Separate and joint toxicity to rainbow trout of substances used in drilling fluids for oil exploration

Paraformaldehyde, capryl alcohol, and 5 surfactants were lethal to rainbow trout at less than 100 mg/litre or μlitre/litre. They were the most toxic of 21 tested materials likely to be present in used well-drilling fluids. Least toxic were bentonite, barite, and several organics. Simulated ‘13-lb’ drilling fluid was lethal at 1·8% concentration, used fluids from the Mackenzie delta at 0·83, 4·2, and 5·3%.

There was no consistent picture for joint action in mixtures. Used fluids were lethal at 0·71, 1·2, and 0·83 times the values predicted from an assumption of additive toxicity of approximated components. Those results were in statistical agreement with additivity. The LC50 of the simulated fluid was 1·5 times the value predicted from its seven components, significantly less-than-additive. When the seven most toxic components were added singly to simulated fluid, about half the results were not different from additive joint action. However, for every component plus fluid, there was antagonism in at least one of three proportions tested. Strongest antagonism was 3·7 times the predicted value for the surfactant B-Free added to fluid. Decreased toxicity may have resulted from sorption of components on bentonite, or different toxic mechanisms for fish. Six of the seven most toxic components lost toxicity upon ageing for 16 days in water, while B-Free increased sharply in toxicity.