Oil spilling from the stricken ship Rena may cause short-term environmental damage, but the bigger headache may be containers floating just below the surface, an expert warns.
There were 1368 containers on board Rena before it ran onto the Astrolabe reef off Tauranga last week and started to spill fuel oil. The ship was thrown about in rough seas and 70 shipping containers were lost overboard on Tuesday night.
Following the experience with containers falling off the ship Napoli off the English coast in 2007, Simon Boxall, from the University of Southampton's National Oceanography Centre, said floating containers were potentially more worrying now than leaking fuel oil.
"Once they break away from the ship they present a hazard to shipping - often floating just below the surface and difficult to see and track."
The containers can remain afloat for weeks at a time before they fill with water and sink, and their contents should also be concern, Dr Boxall said.
"This could range from household goods to chemicals and in the case of Napoli there were several tonnes of herbicides amongst other materials."
Dr Boxall said that with just 350 tonnes of fuel oil spilled so far from the Rena when it was 12km offshore, the damage will be short term.
"The focus should be on securing the ship and its cargo and dealing with oil as and when it reaches shore."
The blobs of oil - emulsified oil often called mousse - can be collected by hand from the beaches, he says.
Dr Boxall also said there should not be any undue concern about using the oil dispersant Corexit 9500 as it was one of the less toxic dispersants, and only two tonnes had been used so far.
Chemicals in the Rena's cargo include hydrogen peroxide, potassium nitrate and alkysulphonic liquid and trichloroiscyanuric acid, which is highly toxic in water.
There is also ferrosilicon, which emits flammable gas when in contact with water.
However, Victoria University's Professor Neil Curtis told NZ Newswire that would happen slowly, and overall the chemicals were unlikely to cause any great problems if they fell into the sea because they would be so highly diluted.
"I can't imagine there's anything to worry about, compared to 1700 tonnes of oil."
Maritime New Zealand said none of the 11 containers listed as containing hazardous goods had gone into the sea.
NZN