Assessment of heavy metal and soil organic carbon by portable X-ray fluorescence spectrometry and NixPro™ sensor in landfill soils of India

Landfill agricultural soils are common worldwide and pose risks to human health via heavy metals common in such systems. Similarly, anthropogenically deposited organic matrices may lead to methanogenesis in saturated landfill systems. In contrast to traditional methods of landfill agricultural soil characterization, the present study employed a combination of portable X-ray fluorescence (PXRF) spectrometry, NixPRO color sensor, and geostatistical methods to identify hotspots and total spatial area in excess of environmental thresholds in landfill soils of Kolkata, India. Of the 335 points surveyed, Zn, Mn, Cu, and Pb exceeded threshold limits at all sites. Indicator and ordinary kriging were employed to show the spatial variability in elements characterized in this study. Four different pollution indices were calculated and spatially compared: geo-accumulation index (Igeo), enrichment factor (EF), contamination factor (CF) and ecological risk factor (ER). Organic carbon levels ranged from 1.27 to 10.57% with a mean of 4.02%; levels substantively higher than naturally occurring soils of the area indicative of anthropogenic enrichment. The NixPro™ sensor predicted soil OC reasonably well (validation R² = 0.70) without using depth as an auxiliary predictor (a common approach in earlier studies). Summarily, the use of these two portable sensor systems was shown effective at characterizing landfill agricultural soils. Future work should extend the results of this study to in-situ testing and a wider diversity of soil types to corroborate the conclusions presented herein.