Hot Water Treatment and Steam Flushing/Stripping
This process injects hot water and steam into an aquifer to mobilize volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs). Vapors rise to the near-surface soil where they are removed by vacuum extraction and then treated. Similar to Enhanced Soil Vapor Extraction and Dynamic Underground Stripping in many ways, hot water treatment entails the injection of hot water into the subsurface to mobilize coal tars. The hot water is injected at the perimeter of the tar formation and withdrawn from the center of the formation. In this way, the site is hydraulically isolated. Tar is mobilized by the hot water and flows to the recovery wells, where it is removed from the subsurface along with the hot water.
Maintaining a cold water cap over the area can control volatilization of the lighter tar constituents. Any volatile organics condense once they reach the cold water. At the surface, the water is separated from the coal tars, reheated, and then re-injected into the subsurface.
Limitations and Concerns
The process injects a large amount of water into the aquifer. Monitoring must be in place to ensure that waste is not being mobilized outside of the treatment zone.
The addition of hot water in the subsurface can cause an increase in dissolved iron in groundwater. When brought to the surface for treatment, iron precipitate may cause coal tar separation problems, and it may foul the treatment unit and injection wells.
Coal tars contain a mixture of very toxic chemicals, such as benzo-a-pyrene. It is important that these constituents be properly disposed of after recovery, and that this process does not accelerate their mobilization in the environment.
Soil type, contaminant characteristics and concentrations, geology, and hydrogeology play an important role in process effectiveness.
The process can be used to remove large portions of oily waste accumulations and to retard migration of organic contaminants in groundwater. The target contaminant groups for hot water treatment are SVOCs (coal tars) and fuels. VOCs also can be treated by this technology. Large deposits of underground tar exist as a result of manufactured gas plant operations. Many such facilities were built and operated in the 19th Century to provide gas for heating and lighting, and byproducts such as tars were injected or placed in the ground.
Technology Development Status
Hot water or steam flushing/stripping is a pilot-scale technology.
Other Resources and Demonstrations
See related technology Dynamic Underground Stripping.
See http://www.clu-in.org/download/citizens/insitutherm.pdf for a description of thermal technologies, including hot water treatment and steam flushing.