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

在日本，泥岩开挖场的排水流入造成的地表水和/或地下水污染是一个严重的问题。人们已经认识到，这一问题是由于泥岩中所含黄铁矿分解产生的硫酸盐排放造成的。海相泥岩一般包括沉积和成岩过程中形成的原生框架状黄铁矿。泥岩中的黄铁矿在风化过程中通过分解向环境中排放酸性水。由于黄铁矿中含有微量元素砷、硒等无机有毒物质，泥岩酸性水中含有这些有毒物质。因此，从处置泥岩的岩渣处置场排出的含有有毒物质的酸性废水，恐怕会对地表水造成污染。排土场排水中实际上含有无机有毒物质，且这些物质的浓度随时间的推移而降低。为了解有毒物质在排水中的行为，对泥岩样品进行了批量浸出试验和柱浸试验。在间歇浸出试验中，随着时间的延长，渗滤液的pH值降低，电导率（EC）增加。在柱淋溶试验中，排水pH值升高，EC值降低。这些浸出试验由PHREEQC建模和分析。用动力学理论模拟了黄铁矿、方解石、钙长石溶解、石膏结晶条件下的浸出试验结果。因此，可以通过数值模拟的方法对泥岩排渣排水水质进行预测。根据泥岩的地球化学特征和淋溶特征，结合排渣场排水的数值研究，可以对泥岩排渣场排水的化学危害性进行评价。

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.