Pollution Detection by Magnetic Susceptibility Measurements Aided by the Stemflow Effect

The applicability of magnetic susceptibility measurements was tested for the detection of industrial pollution by fly ash in topsoil in a beech stand in the vicinity of a steel factory at Široka near Oravskyỳ Podzámok (Slovakia). The first trial measurements already showed that there is at least one spot near the trunk of each tree in which the susceptibility is significantly higher than anywhere else beneath the crown of the same tree. Detailed measurements showed that the susceptibility anomaly is associated with stemflow, and extends to some distance on the slope side of the trunk. Elsewhere beneath the crowns, at spots unaffected by stemflow, the variation of susceptibility is moderate. Samples were collected systematically from the topsoil around five trees in the beech stand, and another one elsewhere exposed to direct flow of fly ash from Široka. The results of measurements of low and high frequency susceptibility allow to conclude that in the beech stand where the ground is effectively shielded by the canopy from direct precipitation of pollution particles, all susceptibility values unaffected by stemflow are dominated by fine-grained magnetic particulates having formed by biogenic processes during pedogenesis, while those pertaining to the stemflow zone are dominated by larger multidomain particles conveyed to the ground in the stemflow. So, in this particular environment, it is the stemflow and its effect that makes magnetic pollution detectable by using susceptibility parameters. The case of the site exposed to direct precipitation of pollution particles is different in that that every susceptibility value irrespective of being small or large is dominated by large multidomain pollution particles. The heavy metal concentration of the soils investigated is elevated with respect to background levels of the Geochemical atlas of Slovakia, and have strong positive relationship, basically established by the effect of stemflow, with magnetic susceptibility. Without the stemflow effect the variation either in susceptibility or heavy metal concentration would not be sufficient to recognise existing relationships between them in the topsoil of the beech stand selected for the present study.