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concentration and clay content in foundry sands. For source zinc concentrations
less than 400 mg/L and seepage velocities less than 0.1 m/d, PRBs with less
than 1 m thickness can be constructed with foundry sands provided the clay
content is higher than 5%.
9.4 LEACHING
Results of the water leach tests and the total elemental analyses showed
that all of the foundry sands are Category 2 materials per Section NR 538 of the
Wisconsin Administrator Code. However, tests on Peerless iron, torpedo sand,
and a typical fill material indicate that these materials, which are commonly
placed below the groundwater table, also are Category 2 materials. Thus, using
foundry sand as a PRB medium should pose no greater risk than that imposed
using conventional construction materials.
Additional column leaching tests were conducted to determine
characteristics of the leachate under flow conditions more representative of the
field. Effluent from these tests was analyzed for three metals (Fe, Cr, and Pb). Cr
and Pb in the effluent were always below MCLs. For Fe, however, several pore
volumes of flow were often required to meet the MCL.
The number of pore volumes required to meet the MCL (PVER) was found
to depend on the initial concentration of Fe in the effluent and the partition
coefficient. A method was developed to predict the PVER using properties of the
foundry sands and the results of water leach tests and column leach tests.