Pre- and post-monsoon variations in the magnetic susceptibilities of soils of Mumbai metropolitan region: implications to surface redistribution of urban soils loaded with anthropogenic particulates

Abstract

Mumbai metropolitan region (MMR) in India represents one of the most industrialized and thickly populated areas of the monsoon dominated Asian region. We present here pre- and post-monsoon magnetic susceptibility variations in the top-soils representing sampling domains of industrial, heavy traffic and forested areas within MMR. The rock magnetic studies (including isothermal and anhysteric remanent magnetization and hysteresis loop analysis) infer predominant pseudo single domain to multi domain grains in an overall ferrimagnetic dominant mineralogy of the soils. The susceptibility-temperature variations (from −190 to 700°C) infer maghemite (γ-Fe2O3) as the chief mineral component of pedogenic origin, and the pure magnetite (Fe3O4) is of anthropogenic nature. Spatial distribution of ferrimagnetic concentration is in agreement with polluting sources. The post-monsoon redistribution pattern is greatly controlled by the surface runoff and topographic conditions. The study demonstrates that in a ferrimagnetically reach substrate like MMR, the spatial distribution patterns derived from routine concentration- and grain-size-dependent rock magnetic parameters integrated with topographic and seasonal attributes yield significant information on the style and surface re-distribution of anthropogenically loaded soils and sediments to identify its seasonal dumping. Alternatively, knowing the source of signal, the magnetic susceptibility can be further used as a robust parameter to produce detailed maps to monitor the pollutions in urban areas.