Emissions of two- and four-stroke outboard engines—II. Impact on water quality

Toxicity and odour threshold concentrations of water polluted by the emissions of outboard engines were analysed quantitatively using a test stand arrangement. Acute toxicity was determined by Daphnia magna and the bioluminescent bacterium Vibrio fischeri. The emissions of two-stroke engines into the water proved to be much more toxic than those of four-stroke engines of equal power. When a catalyst was used, toxicity was no longer detectable in the assays we tested. The use of different lubricating oils had only marginal effects on toxicity while the use of different fuels, unleaded gasoline and 96% ethanol, showed declining toxicity in the order stated. The toxicity of water polluted by a four-stroke engine and a catalyst equipped two-stroke engine was negligible after a period of 14 days while toxicity of water contaminated by a two-stroke engine persisted for more than 14 days. The acute toxicity of the test stand water could be explained by the presence of aromatic compounds. Mixtures of the major components (3-ethyltoluene, 4-ethyltoluene, pseudocumene, indene, benzaldehyde, naphthalene) when tested in concentrations in which they were observed in the test stand water gave a similar inhibition response in the bioassays as polluted test stand water. The effect of two-stroke engines on the odour threshold concentrations of water was found to be severe; e.g. a 15 kW two-stroke engine that operates for 1 h makes 11,000 m3 of water undrinkable. The odour threshold concentrations of C3 benzenes were determined.