Relevant Magnetic Parameters and Heavy Metals from Relatively Polluted Stream Sediments - Vertical and Longitudinal Distribution Along a Cross-City Stream in Buenos Aires Province, Argentina

Abstract

Previous magnetic and geochemical studies of stream sediments in the Northeast of Buenos Aires Province, in the Greater La Plata area, have revealed a high level of trace elements, especially lead and zinc. Continuing these preliminary works, new sampling sites were considered in order to improve and increase the magnetic mapping of this area. This part of the study focuses especially on the magnetic properties of sediments from a cross-city stream. Such stream receives a variable pollutant load from urban, industrial and diffuse sources. Concentration-dependent magnetic parameters, magnetic carriers (para, anti and ferrimagnetic) and their features (e.g. softer or harder magnetic carrier, grain size distribution, etc.) have been calculated using magnetic measurements. According to magnetic parameters and heavy metal loads two main distinctive groups of sediment-cores are distinguished. The vertical distribution of sediments reveals a recent anthropogenic influence, possibly, belonging to the last 20-40 years. Four out of all the magnetic parameters (magnetic susceptibility, anhysteric remanent magnetisation, S-ratio and κ ARM/κ) were chosen in order to investigate their relationship with contents of heavy metals. In all the cases, good positive correlations were obtained using linear regression. However, more significant correlation factors were achieved for grain size and magnetic feature-dependent parameters (κ ARM/κ and S-ratio) than for magnetic concentration-dependent parameters. Therefore, the κ ARM/κ and the S-ratio seem to be the most relevant magnetic parameters to describe the vertical and longitudinal distribution of heavy metals present in these stream sediments. The non-significant relationship between heavy metals and concentration-dependent parameters could be explained taking into account the discrimination of the two distinctive groups. The group belonging to the Coastal Plain shows a clear linear trend between heavy metals and magnetic susceptibility and anhysteric remanent magnetisation.