HINKLEY >> By the side of a washboard-rough dirt road, in a garage piled high with rocks neatly stored in pizza-sized boxes, the long-awaited study has begun to determine how much of the world's largest chromium-6 plume is the result of a San Francisco-based utility's operations and how much was put there by nature. The findings of this pioneering scientific effort that will span five years could add or subtract multiple millions of dollars from the ultimate environmental cleanup cost in this High Desert community.