Mercury contamination in shallow groundwaters of Osaka Plain related to the recycle in the plate convergent margin

Abstract

Concentrations and stable isotope ratios of Hg of the groundwater and deep-sea cored sediments from Nankai Trough (from ocean floor down to 2200 m depth) were analyzed in order to reveal the relationship between Hg contaminated groundwater occurring along the active faults in Osaka Plain and dehydrated slab fluid from the subducting Philippine Sea Plate. Except 1 sediment (2150 m depth, 850 ppb), Hg concentrations ranged from 30 to 140 ppb, and δ²⁰²Hg and δ¹⁹⁹Hg were -0.8 to -0.2‰ and -0.2 to +0.05‰, respectively. δ²⁰²Hg and δ¹⁹⁹Hg of the five groundwaters were -0.8〜-0.4‰ and -1.0〜-0.5‰, respectively. All Hg isotope variation can be explained by the small mass dependent fractionation. The narrow and similar ranges of the Hg isotope ratios of deep-sea sediments and groundwaters may suggest the same origin of the mercury, which is recycling in the convergent margin.