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Current Findings The primary objective of this study is to conduct an in-depth geoenvironmental assessment of the potential for ground water contamination by fly ash stabilized soils, such as soft subgrades in highway construction. To achieve this objective, four different tasks were conducted: (1) water leach testing (WLT), (2) laboratory column testing, (3) field lysimeter testing, and (4) numerical modeling. Testing completed to date has focused on one fly ash and two soils. The completed testing program will include three fly ashes and four subgrade soils representing a wide range of conditions that might be encountered in Wisconsin. WLTs on the fly ash have shown that concentrations of contaminants of concern (Cd, Cr, Se, Ag, and SO4-2) in the leachate are lower than those in the Category-4 standard stipulated in Section NR 538 of the Wisconsin Administrative Code. Thus the fly ash can be used for soil stabilization in confined geotechnical fills in Wisconsin. The concentrations of metals in the leachate from soil-fly ash mixtures tend to be lower (1.5 to 2.5 times) than those from fly ash alone, but are appreciably higher than the concentrations expected based on a simple dilution calculation. Column leaching tests conducted in the laboratory showed that the hydraulic conductivity, pH of the effluent, and initial effluent concentration of soil-fly ash mixtures increase with increasing fly ash content. The porosity also tends to increase with increasing fly ash content. In contrast, the dispersion coefficient for metals and the dispersivity decrease slightly with increasing fly ash content. The release pattern for metals from the soil-fly ash mixtures appears to be adsorption-controlled. The pH of the pore fluid has shown to be persistent for at least 30 pore volumes of flow, which corresponds to at least 30-yrs of flow in the field.
Lysimeters have
been installed at two field sites to monitor the liquid flux and concentration
of metals in leachate from fly ash stabilized soils and a typical pavement
section. Monitoring has shown that
the liquid flux is approximately 4-6% of average annual precipitation, and is
comparable to that from a typical pavement section. Concentrations of most of the metals of concern are higher in
leachate collected from fly ash stabilized soils than typical pavement sections,
and have not varied significantly to date. Concentrations of metals in the field leachate agree well
with concentrations in the effluent from the column leaching tests conducted in
the laboratory, which suggests that transport parameters obtained from the
column leaching tests can be used to predict field conditions. A numerical model has been developed to simulate typical field scenarios where subgrade is stabilized with fly ash. Simulations conducted using transport parameters obtained from the column leaching tests showed that the maximum concentration decreases abruptly 5 times in the first meter, and then decreases more gradually at deeper depths. The maximum concentration at a given depth is independent of the retardation factor, and decreases with higher dispersion and lower thickness of the stabilized layer. The time to reach the maximum concentration at a particular depth is independent of the thickness of the stabilized layer, and increases with lower dispersion and higher retardation factor.
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