Magnetic measurements and geochemical characterization of the road-deposited sediments (RDS), Allahabad city, India: impact of climatic perturbations on sediment composition and provenance tracing

Abstract

Magnetic measurements provide quantitative data on urban pollution (including heavy metal content) which correlate significantly with geochemical analysis and better constrain the source character of pollutants. The present study is aimed to map the distribution of heavy metals in road-deposited sediments (RDS) and identify the possible provenance of the sediments during the pre-monsoon and post-monsoon (mainly fog) periods of Allahabad city, India. In all, 442 RDS samples from 60 representative locations were collected over 3 years, and the different magnetic parameters and metal concentrations were determined. The magnetic minerals were identified using X-ray diffraction (XRD) and backscatter electron-scanning electron microscopy (BSE-SEM) while principal component analysis (PCA) was used for the provenance tracing of the RDS. The samples record high magnetic susceptibility (χlf; $$ \overline{\mathrm{x}}: $$ 174.44–604.36 × 10−8 m3 kg−1), saturation isothermal remnant magnetization (SIRM; $$ \overline{\mathrm{x}}: $$ 1586.30–2421.51 × 10−5A m2 kg−1) and anhysteric remnant magnetization susceptibility (χARM; $$ \overline{\mathrm{x}}: $$ 461.64–1099.68 × 10−8 m3 kg−1) values independent of seasonal change. The magnetic minerals in the sediments lie in pseudo-single domain (PSD) and multi-domain (MD) from anthropogenic sources while stable single domain (SSD) is associated with both natural and anthropogenic sources. PCA of magnetic proxies with heavy metal concentrations suggests the derivation of the RDS from multiple (crustal, anthropogenic, traffic, and industrial) sources. The XRD analyses confirm the presence of magnetite and hematite, in addition to quartz, feldspar, gypsum, apatite, calcite, and dolomite in the RDS. Limited geochemical analysis of the RDS comprising Na, Mg, Al, K, Ca, Ti, Mn, Fe, Sc, V, Cr, Ni, and Rb suggests a predominance of anthropogenic load in RDS compared to geogenic sources. The magnetic measurements coupled with XRD and BSE-SEM analyses suggest the dominance of magnetic grains and heavy metals in pre-monsoon samples compared to those collected during the fog period. The geochemical data also support the seasonal control for the elevated heavy metal concentration in the RDS and their derivation from geogenic as well as anthropogenic sources.