The Waikato River has not suffered any lasting effects from the chemical truck fire on Cobham Drive late last month, this week’s Environment Waikato Regulatory Committee heard.

Reporting on the incident and the river monitoring and testing which followed, Programme Manager Chris McLay said the day after the spill, staff found a highly visual pink foamy plume in the river at two outlets and minor fish kills in the two effected streams which flowed into the river. There were no fish kills in the Waikato River itself, and the pink plume disappeared about 50 metres downstream due to dilution.

While the pink dye was visually striking, it was a marker for animal health products and of no health concern. It proved useful for tracking the potential spread of contamination by other chemicals on the truck. An experiment to model the travel time of contaminants downstream from Fairfield Bridge was carried in 1975, using a much larger quantity of a very similar dye.

A rough inventory of chemicals in the truck indicated 242 kg of Formalin, 360 kg sodium hydroxide, 200 kg corrosive biocide, 44 kg pesticide, 2200 kg phosphoric acid, 250 kg ethanol/methanol 130 kg of aerosols and 4700 litres of paint.

Staff were initially concerned about the potential effects of the pesticide, biocide and solvents from the paint, which could have exceeded drinking water guidelines if the whole truck load of chemicals had been dumped in the river, and dilution was restricted. However, subsequent testing showed that although chemicals from the spill were detected in the discharge, none were detectable in the river at any time, due to dilution.

Chemicals detected in the discharge included copper, zinc and solvents from various paints, phosphoric acid, and low levels of a pesticide used in sheep docking. Concentrations of all chemicals in the discharge decreased rapidly with time, and were undetectable or back to normal levels within 60 hours of the spill, he said.

Remaining concern focused on potential contamination of soil adjacent to the truck fire, and sediments of the two effected streams. Further testing showed that soil within about five metres of the fire was contaminated with copper and some hydrocarbons, but not to levels that would pose a threat to human health.

Sediments of the two effected streams showed much lower levels of residual contamination than expected. Aquatic organisms of various types had already begun to move back into affected parts of the stream. Phosphoric acid in the spill prevented a substantial amount of residual contamination by lowering the pH, he said.

“Ironically, two of the chemicals carried on the truck on the night of the fire proved invaluable in this case: the marker dye for quickly estimating the extent of contamination, and the phosphoric acid for minimising any permanent contamination of the stream sediments,” he said.