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| Ellwood,
B. B., and B. Burkart, 1996, Test of hydrocarbon-induced magnetic patterns
in soils: the sanitary landfill as laboratory, in D. Schumacher
and M. A. Abrams, eds., Hydrocarbon migration and its near-surface expression:
AAPG Memoir 66, p. 91-98.
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Abstract
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.
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