Seventeen years ago, the plume resulted in an out-of-court lawsuit settlement with 650 residents for $333 million.
The utility also continues to offer home buyouts, bottled water to residents and water-filtration devices.
As the years go by, PG&E says it has spent more than $800 million remediating the plume, settling lawsuits, paying attorneys, buying houses and farms, drilling new sampling wells and for frequent testing of those wells, which number more than 500.
How the cleanup is doneScientists from around the world come to see the massive cleanup project designed to contain Hinkley's environmental nightmare.
There's not a lot about the equipment used here that would necessarily wow anyone. In fact, if this project were shrunk, mounted on cardboard and entered in a science fair, any hope of a ribbon would ride on the report attached to the model.
There's no breakthrough in mechanical or electrical engineering.
PG&E has attacked the core of its chromium-6 plume with a strategy that mirrors the successful use in World War I of static lines of fortifications -- trenches and bunkers -- to hold back an advancing enemy.
This strategy failed disastrously in highly mobile tank warfare of World War II, but on the environmental battlefield of today, a half-mile-long barrier is decimating the chromium-6 lurking in the underground water.
How ethanol beats back chromium-6PG&E is on the attack at 58 entry points where ethanol is injected into the ground to stop the northward migration of the most concentrated part of the plume. The ethanol, a carbon food source, is used to feed naturally occurring microbes in the soil, so they thrive and multiply. These bacterial hordes suck oxygen out of the groundwater, setting up a chemical reaction that converts the carcinogenic chromium-6 into its more benign relative, chromium-3.
PG&E points to success using ethanol at the plume's northern boundary, where the chromium-6 is most potent.
Related: Buyouts and waterbottles
"Eighty percent of the chromium is here," said Kevin Sullivan, a PG&E environmental engineer, who heads up the company's Hinkley remediation program.
"Here" means the 284.5-acre site that surrounds the Hinkley compressor station, which from 1952 to 1964 used chromium-6 to protect metal and kill microbes in cooling tower water. Periodically, the water would be dumped into unlined ponds, where it seeped into the groundwater.
Using twin 10,800-gallon tanks of ethanol, the worst part of the plume has been beaten back from 9,030 parts per billion to 3,100 parts per billion.
That level is still 62 times California's legal limit for chromium in drinking water -- 50 parts per billion.
Had the Hinkley plume been located in a green area, like Oregon for example, there would have been sufficient numbers of microbes to begin this conversion process as the chromium-6 entered the soil, Sullivan said, explaining how geography has been a contributing factor to the seriousness of the Hinkley plume.
Hinkley is hot in the summer, so the compressor station went through cooling tower fluids faster than a similar operation would be in a cooler climate, he said.
On a tour of PG&E remediation efforts, Sullivan pointed to the half-mile barrier about a half-mile north of PG&E's compressor station where the utility company's most recent plume map shows distinct separation of the plume into northern and southern components. PG&E officials credit it, in part, to the effectiveness of the alcohol-remediation efforts.
"I believe that PG&E has adequately demonstrated hydraulic capture of the chromium plume in the groundwater south of Thompson Road based on information submitted in ... monitoring reports," Patty Z. Kouyoumdjian, executive director of the Lahontan Regional Water Quality Control Board, wrote in a letter last month. The Lahontan water board is one of nine regional agencies that oversee surface and underground water resources in the state of California.
While the main area of apparent plume expansion to the north appears to have been corralled by PG&E's technology, water board officials are not sure a relatively new finger-like extension of the plume's western edge is being controlled by the injection of fresh water into the aquifer. The water board's staff and PG&E's environmental engineering staff disagree on what might be occurring to cause a protrusion on the western edge of the plume.
Drip-drag irrigationA second method is used where groundwater registers chromium-6 contamination in smaller amounts than in the plume's core. The contaminated water is pumped up and deposited on the ground via center-pivot irrigation. These are composed of a long series of circular pipes mounted on a self-propelled wheeled structure, with sprinklers positioned along its length, generally high on the structure so that water is dripped on the ground.
To do what is called drip-drag irrigation, PG&E has installed long hoses with nozzles that touch the ground so water is applied at the surface.
Not only does this eliminate the risk for inhaling chromium-6, but it also reduces evaporation.
"I think, looking ahead, California will move to requiring this kind of delivery system to conserve water resources," Sullivan said.
Hinkley has no central water supply system. Each household depends on water from a well site located on the owner's property, a huge factor in compounding the plume's harmful effects on this community, Sullivan said.
PG&E, at the urging of the Lahontan water board, has developed a sophisticated -- and expensive -- water treatment system to "decouple" residents from the plume.
The board required the household water system to filter out all chromium down to the level of 0.06 parts per billion, which is the lowest level that can be measured with today's analytical tools.
"We have had requirements put on us that no one else in the country has to meet," said Jeff Smith, a PG&E spokesman based in Fresno.
In setting that upper limit for chromium so low, the board effectively mimicked the public health goal set by the state's Office of Environmental Health Hazard Assessment of 0.02 parts per billion. The public health goal is not meant to be an enforceable rule. It is the level where there will no more than one case of cancer per million people who drink two liters of water every day for 70 years. That "one-in-one million" risk level is widely accepted by doctors and scientists as the "negligible risk" standard, OEHHA, says on its website.
The public health goal is too low to measure with today's technology.