Toxicity of sediment-incorporated drilling fluids

The 24, 96, or 168h LC₅₀s of four used drilling fluids or barite incorporated into sediment were determined in toxicity tests with lancelets (Branchiostoma caribaeum), a benthic chordate. Concurrent to LC₅₀ determinations, EC₅₀s were calculated from the number of lancelets that did not burrow into contaminated sediments. Observations of the burrowing behaviour allowed quantitation of effects after 24h exposures to each of the drilling fluids whereas lancelet mortality was sufficient to calculate 24h LC₅₀s for only one drilling fluid. Drilling fluids were less toxic to lancelets when incorporated into sediments than to mysids (Mysidopsis bahia) or benthic invertebrate communities in water-column exposures.