A new method for testing toxicity of drilling fluid; effect on growth of mussels

Four different oil-based drilling fluids were microencapsulated in a acacia/ gelatine structure. The microcapsules (size 1–10 μm) are dispersed in natural seawater in different concentrations, and fed to mussels in a running seawater system. One water-based drilling fluid was dispersed in seawater without microencapsulation. The growth in length of the exposed mussels was measured every 24 h for five days. Among the oil-based drilling fluids, the EC50 (5d) varied from < 1 to 66 ppm, and for the water-based drilling fluid EC50 ≫ 1000 ppm. There is no correlation between toxicity of the drilling fluids and their oil/water ratio, or between toxicity and the total content of aromatics in the base oils. The mussel test shows a markedly lower toxicity threshold than other tests with the same drilling fluids (Balanus-test, Microtox, Skeletonema-test). The results show that ingestion and digestion of the microencapsulated non-water soluble components of the drilling fluids may have a large impact on the overall toxicity, and that realistic estimates of specific toxicity have to include the effect of both particulate and dissolved fractions.