The Hydrogeological Impact of Heavy Metal-Laden Ash Landfilling at the WLSSD Landfill, Rice Lake, Minnesota

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The Hydrogeological Impact of Heavy Metal-Laden Ash Landfilling at the WLSSD Landfill, Rice Lake, Minnesota

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Starting in August of 1985 the Western Lake Superior Sanitary District (WLSSD) began processing garbage to produce a combustible fuel for use in incinerating heavy metal-laden sewage sludge. The end product of the incineration process is a fine ash which is ultimately deposited at the WLSSD landfill, Rice Lake Township, Minnesota. To assess the environmental implications of the ash landfilling, a hydrogeologic study of the landfill area and a heavy metal adsorption/transport study were conducted. The WLSSD landfill is situated in a complicated ice stagnation complex known as the Highland Moraine. Sediment analyses indicate that the site is located in the silt-rich (sand/silt/clay ratio: 45/46/9) Upper Cromwell Formation. Subsurface exploration data reflect the heterogeneity of the glacial deposits at the study site. Resistivity survey results and well log data suggest that the silty sand unit underlying the study area is overlain by a silt and clay-rich layer in the western and southwestern portions of the site. Previous studies indicate that this fine-textured layer places a slight confining pressure on the water-bearing silty sand unit. Water-level measurements made from observation wells reveal that groundwater flows from a groundwater divide toward the Wild Rice Lake Basin to the northwest at about 0.28 m/year, and the Miller Creek Basin to the southeast at about 1.05 to 1.46 m/year. Flow lines suggest that much of the groundwater flowing through the refuse is intercepted by the leachate collection system located in the southeastern edge of the landfill. Laboratory batch adsorption studies of heavy metals indicate that the local sediment has the highest affinity for Pb, followed by Cu, Cd, Zn, and Ni. The heavy metal adsorption data conformed to the Langmuir adsorption isotherm, suggesting that the heavy metal adsorption follows a kinetic rather than thermodynamic adsorption path. Elution curves produced from the step input of a heavy-metal spiked solution into silt loam and sandy loam sediment columns imply that Cu, Cd, Ni, and Zn migrate through the respective sediments at about the same rate, while Pb migrates through' the sediments at a significantly slower rate. These experiments suggest that the silt loam is more effective in retarding the heavy metals than the sandy loam. Sediment texture appears to be the main controlling factor in heavy metal attenuation: the finer grained a sediment, the more effective it is in attenuating heavy metals. An error-function model fit to the elution curves predicts that Cu, Zn, Cd, and Ni solute profiles migrate about 14 percent as fast as the groundwater in the silt loam and about 28 percent as fast as the groundwater in the sandy loam. Pb, which is strongly attenuated by the soil, was calculated to migrate about 95 percent slower than the groundwater in the silt loam and about 86 percent slower in the sandy loam. Due to the small amounts of heavy metals leached from the ash in water and acid extraction tests performed by WLSSD personnel, the sediments' relatively high affinity for Pb, Cu, Cd, Ni, and Zn, and the low groundwater flow rates observed at the study site, it is unlikely that the groundwater environment will be adversely affected by ash landfilling. To insure groundwater protection, three main steps should be taken: (1) the ash should be landfilled within areas where the groundwater flow direction is towards the leachate collection system; (2) the ash should be landfilled in areas where the underlying sediment has a high percentage of silt and clay; and (3) the ash should be kept separate from the other waste to minimize heavy metal chelation.


A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Martin Walter Wangensteen in partial fulfillment of the requirements for the degree of Master of Science, May 1988.

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Wangensteen, Martin Walter. (1988). The Hydrogeological Impact of Heavy Metal-Laden Ash Landfilling at the WLSSD Landfill, Rice Lake, Minnesota. Retrieved from the University Digital Conservancy,

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