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Assessment of the Performance of Engineered Waste Containment Barriers
TABLE 2.1 Major U.S. Federal Statutes Governing Waste Classification and Containment
Municipal solid waste
Focuses on residential and commercial refuse such as food, paper, glass, plastic, textile, grass, wood, and metal
Resource Conservation and Recovery Act (RCRA), as amended (42 USC 6901 et seq.) Subtitle D
Focuses on hazardous waste such as refining and manufacturing by-products, paint, solvents, pesticides, and ashes; hazardous wastes must be stabilized before disposal in a containment system
RCRA, as amended (42 USC 6901 et seq.) Subtitle C
Wastes associated with cleanup of abandoned hazardous waste sites
Applies to hazardous contaminated soils and liquids removed from the ground or generated from treatment
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) (1986), as amended (42 USC 9601 et seq.)
Low-level radioactive and mixed waste from production of nuclear power (Nuclear Regulatory Commission) and weapons (Department of Energy)
Requires each state to provide disposal capacity for commercial low-level radioactive waste, such as protective clothing, filters, containers, tools, laboratory equipment, and piping
Low-Level Radioactive Waste Policy Act as amended in 1985 (42 USC 2021b et seq.)
Uranium tailings and other contaminated materials at uranium mill processing sites and adjacent properties
Applies to mining waste rock, tailings from ore processing, industrial waste, and waste water
Uranium Mill Tailings Radiation Control Act of 1978 (42 USC. 2022 et seq.)
as dilution can decrease downstream contaminant fluxes to levels that meet regulatory standards. While not the main purpose of a resistive barrier system, some resistive barriers contain reactive materials that transform contaminants as they pass through the barrier.
Capacitive barriers function by retaining contaminants or contaminant transport media by sorption processes or retention in pore spaces. An example is a layer in a cover barrier that stores water and supports vegetative growth on the cover (Figure 2.1). A capacitive barrier retains the contaminants and transport media by processes such as adsorption, redox reactions, and/or precipitation in pore spaces. A capacitive barrier that works by retention alone will merely delay the eventual breakthrough of contaminants unless the retention capacity exceeds the contaminant mass. Therefore, capacitive barriers usually require a supplemental mechanism if they are to be effective over the long term. Such mechanisms include (1) resistance to transport (described above); (2) reactive treatment, in which the contaminant reacts with barrier media and is transformed to a harmless substance; and (3) gradient reversal, in which the stored contaminant or transport media is released from the barrier back in the
FIGURE 2.1 Schematic illustration of functional mechanisms for engineered barriers.
direction of its origin (e.g., release of stored water from soil covers by evapotranspiration). The use of zeolites in barriers for radioactive waste is an example of a capacitive barrier that also relies on other mechanisms to reduce contaminant transport: the zeolites retain the radioactive isotopes while radioactive decay processes reduce their potential impacts.
Advective barriers rely on advective flow to control the migration of contaminant transport media. In an advective barrier, gradients are introduced to generate a flow counter to the indigenous direction of contaminant transport. An example is a groundwater pumping system surrounding the waste that creates an inward gradient so that liquid flows into and not out of the system (Figure 2.1). Advective barriers also include pneumatic barriers, where a suction pressure is applied to control landfill gas transport.
Extractive barriers are used in conjunction with advective transport to remove contaminated liquid or gas for treatment and/or disposal. Examples are leachate and gas collection and removal systems that remove contaminated liquid and gas from inside the barrier system (Figure 2.1). Extractive barriers include extraction wells, trenches, and blanket leachate collector systems.
2.2.2 Barrier System Orientations
Final covers are the most common type of engineered barrier system. They are used for both engineered containment systems and nonengineered contaminated sites and dumps to keep waste and contaminants in and to keep potentially infiltrating water out. Most are resistive barriers, and many incorporate capacitive and extractive barriers to control surface water infiltration (Table 2.2). Enhanced capacitive covers, also referred to as alternative covers because they can