Vertical Variation of Lead Content in Sediment Collected from Man‐made Tamagawa Dam Lake in Akita Prefecture, Japan

Abstract

Alternation layered lake sediment that had accumulated over a period of 22 years, from 1990 to 2012, was collected from the Tamagawa Dam Lake, which is located in the northeastern part of Akita Prefecture. The lake water is acidified (pH = 4.1) by the inflow of high‐acidic thermal water (pH = 1.2) from the Obuki Hot Spring, the main hot spring in the Tamagawa Hot Spring area. The vertical variations in Si, Al, Fe, Ti, and Pb contents of the sediment were determined by XRF, EPMA, and LA‐ICP‐MS in order to clarify the sedimentation processes of the lake sediment. The layers consisting of the sediment could be mostly classified into three types: dark brown, yellowish brown, and reddish layers. The contents of Si, Al, Fe, and Ti corresponded to the variation in color of each layer in the sample. Based on the Si, Al, and Fe contents in the sediment of Tamagawa Dam Lake, the sources of these elements were classified as detrital origin in dark brown and yellowish brown layers (Si, Al, and Ti) and chemical precipitate origin in reddish layers (Fe). Detrital components were derived from volcanic rocks in the watershed of Tamagawa Dam Lake. The variation of Pb content did not accord with the color of layers. The content of Pb in the sediment of Tamagawa Dam Lake ranges from 45 to 522 ppm. The vertical variation of Pb content in sediment corresponded to the temporal variation of Pb content in thermal water from the Obuki Hot Spring from 1990 to 2012. A large influence of hydrothermal activity of the Tamagawa Hot Spring on the vertical distribution of Pb was found during the active period of the Obuki Hot Spring, resulting in high Pb content within layers at 5–8 cm from the bottom of the sample. T‐Fe2O3‐rich reddish layers were also found in this range. Therefore, it is assumed that T‐Fe2O3 and Pb originated from Obuki Hot Spring and precipitated in Tamagawa Dam Lake. However, no correlation was found between T‐Fe2O3 and Pb contents for any of the dark brown, yellowish brown, and reddish layers. Some layers with high Pb content were also found to have high SiO2 and Al2O3 contents. These findings indicate that there are several possibilities for the sedimentation process of Pb. The sedimentation of Pb as well as that of T‐Fe2O3 in Tamagawa Dam Lake provides a good example of the accumulation of elements by chemical precipitation away from the source of elements.