HYDROGEN SULFIDE (H2S) HAZARDS

Low levels of H2S gas may be encountered in cores in which microbial sulfate reduction is occurring or in cores from hydrothermal vent areas in which seawater sulfate has reacted with high-temperature intrusions. H2S is a highly reactive (and toxic) gas, which is normally maintained at negligible concentrations by rapid reaction with iron or other metals present in the sediment. In some cases, such as iron-deficient carbonate sediments and where gas hydrates or sulfate-rich brines are present, H2S concentrations may be sufficient to cause a toxic gas breathing hazard, especially when cores are being handled in confined or poorly ventilated areas. In these situations, the JOIDES Resolution is equipped with H2S sensors on the rig floor, the core receiving platform, and in the interior core-handling areas. H2S levels of 10-20 ppm should activate audible and visual alarms and bring about an H2S alert. H2S odor is apparent at levels below those detectable by sensors or analytical instruments. If H2S presence is suspected, the hydrocarbon monitoring techniques should include analysis by thermal conductivity detector gas chromatography (NGA) to provide information on concentration levels and any trends of H2S occurrence in cores.

Procedures for responding to an H2S alert are fully described in ODP Technical Note 19, Revised H2S Drilling Contingency Plan (Foss and Julson, 1993 [currently under revision]).

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