Study on drainage behaviors of toxic substances from mud sediments at excavation engineering

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The pollution of surface and/or ground water caused by the inflow of drainage water from mudstone excavation sites is a serious issue in Japan. It has been recognized that this issue results from the discharge of sulfate generated by the decomposition of pyrite contained in the mudstone. In general, marine mudstone includes the primary framboidal-type pyrite formed during sedimentation and diagenesis. The pyrite in mudstone discharges acid water into the environment by decomposition during the weathering process. Because the pyrite contains inorganic toxic substances such as arsenic and selenium as minor elements, the acid water from mudstone includes these toxic substances. Therefore, it is afraid that the surface water will be polluted by the acid water drainage containing toxic substances from rock muck disposal site where mudstone are disposed. The drainage water from mudstone-muck disposal site contains inorganic toxic substances in actually, and the concentrations of these substances decrease with time. To understand the behaviours of toxic substances in drainage water, mudstone samples are tested by batch-leaching test and column-leaching test. In the leachate at batch-leaching test, the pH is reduced and the electric conductivity(EC) increases with time. Oppositely, in the drainage water at column-leaching test the pH increases and the EC decreases. These leaching tests are modelled and analyzed by PHREEQC. The results of leaching tests are simulated under the condition, that pyrite, calcite and anorthite are dissolved and gypsum is crystallized in kinetic theory. Therefore, it is possible to presume the water qualities of drainage water from mudstone rock muck by numerical study. From these results, we can evaluate the chemical hazard of drainage water from mudstone rock muck disposal sites based on the geochemical characteristics and leaching features of mudstone and the numerical study for drainage water of rock muck disposal sites.