Abstract
The pollution of surface/ground water caused by the inflow of acid water including heavy metals poses serious environmental problems around the mine areas and excavation sites. The acid water is generated by decomposition of sulfide minerals such as pyrite. Because the Hakkouda Tunnel in northeast Japan passes near the mine area, it is feared that water pollution around this tunnel occur due to the acid water drainage from rock muck disposal site. Therefore, at excavation of this tunnel, the rock muck was disposed after evaluation of acid water drainage ability. We assessed the evaluation method of the ability of acid water drainage of rock muck from the results of long-term leaching test of rock samples and the leachate water quality monitoring at rock muck disposal sites.
From water qualities of leachates drained from disposal sites, we contend that the evaluation method could discriminate the rock muck that have high probability of acid water drainage from the others rock muck. S/Ca mole ratio was most effective to judgment in acid water drainage ability of mudstone. In the case of volcanic and pyroclastic rock, most contributory item was sulfur content in rock.
The leachate water from the controlled rock muck disposal site did not become acid, and heavy metal content of the leachate water was lower than the standard of discharged water. Therefore, it seems that the evaluation method of rock muck overestimated the ability of acid water drainage. However we consider that the method was reasonable in consideration of recent social situation for the environment around the civil working site.
Based on the results in the Hakkouda Tunnel, we suggest the examination for evaluation of ability of acid water drainage from rock muck on the civil working around the mine areas as follows. First, the distribution area and/or length of rocks with high-risk of acid water drainage and the volume of controlled rock muck must be estimated correctly by the horizontal drilling examination in advance. Next, at the excavation, the geology of cutting face must be checked and the water quality of leachate water from rock muck disposal site must be monitored. It is effective that the rock muck having high risk of acid water drainage is disposed in the controlled disposal site similar to the industrial waste disposal facility. However, it is necessary to examine the structure of disposal site that have high economical efficiency and same effectiveness concerning acid water drainage controlling.
