Acute phytotoxicity of oil-based drilling muds

Cultures of the marine diatom Skeletonema costatum (Grev.) Cleve exposed to sea water extracts of various oil- and water-based muds and cuttings showed reduced photosynthetic capacity within a few minutes; mortality was generally low on the first day. Growth rate was the most sensitive indicator of toxicity, falling to zero even for cultures which appeared normal and physiologically intact. Exposure to aqueous solutions of 100–1000 ppm of diesel-based muds reduced growth rate significantly, while water-based muds exerted a similar effect only at 100 000 ppm. Various alternative (low aromatic) oil-based muds gave intermediate effects, not only dependent on the base oil, but also on the mud composition and on changes occurring during drilling. The toxicity of the muds and the derived cuttings was clearly correlated, even in 9 months old sediment layers of cuttings. Programme.