Spatial distribution, sources and risk assessment of potentially toxic trace elements and rare earth elements in soils of the Langtang Himalaya, Nepal

Soils in the fragile Himalayan region could be affected by the transport and deposition of potentially toxic trace metals (PTEs) from urban and industrialized areas of South Asia. The transported pollutants could pose a serious threat to the soil quality in the pristine regions at high elevations having minimal direct human influence. Therefore, it is important to understand the geochemical and physical characteristics of soils in this region and determine the extent of their chemical pollution. In order to achieve these objectives, soil samples were collected from different elevation transects of the Langtang Himalaya in Nepal. The samples were analyzed for PTEs and rare earth elements for the purpose of identifying their possible sources and to evaluate their environmental risk in the region. The PTEs and REEs concentrations were measured by ICP-MS (X-7; Thermo-elemental, USA) and total organic carbon (TOC) by TOC analyzer. The results of this study were comparable to those of the world average background soil as well as the Tibetan plateau surface soil. TOC revealed a decreasing trend with increasing elevation. Correlation analysis and principle component analysis (PCA) indicated that most of the elements were highly associated with major crustal elements, suggesting that their primary sources were of natural origin. Furthermore, the geo-accumulation index (I_geo), enrichment factor (EF) and pollution index (PI) analyses indicated that the Himalayan soils represent minimal pollution and the data from this study may be used as background values for the Himalayan region in the future studies. REEs in the soil samples were found to be consistent with an order of average abundance of the Earth’s crust. In addition, the chondrite-normalized REE distribution of the light REE suggested enrichment of LREE and Eu depletion. Moreover, this study emphasized that soils of the Himalayan region could, in future, be under threat of elemental pollution from long-range transport via atmospheric circulation and deposition.