Behavior of Trace Elements in the Tama River

Abstract

Trace elements dissolved in the Tama river were analyzed by an instrumental neutron activation analysis (INAA). River water samples were taken from 15 points of the Tama river, which runs through the Metropolis of Tokyo, about 6 times from 1980 to 1983. The water samples were activated for two methods, a solution activation method a n d a freeze-dried activation method. In the solution method, 30-50 ml samples in polyethylene bottles were irradiated at an irradiation pit (7.5×10¹¹ n.cm^-2.s^-1) in the Musashi Institute of Technology Research Reactor (MITRR). In the freeze dried method, 150-400 ml samples were frozen-dried at 40°C, and the dry residues were irradiated in a pneumatic transfer system (1.5×10¹² n.cm^-2.s^-1) or a central thimble (3.2×10¹² n.cm^-2 s^-1). Gamma-ray spectra for the irradiated samples were measured by a Ge(Li) detector coupled with a multichannel analyzer and a minicomputer (GAMA system). Twenty-five elements (Mg, Mn, Cl, Na, V, Al, Ca, Sm, Br, As, K, Cr, Sb, Sc, Fe, Co, Cs, Zn, Th, Sr, Rb, Se, Ag, Ba, Ni) were analyzed by these methods. As a result, we found that the concentrations of analytical elements at each sampling point were nearly constant independently of weathers and seasons. The concentration of most of these elements increased from the upper reaches through the middle reaches to the lower reaches, especially further increased from Hamura intake dam to the lower reaches. The elements dissolved in the Tama river were divided into two groups. The concentrations of Br, Ni, Mn, Fe, Co, Cl, K and Na changed greatly from upper reaches to lower reaches. The concentrations of Ca, As, Sb, Sr, Al, Mg, V, Ba, Zn and Rb did not change so much from upper reaches to lower reaches. The Br concentration increased from 2 ppb at the upper reaches to 240 ppb at the lower reaches, and the concentration ratio of Cl to Br decreased from 240-900 at the upper reaches to 50-190 at the lower reaches. This result suggests the influence of human activities.