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The crash was reported on Riverview Drive at about 4:40 p.m.
CHP Officer Tom Maguire said it appears as though Mary Williams, of Bethel Island, crashed into a tree and was ejected from her dirt bike after she made a left onto Riverview Drive at a high rate of speed.
"According to witnesses, she was using the area as her personal racetrack," Maguire said.
She suffered major head injuries and was airlifted to John Muir Medical Center in Walnut Creek.
Maguire said Williams was not wearing a helmet at the time of the crash and that alcohol appears to be a factor.
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