Magnetic biomonitoring of roadside pollution in the restricted Midagahara area of Mt. Tateyama, Toyama, Japan

Magnetic biomonitoring techniques and in situ topsoil magnetic susceptibility measurements have been shown to be rapid, cost-effective, and useful methods for investigating roadside pollution. However, combustible vegetation in samples makes it very difficult to use them in high-temperature magnetic experiments although the thermal alteration of spontaneous magnetization is a fundamental magnetic property and can be used to identify reliably the magnetic minerals. Here, we report the first magnetic biomonitoring results of dust deposited on plant leaves along the Tateyama-Kurobe Alpine route at the highly protected Midagahara wetland areas of Mt. Tateyama in Toyama, Japan. In-field magnetic susceptibility from 15 sites (161 points) shows higher susceptibilities near the roadside. Dust deposited on the leaves of Sasa kurilensis, or dwarf bamboo, has been wiped off at 12 sites (64 samples) with a commercial ethanol wipe sheet or silica wool damped in ethanol and subjected to rock magnetic analyses. Thermomagnetic remanence curves and low-temperature behaviors for samples collected near the roadside using silica wool show clearly that the main magnetic mineral in the dust is partially oxidized magnetite. Further, detailed rock magnetic analyses and elemental analyses of leaves in the study area indicate that (a) the magnetic mineralogy on leaves’ surface is consistent throughout the study area and (b) higher saturation isothermal remanent magnetization intensities as well as higher concentrations of Pb, Fe, Cr, and Y are observed near the roadside, i.e., the closer to the roadside, the more anthropogenic materials, including partially oxidized magnetite, are present. Also, microscopic observations show the lack of spherical grains, indicating that dust on the roadside leaves is derived from passing vehicle rather than industrial process. Both rock magnetic and geochemical results show that S. kurilensis would be an excellent candidate for investigating air pollution. Also, the proposed wiping-off method of collecting dust from plant surfaces is an effective non-destructive method that can be applied even in highly restricted collection areas, because detailed magnetic mineralogy, including superparamagnetic minerals, could be determined with the use of silica wool for use in both high- and low-temperature experiments.