Radiological investigations of soil samples from neutral and agrarian lands

Abstract

Naturally occurring radionuclides (NORM) in the terrestrial environment constitute the majority of human radiation exposure. Soil not only acts as a source of environmental radioactivity but also serves as a pathway for radionuclides into the food chain via crops. Rapid industrialization and the widespread use of chemical pesticides and fertilizers have heightened concerns regarding radionuclide and heavy metal (HM) contamination in soils, necessitating a comprehensive evaluation. The Modern strategy to boost soil fertility for agricultural productivity by applying chemical fertilizers like NPK (nitrogen, phosphorus, and potassium), potash can elevate levels of natural radionuclides (238U, 235U, 232Th, 40K), contributing to contamination. Furthermore, artificial radionuclides such as 137Cs also pose a risk. These contaminants are absorbed by crops and enter the human food chain, leading to potential health hazards. A comparative study of soil profiles from neutral and agrarian lands in Ropar, Punjab, India, was conducted to assess natural and anthropogenic radionuclides, heavy metal concentrations, and their potential impact on human health. This thesis investigates radioactivity, heavy metal pollution, and the effects of different land use patterns, utilizing state-of-the-art techniques to analyze the radiological, mineralogical, and physicochemical properties of the soil. The radioactivity assessment was conducted using gamma-ray spectrometry with a carbon fiber endcap p-type high-resolution, HPGe detector. The mean activity concentration of 238U/226Ra, 232Th, 40K, 235U, and 137Cs, measured in neutral land was observed as 58.03, 83.95, 445.18, 2.83, and 1.16 Bq kg−1, respectively. However, in vegetation land, it was found to be 40.07, 64.68, 596.74, 2.26, and 1.90 Bq kg−1, respectively. 238U, 232Th, 235U form consistent hotspots across sites, while 40K and 137Cs show distinct spatial patterns influenced by land use and soil properties. Further, a comparison of elemental concentrations of radionuclides (238U, 232Th, and 40K), elemental ratios in soil samples from undisturbed and cultivated lands were assessed. The mean elemental concentration of 238U, 232Th, and 40K in uncultivated (undisturbed or neutral) land were found to be 4.70 ± 1.16 parts per million (ppm), 20.63 ± 4.53 ppm, and 1.44 ± 0.15%, respectively, whereas, in cultivated (vegetation) land, the concentrations were 3.25 ± 0.28 ppm, 15.90 ± 0.96 ppm, and 1.92 ± 0.25%, respectively. The study further compared the elemental concentrations and their correlation with different aspects of radionuclides in soil samples. Moreover, the elemental ratios 232Th:238U, 232Th:40K, and 238U:40K have been established to assess the depletion or enrichment of radionuclides within the soil samples from both sites. The soil’s physicochemical parameters (pH, EC, porosity) from both sites were measured, and their correlations with radionuclides were analyzed. The major elemental composition of the soil was determined using energy dispersive X-ray spectroscopy (EDX). Additionally, the surface morphology of soil samples from both locations was analyzed corresponding radioactivity. The radiological hazards associated with radionuclides in soil samples from undisturbed and cultivated lands were investigated to evaluate radiation exposure, assess their roles in hazard quantification, and identify health risks to the local population due to the existing radioactivity in the studied region. In the detailed activity analysis, radium equivalent (Raeq) radioactivity is within the safe prescribed limit of 370 Bq kg−1 for all investigated soil samples. However, the dosimetric investigations revealed that the outdoor absorbed gamma dose rate (96.08 nGy h−1) and consequent annual effective dose rate (0.12 mSv y−1) for neutral land, and the gamma dose rate (82.46 nGy h−1) and subsequent annual effective dose rate (0.10 mSv y−1) for vegetation land, marginally exceeded the global average. Dose contour maps confirm that radiological hotspots mainly arise from 238U and 232Th, with minimal 40K impact. Various heavy metals of health concern, specifically Cr, As, Cu, Co, Cd, Pb, Hg, Se, and Zn, were also assessed in soil samples utilizing the ICP-MS technique. Pollution Load Index (PLI) and Ecological Risk Index (RI) revealed that vegetation land was more anthropogenically contaminated than neutral land, with high contamination from As, Cd, and Hg. Contour maps of Ei indicate limited spatial overlap of As, Cd, and Hg. Overall, both land types exceed the highest PLI and RI grades, with cultivated land exhibiting markedly higher levels.