Spectroscopic Analysis of Tungsten Mining-Associated Heavy Metals and Radionuclides and Evaluation ofImpact on Agricultural Soil in Selected Areas of Rwanda

Abstract

The aim of this studywas to evaluate the impact of tungsten mining on degradation of agricultural soil quality in selected regions of Rwanda. The determination of heavy metals (As, Pb, Mn, Cu,Zn, Co, Cd, Ni, Cr and Hg) content and radioactivity measurements of primordial radionuclidese38U, 232Th and theirdaughters and 40K) concentrations was performed. Energy Dispersive X-Ray Fluorescence (EDXRF) and HPGe gamma-ray spectroscopy methods together with multivariate analysis technique were used in this study. Soil samples of tungsten minerals, tailing and agricultural soils were collected from Nyakabingo, Gifurwe, Rwinkwavu and Ntunga and analyzed forradioactivity of 226Ra (238U), 232Th and their daughters and 40K and heavy metal contents respectively. External exposure dose rate was evaluated from mathematical models and theresults compared to the global averages. Since run- off was assumed to be the main source of contamination dispersion downstream from the mines, control samples were collected upstream for analysis and the results compared. Tile elemental contents in soil samples were classified into two groups; namely major constituents of soil, such as Fe, Mn, Ti and Zr and heavy toxic metals associated to tungsten mining, i.e W, As and Pb which were found to be in traces, perhaps is due to the association of arsenopyrite and tungsten minerals in quartz veins. The presence of Wand tailings is attributed to poor extraction processing methods used by the artisanal miners. Arsenic (As), in agricultural soil, was found in the following concentrations; 157 mg/kg, 163 mg/kg, 98 mg/kg and 55 mg/kg in Nyakabingo, Gifurwe, Rwinkwavu and Ntunga respectively. This is above intervention value (or clean -up criterion) of 50 mg/kg. The average dose rate for farm land around the mines was found to be 136.7 n Gyfh (2.5 times above the world average). This may be attributed to high activity in mine tailings and in the soil samples from the adjacent areas of these mines. Multivariate analysis techniques, such as Principal Components Analysis (PCA) with VARIMAX rotation and Hierarchical Cluster Analysis (HCA), were used to explore the general trends and interrelationships in the data sets, from which arsenic and radioactive soil contamination was found to be strongly associated with tungsten mining activities.