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increases. Also, at the same clay content, lower hydraulic conductivity was
obtained at higher dry density, which was anticipated.
PRBs typically require that the hydraulic conductivity of the medium be at
least that of the aquifer (Starr and Cherry 1994). Thus, foundry sands need to be
selected that have hydraulic conductivity consistent with the hydrogeologic
setting in which the PRB is to be installed. Most of the sands that were tested are
suitable for aquifers with hydraulic conductivity of 10-4 cm/s. For more permeable
aquifers, granular materials might be added to increase the hydraulic conductivity
and reactivity of a foundry sand. This alternative was evaluated by conducting a
series of tests with Sand 10. As shown in Fig. 3.12, the hydraulic conductivity
increased linearly when Peerless iron was added to Sand 10. A less costly
alternative would be to add gravel, crushed glass, or sand to the foundry sand
provided additional reactivity was not necessary.