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ANCHORAGE - The network of pipelines spanning Prudhoe Bay, the nation's most productive oil field, was widely criticized last year following a pair of high-profile crude oil spills.
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Now an upcoming state study shows that all kinds of spills, from crude to gasoline to tainted water, have occurred with regularity during the past decade on the remote wetlands of the Arctic tundra.
Oil companies reported about two million gallons of toxic spills from 1995 to 2005 at the massive Prudhoe Bay field and surrounding region known as the North Slope, according to the report provided to The Associated Press by the state Department of Environmental Conservation.
The study worries environmentalists who say the spills are chronic and have damaged the vast network of lakes and ponds to an unknown degree. But state environmental officials and some scientists say there is scant evidence that spills have caused long-term damage.
"The immediate effects really look bad, but usually they disappear pretty quickly," said Jay McKendrick, professor emeritus at the University of Alaska Fairbanks. He advises oil companies on replanting techniques following a spill. "The long-term effects on the tundra are pretty minimal."
Home to polar bears, grizzlies, caribou and millions of migratory birds, the 89,800 square mile North Slope encompasses what environmentalists call one of the most delicate habitats in the world.
The region is slightly larger than Minnesota and stretches from the Arctic Ocean south to the Brooks Range. Besides Prudhoe Bay, it contains nearly two dozen other fields, most operated by BP PLC, ConocoPhillips and Exxon Mobil Corp.
A large breach of seawater, which can kill slow-growing tundra plants, accounted for about half of the reported amount spilled. The 994,400-gallon spill occurred in March 1997 on a gravel pad in the eastern section of Prudhoe Bay and did not cause lasting damage, officials said at the time.
Salt water is found in oil and gas deposits beneath floor of the Arctic Ocean and wells to the surface through undersea pipelines. It is separated out and reinjected into oil formations.
Salt water that had not been separated from the oil and gas mixture was second with 349,274 gallons spilled. Crude oil was third, with 103,397 gallons reported.
Of the 4,481 spills reported, 89 percent of them measured less than 99 gallons. Environmentalists worry because few studies have documented their cumulative effects.
"What's important is that many of the spills may be small, but they're chronic and they're continuing to happen," said Pamela A. Miller, Arctic coordinator for the Northern Alaska Environmental Center.
In addition, there is little data on water quality before exploration and construction began at Prudhoe Bay in the late 1960s, making changes difficult to measure, Miller said.
"There hasn't been major documented damage, but it doesn't mean that's something to ignore," Miller said. "If there's a major spill, it has the potential to harm bowhead whales, polar bears and migratory birds."
One study by the National Research Council in 2003 found no evidence that spills caused enduring harm to the tundra. That study said damage to plants and animal populations from ever-expanding infrastructure is much more lasting and widespread. Of the 250 sites the state has labeled "contaminated" on the Slope, most are permanent installations such as roads, drilling rigs, pipelines, buildings and gravel pads.
"There are certain plant species up there that tend to have tolerance to oil so they come back and take care of the vegetation problem pretty quickly," McKendrick said.
Several state officials said the spills' effects on wildlife have been minimal.
"Based on spills reported, we haven't really noted any lasting impacts to wildlife per se," said Leslie Pearson, prevention and emergency response manager for the Department of Environmental Conservation.
Minor spills have occurred in the eight Alaska Native villages in the region, and military and aviation companies have also been responsible. But the bulk of pollution has come from the oil industry. Crude from the Slope makes up about 15 percent of U.S. production and seven percent of consumption.
The largest crude oil spill on the North Slope occurred last year, outside the time period studied. Any long-term effects of the 201,000-gallon spill from transit lines at Prudhoe Bay, discovered in March 2006, have yet to be documented. A smaller spill in August prompted BP, which manages Prudhoe Bay, to temporarily halve production at the field and replace 16 miles of pipelines at Prudhoe Bay.
DEC officials sheared off the oiled mat of vegetation damaged in the winter spill and replaced it with pads of tundra removed during construction on a mining site.
The site, which is just under two acres, is in the early stages of recovery.
"There are patches of green sprouts alongside significant areas that are still black, with nothing growing," said Tom DeRuyter, an environmental program specialist for the agency.
The U.S. Environmental Protection Agency and the Justice Department are investigating whether BP's pipeline maintenance program violated the Clean Water Act.
"Most of our spills on the North Slope are small drips and leaks, but we are also prepared to respond to the most catastrophic event," BP said in a statement.