Occurrence of hydrogen sulfide gas (H2S) is one of the most important engineering geological hazards during tunneling. Its hazards and consequent challenges are very difficult and costly to solve. During site investigation, one of the tasks for engineering geologists is prediction and evaluation of the risk of H2S gas in the underground spaces. In this study, water conveyance tunnel of Aspar, which was excavated in H2S-bearing environments, is discussed. The tunnel is excavated in the hydrocarbon formations. Applied experiments suggest that geological formations pertaining to hydrocarbon resources are crucial in formation and reservation of H2S gas. This paper briefly discusses hazards and geological sources of H2S, as well as remedial measures for decreasing the risks and problems in excavation of the tunnel. To predict the risk of H2S gas in the underground spaces, it is possible to use some precursors such as: sulfur springs, organic traces, organic argillaceous rocks, exposure of H2S odor from fresh surface of rock and smell of H2S during boreholes drilling. Controlling the inflow of groundwater into the excavation, diluting the concentration of H2S, training the personnel and utilization of some proper safety equipment have been used to mitigate risks and problems in tunnel excavation.

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