Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.  All Rights Reserved.
 

The magnetic susceptibility of soils has been studied at a sanitary landfill site, where upward-fluxing methane gas has caused changes in the magnetic mineralogy of the capping soils. Soil used as a cap on the Hillsboro, Texas, sanitary landfill was put into place 1, 10, and 20 years before sampling for this study. After 1 year in place, the susceptibility of the capping soil dropped below that of control samples not exposed to methane flux. Magnetic susceptibilities increased progressively from the control soils to the 10- and 20-year-old samples, with the highest values at depths of ~40 cm below the soil surface. New authigenic minerals accumulated in landfill caps, with longer exposure to infiltration during reducing conditions producing greater magnetic effects. Calcite along with maghemite, the principal authigenic magnetic mineral, accumulated below the 40-cm level, iron and calcium having dissolved from the upper soil of the landfill cap. Calcite also accumulated during times of soil desiccation, forming a barrier to fluid transfer. Landfill caps that have distinct zonation of Fe(II) minerals beneath those of Fe(III) are likely to have a well-established CaCO₃ barrier that separates redox environments.

Magnetic anomalies appear in capping soils exposed to high upward flux of methane and periodic infiltration of water, which produce a reducing environment favorable to the growth of magnetotactic bacteria. When the level of microbial catalysis is high, Fe(II) dissolved from the upper levels is transported deeper into the soil where it can reprecipitate as magnetic oxide or sulfide. Precipitation of nonmagnetic Fe(II) phases during wet winters followed by oxidation to magnetic phases during dry summers may take place, as observed in normal soils. Our study demonstrates that sanitary landfills can be used as convenient laboratories for studies of natural soil magnetism and are effective model systems for the study of magnetic effects in soils above areas of light hydrocarbon flux, such as petroleum reservoirs.