Attenuation liners

The prineiples underlying the development of attenuation liners have been previously described in a related paper (Thornton et al. 1993) and some preliminary results of the research programme have been presented (Bright et al. 1996). The present paper summarizes the results of the research and implications for the wider application of these liners. 

In principle the material properties identified in Table 1 allow an attenuation liner to be engineered for any given landfill. The ideal liner recipe which maximizes the attenuation of the key contaminants in leachate is deduced to be high clay and organic carbon content plus adequate alkaline pH buffering capacity. 

For compacted, low-permeability soil liners, the U.S. EPA draft guidance recommends natural soil materials, such as clays and silts. However, soils amended or blended with different additives (e.g., lime, cement, bentonite clays, and borrow clays) may also meet the current selection criteria of low hydraulic conductivity, or permeability, and sufficient thickness to prevent hazardous constituent migration out of the landfill unit. Therefore, U.S. EPA does not exclude compacted soil liners that contain these amendments. Additional factors affecting the design and construction of CCLs include plasticity index (PI), Atterburg limits, grain sizes, clay mineralogy, and attenuation properties. 

To determine the source intensity in practice and to correct for the attenuation in the liner walls first no core and then an empty core liner are placed between the gamma ray source and detector to measure the intensities (1 ) and (1, ). The difference (1 - 1, ) replaces the source... 

Impact attenuation is initiated by the outer shell, which delocalizes and spreads the direct load into the liner to manage impact energy. To consistently maintain its protective performance and systematic function, the shell must provide the necessary stiffness, toughness, and stability across a wide array of end-use impact conditions. As a result, sports helmets are most commonly constructed with an injection-molded rigid polymer outer shell. The selected thermoplastic polymers influence the level of impact protection and are often divided into three performance categories high-impact resistance, moderate-impact resistance, and general purpose. 

Thornton, S.F.,Lemer, D.N. andTellam, J.H. (2001). Attenuation of Landfill Leachate by Clay Liner Materials in Laboratory Columns. Waste Manage. Res., 19 70. 

Before fire testing, the Fire Science Group tested lined containers for their neutron-absorption properties. After exposing each container to flames for 10 to 20 min, they reevaluated its neutron attenuation. For a Cf fission spectrum moderated by 25 cm of DjO, the liner reduced the low-energy neutron flux by an average factor of about SO before and after fire testing. These tests also showed the neutron-absorption properties of the liner were not greatly affected despite the partial collapse and loss (as much as 8%) of absorber in the form of steam. 

For mass transport in composite liners, which will be dealt with in the next section, further simplifying assumptions can be made. In a perfectly installed composite liner no advective mass transport and associated dispersion effects can take place. The discussion of degradation processes and natural attenuation effects is well beyond the scope of this book and therefore neglected. Equation 7.24 is then simplified to the common diffusion equation ... 

As can be noted from the schematic diagram, if the solute pulse is retarded, the area under each of the retardation pulse curves remains constant as the pulse travels downward through the unsaturated zone toward the aquifer. This indicates that the total contaminant load is transferred to the aquifer. By contrast, the retention pulse shows decreasing areas under the pulse curves. This indicates partitioning by chemical mass transfer and irreversible sorption, and resultant dilution of contaminant concentrations. If proper design procedures are used, no transfer of contaminants to the aquifer should occur, thereby fulfilling the requirements for contaminant containment using the natural attenuation capability of the soil substrate liner system. 

In terms of its use as liner material, kaolinite has a lower affinity for water, has lower dispersive properties and is more permeable. However, due to its low CEC (3-15meq/100g) and surface area (10-20m g ) it is less electro-ehemically active and also exhibits the least plasticity. The higher permeability of kaolinite has led to fears of shortened breakthrough time. However, as this clay exhibits less attenuation, its behaviour may be less affected by the interaction with organic/inorganic chemicals, and by moisture variations. 

A high clay mineral content is correlated with an increased particle surface area and sorption capacity for both inorganic and organic contaminants. Permeability is also reduced, increasing the residence time of leachate within the liner, and is important for contaminants (e.g. XOMs) which may be attenuated by processes (e.g. degradation) which are kinetically controlled... 

The quartz sand was used to dilute the active substrates and also as a control experiment. Powdered Chalk was used as a pH buffering agent and its performance when added as a layer or admixed with the host liner substrate evaluated. This array of experiments enabled an assessment to be made of impacts of variations in one or several key substrate properties on the liner performance, and also to test the robustness of the methodology against a range of materials. The column experiments were run for up to 15 months and a selection of leachate-loaded liner recipes were flushed with oxygenated water to examine attenuation reversibility effects. Some exhausted liner recipes were also... 

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