Open marine water (salinity 30–35°/00) is the environment where dispersants are used most frequently in oil spill response. In the Azerbaijan sector of the Caspian Sea, offshore oil and gas reserves are being developed in areas where salinity ranges from 10 to 12 °loo. Because salinity can affect dispersant efficacy and toxicity, the effectiveness and aquatic toxicity of six commercially available dispersants were tested using Azerbaijan crude oil, Caspian species and 12°/oo seawater.

Effectiveness for the dispersants tested with Chirag crude oil and Caspian seawater ranged from 72% to 86%, using USEPA's baffled flask method. Dispersant toxicities were in the ranges: diatom (Chaetoceros tenuissimus) 72 hr EC50 (effective concentrations inhibiting growth rate by 50%) 18 to > 100 mg/l; copepod (Calanipeda aquae dulcis) 48 hr LC50 (effective concentration for immobilizing 50% test organisms) 12 to 49 mg/l; amphipod (Pontogammarus maeoticus) 48 hr LC50 (concentration lethal to 50% test organisms) 50 to > 100 mg/l.

For dispersant use, the key toxicity concern is that of dispersed oil, not dispersant. Aquatic toxicity was determined for water—accommodated fractions (WAFs) of Chirag crude in Caspian seawater. Toxicity results for the WAFs were: diatom 72 hr EC50 > 10,000 mg/l nominal; copepod 48 hr LC50 3.9 mg/l; amphipod 48 hr LC50 >15 mg/l. Chirag crude was mixed with dispersant at 20:1 oil: dispersant ratio and resulting WAFs were tested for toxicity. Results were: diatom 72 hr EC50 < 18 to 208 mg/l nominal; copepod 48 hr LC50 2.1 to 37 mg/l; amphipod 48 hr LC50 20 to 89 mg/l. Dispersant and dispersed oil toxicity for Caspian species are similar to published toxicity data for marine species tested at typical ocean salinity.

Prolonged exposure (24 to 96hrs.) to constant concentrations of dispersant or dispersed oil used in laboratory tests may overestimate potential field toxicity, where dilution and mixing can decrease concentrations to low ppm's within hours of application. Dispersant use decisions for any Caspian Sea oil spills will focus on net environmental benefits of moving oil into the water column where it can be quickly diluted compared to potentially greater impacts from oil reaching nearshore environments.

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