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R9 Laboratory SOP 1000
SUSPENDED PARTICULATE PHASE OF DRILLING MUDS TOXICITY TEST USING
Mysidopsis bahia
Summary
This procedure describes the storage, handling, preparation, and toxicity testing of the suspended particulate phase (SPP) of drilling mud samples from offshore oil and gas facilities. The SPP is the phase used to simulate water-column effects present during the disposal of drilling mud. Exposure of mysids (Mysidopsis bahia) to several concentrations of this SPP is used to determine the concentration of suspended particulate phase drilling mud that results in a 50% reduction in survival relative the control (LC50) of exposed mysid shrimp. The procedure follows methods specified in the 40 CFR Part 435 Federal Register Volume 58, Number 41, March 4, 1993 (Attachment E) for NPDES oil and gas facilities. U.S. EPA Region IX Laboratory Section staff perform toxicity tests concurrent with permittee tests to support the NPDES Compliance Section.
A 1:9 drilling mud to seawater mixture is prepared by mixing whole drilling mud with 20 ppt seawater. This 100% Suspended Particulate Phase (SPP) solution is then pH-adjusted to 7.8 ± 1 unit. The 100% SPP is diluted to the appropriate concentrations bracketing the NPDES permit limit. Ten mysids in each of three replicates are exposed to each SPP concentration for 96 hours. The median lethal concentration that causes 50% mortality in the test organism population after this 96 hour period of exposure is calculated statistically. A parallel positive control test is conducted with sodium dodecyl sulfate.