Grain Size

The movement of fine grained material within the barrier, either through washing of existing grains or the generation of new fine-grained particles via freeze-thaw shattering, can lead to zones of enhanced flow (leading to premature contaminant breakthrough) or clogging and reduced flow (leading to groundwater bypassing the barrier). A 1-h shake test of 24 zeolites in water showed a 1-18% loss of mass from the 65 to 40 mesh (212-420 |im) size fraction. Similarly, a 21-pore volume wash-through test using clinoptilolite zeolite revealed a 2.7-4.3% loss of mass, although these values were reduced by pre-washing to remove fines or calcining to increase grain strength (Zamzow and Murphy 1992). Low ionic strength solutions enhanced the loss of fines due to electrostatic repulsion, and shear by the slow-flowing (0.609 m per day) rinse water played only a minor role in particle movement (Abadzic and Ryan 2001).

The addition of freeze-thaw activity does not seem to create a very large additional loss of zeolite material. The <0.15 mm fraction of a 85% sand:15% ZeoponiX clinoptilolite amendment mixture, subjected to 20 freeze-thaw cycles, increased only 1.3% (Li et al. 2001) to 1.5% (Li et al. 2002) by mass, mainly at the expense of the >250 |im fraction. Similarly, clinoptilolite zeolite grains subjected to 60 freeze-thaw cycles under both drained and saturated moisture conditions, led to the <250 |im fraction increasing from 1 to 3% by mass (Gore et al. 2006). The significance of the creation of fines due to freeze-thaw activity and their redistribution or removal by flow can only be fully understood by the assessment of their three-dimensional arrangement and the resultant hydraulic characteristics of the media.

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