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.

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