"It is important that we have a clear and up-to-date understanding of the chromium plume boundaries," Patty Kouyoumdjian, the executive officer of the Lahontan Regional Water Quality Control Board, wrote in a letter to Kirk Howard, PG&E's vice president of gas transmissions and distribution.
"This critical information will guide us as we clean up groundwater pollution from the PG&E compressor station and will ensure protection of public health in the community," she wrote.
Separately, the water board on Friday cited PG&E for providing bottled water to residents that exceeded chromium 6 levels set by the agency.
The board also cited PG&E for failing to provide evidence that the whole-household water system it has installed in two Hinkley homes met the same chromium standard.
The agency was seeking corrective actions only - fines were not part of the official notice of violation.
In the 1950s and 1960s, PG&E used the chemical chromium 6, also called hexavalent chromium, to prevent rust and algae buildup in its Hinkley cooling towers.
The chemical, widely used before its cancer-causing properties were known, was discharged into unlined ponds and from there entered the groundwater.
The order to PG&E, released on the water agency's website late Wednesday, said that based on a conservative migration rate of 2 feet per day, the length of the plume could be 7.32 miles from north to south.
The plume is known to be at least 6 miles long and 2 miles wide.
The northern boundary of the plume has been expanding rapidly in recent years, although some water board scientists believe the plume has been much larger than believed for some time.
Hinkley resident Daron Banks said, "I think the order is on the right track ... The intentions are good."
PG&E is reviewing the order, said spokesman Jeff Smith. "We always try to be responsive to the water board," Smith said.
The order sets the following deadlines:
Among the points PG&E will be reviewing is whether it can meet the deadline schedule.
In the past, well sampling on the northern edge of the plume has been significantly delayed because of concerns about desert tortoises and other endangered species, Smith said.
Late last year, the water board directed PG&E to submit a plan for the definition of and monitoring of manganese plumes created from the treatment of heavy concentrations of chromium 6 near its Hinkley compressor stations.
Manganese is a by-product from the injection of ethanol into the ground, which reduces chromium 6 to the relatively harmless chromium 3.
However, according to water agency documents, the manganese plume potentially threatens nearby domestic wells.
PG&E has a Feb. 15 deadline to produce its plan to study the manganese plume.
The water agency plans a public meeting at Hinkley's combined elementary and junior high school in March so the community can give feedback on its orders - and PG&E's responses - for the expanded plume monitoring and its order to define the manganese plume, said Lauri Kemper, assistant executive officer.
Water-borne manganese has been associated with mental impairment and reduced intelligence. The chemical may play a role in the development of some cases of Alzheimer's, Parkinson's and other neurodegenerative diseases, according to health officials.
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