Abstract: A Test of Hydrocarbon Induced Magnetic Patterns in Soils: Utilization of the Sanitary Landfill as a Laboratory
Burke Burkhart, Brooks B. Ellwood
Magnetic susceptibility of soils has been studied in sanitary landfills, where upward fluxing methane has caused changes in magnetization of capping soils that were put into place, one, ten and twenty years before sampling. Susceptibilities increase progressively from uncontaminated control soils to samples with 20-year old caps, with the largest changes found below 40 cm depths. Authigenic minerals accumulate in landfill caps, with longer exposure to infiltration during reducing conditions producing higher magnetizations. Calcite and maghemite accumulate below 40 cm, iron and calcium having dissolved from above. Calcite accumulates during times of soil desiccation, forming a barrier to fluid transfer through caps that have distinctive zonations: Fe(II) accumulations abov and Fe (III) highs below. Also found are well established CaCO3 barriers separating redox environments.
Magnetic anomalies appear in caps exposed to high upward methane fluxes and periodic water infiltration, thus causing reducing environments favorable to magnetotactic bacterial growth. When microbial catalysis is high, Fe (II) is dissolved and transported deeper into the soil where it may re-precipitate as oxide or sulfide minerals phases that may or may not be magnetic. Precipitation of essentially non-magnetic Fe (II) phases during wet winters, followed by oxidation to magnetic phases during dry summers, has been previously observed in normal soils.
Our study demonstrates that sanitary landfills are useful for studies of natural soil magnetism, and are effective model systems for the study of magnetic effects above areas of light-hydrocarbon flux, such as petroleum reservoirs.
AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas