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Physical Containment <br /> f <br /> Adverse geologic conditions may render remediation by extraction and aboveground <br /> treatment ineffective or prohibitively expensive. In these instances, the dissolved <br /> hydrocarbon plume can be effectively isolated by installation of impermeable barriers. <br /> Barriers generally consist of a trench excavated around the perimeter of the plume and <br /> backfilled with a mixture of soil and bentonite or cement and bentonite. Grout curtains can <br /> also been installed by injecting cement into soil through borings. To be effective, a barrier <br /> should extend from the ground surface to a natural impermeable layer. In the absence of <br /> a natural impermeable layer, cement can be injected through soil borings to form a grout <br /> blanket. <br /> Dissolved hydrocarbon plumes can also be contained by creation of artificial hydraulic <br /> gradients. The gradients are induced by extraction of ground water from wells or injection <br /> of water into the subsurface through ponds, trenches, or wells. Extraction produces a cone <br /> of depression in the groundwater surface that prevents migration of ground water <br /> containing dissolved hydrocarGons. Injection produces an artificial mound around the <br /> perimeter of the plume that has a similar effect. The disadvantage of this method is that <br /> the dissolved hydrocarbons are isolated but not removed from the subsurface environment. <br /> ars <br /> We do not consider physical containment methods to be applicable at this site. Excavation <br /> of deep trenches is not cost-effective and an adequate bottom seal at the site is not <br /> ti apparent. <br /> A,op/ied GenSyWOMS <br />