Seoul City has accelerated its probe into the suspected pollution of the U.S. military base in the capital as part of its efforts to seek the source of oil leaks, officials said Monday.

The Seoul Metropolitan Government said it obtained 18 underground water samples among 32 boreholes in Yongsan Garrison in central Seoul last week. The central government will look into whether the samples contain toxic volatile substances such as benzene, xylene and toluene.

Korean environment officials suspect that the oil leaks, which were detected in 2001 around Noksapyeong Station near the U.S. base, originated from Yongsan Garrison.

If confirmed, Seoul City will demand that the U.S. military officials carry out decontamination work before returning the garrison land next year. The result of sample analysis is expected to be released later this month, officials said.

This is the second sample gathering in Yongsan Garrison following the first in 2001 after the oil leaks. Late last year, Seoul and Washington reached an agreement on on-site sample-gathering over oil detected in the neighboring region.

In 2003, both parties mutually announced that the oil near the subway station could “have likely come from the U.S. military base.”

For the clean-up measures, Seoul City has injected about 7.1 billion won ($6.4 million) in the region around the base, with the majority of the cost reimbursed by the central government through a lawsuit, they said.

According to the government’s report last year, about 12,000 square meters of land near the base is polluted, while more than 700 liters of underground water is contaminated.

Benzene was detected in the water with 0.836 milligrams per liter on average, the report added. This is about 55 times higher than the safe limit of 0.015 milligrams per liter.

Despite the city’s clean-up efforts, the average concentration of total petroleum hydrocarbons of the water also recorded 119.3 milligrams per liter, which is nearly 80 times higher than the standard limit. Its maximum concentration reached more than 4,400 times the limit, it added.

By Lee Hyun-jeong (rene@heraldcorp.com)