The magnetic susceptibility (MS) measurements are used for rapid and cost-effective soil surveys and for accessing heavy metal contamination worldwide. In the sub-Himalayan plains of India, nearly 6.05 × 104 km2 area of most the fertile land occurs as interfluve of Late Quaternary age between the two major glacier-fed rivers (Ganga and Yamuna). The vast areal expanse of interfluve terminates at the rivers' confluence in Sangam (25°25'13″N-81°53'22″E), Allahabad. This is the first study of MS soil survey of the interfluve region at the confluence comprising 490 samples from 49 locations. The MS values are between 8.84 and 261.25 × 10-8 m3 kg-1 and the change is more pronounced (8.84-312.65 × 10-8 m3 kg-1) with increasing depth. A sudden increase in the MS between 12- (11.28-303.32 × 10-8 m3 kg-1) and 14-cm (11.21-238.45 × 10-8 m3 kg-1) depth is observed similar to observations worldwide. The high MS hotspots are aligned parallel to major traffic networks of the city suggesting a major contribution emanating from the anthropogenic load. A significant difference has been noted in the MS values of present-day mid-channel bar sediments of Ganga (25.24 × 10-8 m3 kg-1) and Yamuna (116.47 × 10-8 m3 kg-1) Rivers. The laser-induced breakdown spectroscopy (LIBS) data showed the presence of heavy (Fe, Ti, Cr, Cu, Cd, Zn, and Pb) and light (H, C, N, and O) elements supporting MS data. The concentration of toxic elements predicted by partial least squares regression (PLSR) approach concurs with magnetic measurements. The topsoil MS values increase up to a depth of ~ 6.25 cm suggesting the dominant role of anthropogenic source for the increased heavy metal concentration compared with basement contributions.
Keywords: Heavy metals; Indo-Gangetic Plains; Interfluves; Magnetic susceptibility; Topsoil.