There are several dense non-aqueous phase liquid (DNAPL) sites owned by the Department of Defense (DoD) where source zone treatment could be beneficial. The objective of this project is to conduct a field test of vegetable oil injection into a DNAPL source. The oil injection is aimed at physical and chemical sequestration of the DNAPL in the short-term, followed by enhanced biodegradation of dissolved contaminants in the long-term.
Vegetable oil is a light, viscous liquid that is sparingly soluble in water. It acts as a slow-release electron donor source in the long-term, and vegetable oil injection has been used primarily for this purpose. However, there are additional benefits from injecting the oil into a DNAPL source zone that will be evaluated in this project. When enough oil is injected in the soil, the oil can sequester the DNAPL by physically diverting groundwater flow around the source and thus weakening the plume emanating from the source. Additional sequestration occurs as the DNAPL is solubilized in the oil. These physical-chemical features of the oil injection play an important role in controlling the DNAPL source. The challenge often is in obtaining sufficient distribution of the oil in the subsurface to realize these benefits. Use of emulsifiers and cosolvents can enhance distribution of the oil, but it may be difficult to retain the oil in this form in the source zone as it may migrate out with the groundwater flow. This project will investigate if pre-heating the oil to a moderately high temperature before injection improves its distribution in the soil. Bench-scale tests have shown that heating the oil temporarily reduces its viscosity, allowing it to travel into the soil pores surrounding the injection point. In the soil, the oil cools quickly, its viscosity rebounds, and it remains trapped in the source zone. The ability of the vegetable oil to sequester and treat a DNAPL source through these multiple mechanisms will be tested at a suitable DoD site.
Vegetable oil injection has the potential to be an environmentally benign and relatively low-cost treatment for DNAPL sources, both in the short- and long-term. In the short-term, the physical-chemical aspects of the oil distribution play a major role. In the long-term, its ability to act as a slow-release electron donor will enhance biodegradation of any residual plume. Unlike many other source zone treatment methods, which leave behind some residual DNAPL that may have to be addressed with a secondary treatment, a single vegetable oil injection event could be sufficient for long-term remediation of a DNAPL source.