Hydrogen sulfide (H2S) can be present in produced water discharged from production platforms along the Southern California Outer Continental Shelf (OCS). An assessment of the toxicity of H2S to marine organisms was conducted to determine if adverse effects could occur due to California OCS produced water discharges.
Hydrogen sulfide toxicity is attributable to the undissociated H2S, which comprises approximately 3 - 8% of the total discharged sulfide in seawater. The dissociation of H2S is a function of temperature, pH, and salinity. H2S is not persistent in seawater and rapidly degrades by reacting with oxygen to form nontoxic thiosulfates. The oxidation half-life of H2S in seawater is reported to be between 0.4 to 3 hours. Data in the scientific literature indicate that marine species are less sensitive to H2S than fresh water species. Lethal effects to marine species have been reported to occur at concentrations as low as 20μg undissociated H2S/L in 96-hour laboratory exposures. No observable effect concentrations based on results from chronic toxicity bioassays that assess growth and developmental effects have been reported as low as 4 μg undissociated H2S/L.
Produced water data from three production platforms were analyzed to determine the potential impact of H2S on marine life passing through the effluent plume or residing outside of the mixing zone. Calculations of the undissociated H2S at the edge of the mixing zone, taking into account natural oxidation of H2S to nontoxic thiosulfate, and comparing those concentrations with conservative chronic toxicity values, demonstrate that H2S in California OCS produced water discharges should not present a hazard to marine aquatic life.