Relevance of joint action toxicity evaluations in setting realistic environmental safe limits of heavy metals

The evaluation of types of toxicological interactions existing between heavy metals, which are prominent in effluents of some industrial establishments in Lagos State, Nigeria and the Lagos lagoon sediment was carried out against benthic animals, Tympanotonus fuscatus, Clibanarius africanus and Sesarma huzardi of the Lagos lagoon. In order to determine the type of interactions existing between the metals, acute toxicity tests of the metal compounds when acting singly and in joint action studies, by adopting mixture ratios that depict (i) the proportions of the concentrations of the metal ions in the sediment of the Lagos lagoon and (ii) equitoxic mixtures, i.e. based on the 96 hLC(50) values of the metal compounds under single action studies were carried out in laboratory biotests. The joint action evaluations of the test metal mixtures, prepared on the basis of the proportion of the availability of the metals ions in lagoon sediment and equitoxic ratio against the test animals agreed mainly with the model of antagonism (reduction in toxicity) except for test mixtures prepared on the basis of equitoxic ratio and tested against T. fuscatus, where interactions between the test metals was in conformity with the model of synergism, indicating that the toxicity of the constituent metals in the mixture was enhanced. Furthermore, on the basis of classification of synergistic ratio model, the toxic effect of Pb compound which was the least toxic metal compound in the single action toxicity studies was found to be enhanced (synergised) in the presence of other metals when tested jointly. The significance of the results in setting water quality criteria aimed at protecting aquatic biota was discussed.