Pacific Gas and Electric Co. was ordered by regulators this week to find out why toxic chemicals from a cooling tower spill have leached even deeper into the water supply of the Mojave Desert town of Hinkley.

The order, by the Lahontan Regional Water Quality Control Board, requires the San Francisco utility to dig monitoring wells and determine how cancer-causing hexavalent chromium got into the lower aquifer, where many residents get their drinking water.

"It's a serious issue," Lauri Kemper, the assistant executive officer for the Lahontan board, said Tuesday. "It is a formal investigative order, but it is not necessarily considered punitive. If PG&E were to fail to meet the deadline then they would be subject to penalties."

The four-page order was issued the same day The Chronicle revealed that the plume of contaminated water underneath this San Bernardino County farming community was still expanding two years after the utility had promised to contain it.

The creeping plume of hexavalent chromium, also known as chromium 6, was first exposed by Erin Brockovich, a paralegal at a Southern California law firm whose court battle on behalf of sickened residents against PG&E was recounted in a 2000 movie starring Julia Roberts.

Most people thought the issue had been resolved after a series of multimillion-dollar settlements, but higher than normal levels of the compound were detected over the past year in groundwater more than a half-mile beyond the previous boundary. In August, levels well above background levels were found in the lower aquifer, which until then had been considered clean.

"I am furious. I am fit to be tied," said Roberta Walker, the original plaintiff in the 1992 lawsuit that was settled in 1996 on behalf of 600 Hinkley residents for $333 million. The film "Erin Brockovich" focused on that case.

"PG&E said they started to clean it up, but they stopped and it has continued to move," said Walker, who had to move her family out of their house in the middle of the plume. The family moved 7 miles away. "The plume is now about a mile and a half away from us, heading our way again."

Hexavalent chromium was used by PG&E between 1952 and 1966 to fight corrosion in the cooling towers at a compressor station about 12 miles west of Barstow. The wastewater from the cooling towers was discharged into unlined ponds and the contaminated runoff percolated into the groundwater.

Dozens of townsfolk reported falling sick, and some allegedly died from exposure to the carcinogen. Breast cancer, Hodgkin's disease, lung, brain and gastrointestinal cancers, miscarriages, and kidney and ovarian tumors were blamed on the poisoned water.

PG&E allegedly tried to cover up the mess and its effects, leading to the court battle.

The plume has grown to 2.6 miles long and 1.3 miles wide and extends 0.6 of a mile farther northeast than it did two years ago. Regulators said increased levels of chromium have recently been detected in a dozen more domestic drinking water wells and in the lower aquifer, an area of groundwater separated from the upper aquifer by a layer of clay.

Representatives of PG&E said the levels of chromium 6 in the new locations were never above California's safe drinking water standard.

The goal, however, is to reduce chromium levels to naturally occurring background levels. "These concentrations exceed background concentrations and show an increasing trend over time," wrote Kemper, in support of the control board's investigative order.

Jeff Smith, a spokesman for PG&E, said the utility staff intends to comply with the order.

Meanwhile, anger among the locals continues to grow amid reports of sick residents.