminants in the most severely affected shallow groundwater near Building C1, WithersRavenel recommended phytoremediation, which is the process of using hydrophilic plants to absorb contaminated groundwater and convert the harmful substances into harmless ones or—better still—into chemicals beneficial to plant growth. WithersRavenel designed another pilot test using hybrid poplar trees to determine whether the concentrations of fertilizer constituents in the groundwater would be toxic to the trees. The results indicated that phytoremediation stood a good chance of being successful for the Building C1 site, so WithersRavenel was authorized to plant more than 400 trees in the area. After two years, the trees are at least as healthy as ones growing in non-contaminated settings, and may even be larger than average thanks to the extra nutrients they are absorbing.
By combining construction techniques like horizontal directional drilling with green strategies like phytoremediation, WithersRavenel delivered a project that had a much lower impact on the surrounding environment and was much more affordable than conventional site remediation programs.
North Carolina State Ports Authority
Construction Cost: $500,000