"During three days, we checked whether 30 vessels had the correct documentation. In 12 cases, documents were forged, or the environmental law was violated," Hans Tuinder, head of the Criminal Investigation department for the Marine Police, told Xinhua. Over 22,000 vessels are bunkered in the Netherlands annually, taking in around 13 million tonnes of fuel in total. The port of Rotterdam is a major trading location, the source of around 88 percent of all bunker fuel. A study conducted by the scientific institute CE Delft last year showed some ocean-going vessels were bunkered by fuel oil contaminated with hazardous waste materials. Fuel oil "naturally" contains a variety of hazardous substances in high concentrations.