[H-GM02] Geomorphology

[HGM02-P01] Arsenic in pore waters contained in natural sediment
at the northern part of the Kanto Plain, Japan

★Invited Papers

*Shoichi Hachinohe¹, Sushmita Hossain², Takashi Ishiyama¹, Hamamoto Hideki ¹, Chiaki T. Oguchi³ (1.Center for Environmental Science in Saitama, 2.Bangladesh Atomic Energy Commission, 3.Graduate School of Science and Engineering, Saitama University, Japan)

Keywords:Arsenic, Pore water, Natural sediments

The Kanto Plain is known as the largest plain in Japan, where marine sediments are widely developed because of cyclic iteration of global sea-level changes even 50 km or more inland from the present shoreline. In this area, dependence on groundwater for water requirements is relatively high; in particular, around 40 % of the municipal water supply is dependent on groundwater. Arsenic levels greater than that permitted by the environmental standards of Japan have been detected in groundwater in this area. Therefore, to evaluate occurrences of arsenic and other related elements in pore waters contained in natural sediment layers, we measured the levels of various inorganic chemical substances such as arsenic (As), iron (Fe), and sulfur (S) and major dissolved ions such as sulfate (SO₄^2-), calcium (Ca²⁺), and sodium (Na⁺). Pore waters were collected from sediment samples that were obtained by a drilling from the river bottom down to 44 m depth; pore water samples were obtained immediately after extraction of sediments. The sedimentary facies in the vertical profile are continental, transitional, and marine, including two aquifers. The upper aquifer (15–20 m) contains fine to coarse sand, whereas the lower aquifer (37–44 m) contains fine to coarse sand and gravel. Arsenic and other inorganic elements were measured by an inductively coupled plasma mass spectrometer (ICP/MS) and an inductively coupled plasma atomic emission spectrometer (ICP/AES), and major dissolved ions were measured by an ion chromatograph analyzer. The total content of chemical elements was measured by X-ray fluorescence analysis using solid sediment samples. We obtained the following results. The arsenic concentrations in pore waters in marine silt and clay sediments (approximately 40 mg/L) were about five times higher than that in continental sediments (approximately 8 mg/L). The highest concentration of arsenic (74 mg/L) was detected at a depth of 13 m, which is immediately above the upper aquifer. Visual observations confirmed that this level is under oxidizing conditions. Thus, it regards that arsenic was adsorbed to iron hydroxide in the sediments.