Soil Magnetic Susceptibility for Road Emission Monitoring in Beni Mellal City (Morocco).

Environmental magnetic methods based on the study of variations in magnetic susceptibility and rock magnetic parameters have been successfully used over several decades to characterize and quantify the degree of pollution of air, water, vegetation and land systems. These prompted us to undertake joint magnetic and geochemical investigations of road-side urban soil materials to address the environmental pollution of Beni Mellal city that has been subjected to environmental stress, due to population overpressure and related urbanization.

In order to quantify the degree of environmental pollution using magnetic method, magnetic susceptibility of soil and sediment out-side and along 5 km of the national road N8 (formally named 20 aout) has been measured.

All samples collected near roads have higher values of magnetic susceptibility and heavy metals (especially Pb) contents that those collected far from roads. These results show that both ferrimagnetic minerals and heavy metals come from the same pollution source. The most probable candidate for this type of relation between these parameters is road traffic, road station, and car park for large machine.  The influence of road traffic pollution can be seen in magnetic susceptibility data where high values are found in sample collected near main roads.

In areas far from roads, median magnetic susceptibility is between 30–50 * 10^-5 SI, similar to that observed in the valley filling clastic sediments and hence consistent with geologic or pedogenic origin. Intraverses of in situ susceptibility across roads, a 10-m wide zone situated on either side of the asphalt-paved road exhibits an enhancement zone with maximum susceptibility of 250 and 400 * 10^-5 SI occurring 0.5–1 m from the road edge.

In urban recreational areas, magnetic susceptibility varies with in abroad range (5-50 * 10^-5 SI) with lowest values occurring 50 m from surrounding roads, in areas least disturbed by human activity. Asystematic increase in susceptibility towards the roads sites is observed. Within urban areas, in the vicinity of heavy traffic or industrial sites, the upper 30–50 cm of soil profiles exhibit frequent enhancement in susceptibility, of two orders of magnitude, higher than those expected from geologic input. Such enhancement is attributed to input from anthropogenic sources. Magneto-mineralogical analyses and scanning electron microscopy on magnetic extracts, grain size fractions of bulk samples of road dust and soils, suggest lithogenic magnetite-like minerals and anthropogenic magnetic spherules to be the dominant contributors to the magnetic susceptibility signal.

As clear relations have been shown between the Mössbauer parameters and the chemical neighborhood of iron atoms, the Mössbauer Spectrometry (MS) is expected to be particularly fruitful for the study of geological samples. In this work, we have used the MS in order to establish the different Fe phases present in the samples. The Mössbauer spectra were measured at room temperature on suitable absorbers with conventional source drive and recording device. The spectra, collected for five samples of different magnetic susceptibility, are calculated by superposition of magnetic components and two doublets. The computed values of the hyperfine field, the isomer shift, the quadrupole splitting and the relative subspectral areas of each contribution will be presented and discussed.