USING MAGNETIC SUSCEPTIBILITY TO DELINEATE HYDRIC SOILS IN ILLINOIS: EVIDENCE FOR MAGNETITE DISSOLUTION

Hydric soil identification has become increasingly important for wetland conservation and restoration. Standard field indicators have been used for this purpose, but can be biased or difficult to identify. Magnetite is destroyed under anaerobic conditions and neoformed ultra-fine ferrimagnetic minerals are preferentially preserved under aerobic conditions. Thus, magnetic susceptibility (MS) is higher in well-drained areas than in hydric areas. MS may therefore be a more objective method for identifying hydric soils than those currently used. We examined several sites with different parent materials in central, western, and southern Illinois. At each site, MS readings were taken every 30 feet on transects across transitional hydric boundaries of consistent parent material. MS readings were calibrated with traditional field indicators to obtain a critical MS value for identifying hydric soils. Early data suggest that this value is typically 30x10^–5 SI at central Illinois sites with loessal parent material and is 20x10^–5 SI at southern Illinois sites with alluvial parent material.

In laboratory tests, frequency dependent MS ranged from 0 to 10 %. Further analyses on grain size fractions of soils show the contribution of clay and fine silt (<8um) to MS ranges from 25 – 70 %. In preliminary examination by scanning electron microscopy (SEM), fly ash (spherical industrial particulates) was found to comprise 7 to 30% of the > 8um strongly magnetic particles. Samples with lower MS readings contain less magnetite and show more dissolution of fly ash and natural magnetite when viewed under SEM. The alteration of fly ash implies that significant magnetite dissolution can occur in less than 150 years.

MS values include proportions of their signal from neoformed magnetite and silt-sized detrital magnetite with a secondary contribution from industrial fly ash. Both the formation and dissolution of these magnetic particles affect MS under hydric and non-hydric soil conditions.