Seepage of residual oil previously leaked from PetroChina Lanzhou Petrochemical Co.'s 10.5 million mt/year (210,000 b/d) integrated refinery and petrochemical complex into the water source for water works was behind the unusually high benzene level in tap water in China's Lanzhou city, local media reported Monday.
Local environmental officials on Friday found residual oil in around 800 mt of water from deep pits dug on both sides of the ditch where the water pipeline of the city's sole water supplier Veolia Water, lies, government-backed China Gansu Net reported.
"The channel has been carrying water to Veolia Water's No. 1 and No. 2 plants for decades. Under this ditch lies Lanzhou Petrochemical's waste disposal pipeline," Yan Zijiang, Lanzhou's environmental protection chief was quoted by the official Xinhua news agency as saying.
No leakage was found on PetroChina Lanzhou Petrochemical's waste disposal pipeline, according to a report from National Business Daily.
The residual oil found in the water was believed to contain high levels of benzene, and had previously leaked into the soil following accidents at PetroChina Lanzhou Petrochemical's refinery in 1987 and 2002, according to preliminary conclusions drew by local environmental officials, the report said.
In 1987, an explosion at the refinery's R205 residual oil tank -- part of its 2.5 million mt/year crude distillation unit -- on December 28 resulted in leakage of residual oil from the tank, where 34 mt could not be recovered.
And on April 3, 2002, a fire broke out after an outlet control of the B-113 pump at the same CDU cracked, and the residual oil that was leaked into the soil was left untreated.
The CDU was idled in 2003 and eventually dismantled in 2006.
Refinery officials could not be reached for comment Monday. Benzene levels in Lanzhou's tap water on Friday rose 20 times above the national safety levels, affecting 2.4 million residents, authorities in the northwestern Gansu province's city said in a statement Friday.
Tests carried out in the early hours of Friday showed that the tap water contained 200 micrograms/liter of benzene, far exceeding the national limit of 10 micrograms/liter.
The levels have since dropped back down to where environment officials said were "confirmed safe" at five out of six water monitoring sites by 11 am Saturday (0300 GMT).
Benzene is an aromatic hydrocarbon that can be present in small proportion in some crude oils and produced through the catalytic reforming of naphthenes in naphtha, according to the US Energy Information Administration. It is used as a solvent in the manufacture of detergents, synthetic fibers, petrochemicals, and as a component of high octane gasoline.
PetroChina Lanzhou Petrochemical's refinery now has two CDUs and runs a combination of crude oil from the domestic Qinghai and Changqing fields, and imported crude from Kazakhstan.
It is one of four major refining and petrochemicals bases earmarked for development by state-owned China National Petroleum Corp., parent company of PetroChina, during the 12th Five-Year Plan running from 2011-2015.
But due to emission issues as well as problems with waste disposal, the refinery may have to relocate from its present site at Xigu to an industrial development zone, Platts reported previously in 2012.
However, the Lanzhou government and PetroChina have not yet agreed on a concrete plan for the relocation because of the hefty costs involved, refinery sources said previously in 2013.
