Heavy metal signatures in urban and peri-urban agricultural soils across the Mumbai Metropolitan Region, India

Millennium Development Goals recognized the contribution of urban and peri-urban agriculture (UPA) towards food security, income generation, and livelihood strategies. Given the scarcity of relevant data, the present study was conducted to assess heavy metal load of UPA soils (at 0.00–0.20, 0.20–0.60 and 0.60–1.00 m depth) in Mumbai Metropolitan Region, India by comparing the signatures from soil profiles of three railway gardens (RG1–3) and three farms (F1–3) over 2 years. Potential human health risks of consuming produce from these soils were assessed using the contamination factor, degree of contamination, pollution load index (PLI), enrichment factor, geoaccumulation index (Igeo), and total metal and element content in comparison with different safety standards. Semi-sequential extractions were performed to determine the concentration of available elements and heavy metals for plants. The total concentration of heavy metals (Cr, Ni, and Sr) exceeded the critical thresholds in all surface soils, while the contribution of water-soluble and exchangeable fractions of Cu, Fe, Co, and Cr was negligible across the selected gardens. At the same soil depth, the PLI was highest for RG3 (3.6) at 0.00–0.20 m depth and lowest in RG2 (1.2). The Igeo value for individual elements ranged from 0.08 to 0.12 (Ni), 0.06 to 0.12 (Cr), 0.07 to 0.10 (Zn), 0.10 to 0.18 (Cu), and 0.24 to 0.34 (Co), whereas the value for Mn was 0.01 similar in all gardens. The soil pollution assessments by these indices revealed moderate to considerable (chromium and strontium) heavy metal contamination and accumulation, however, the origin of these metals remain unclear.