SAN FRANCISCO (AP) - A top California oil and gas regulator has resigned amid questions about oilfield contamination of the state's water supplies.

SAN FRANCISCO (AP) A top California oil and gas regulator has resigned amid questions about oilfield contamination of the state's water supplies.

Mark Nechedom, head of the Department of Conservation, submitted his resignation Wednesday, and the state confirmed it on Friday.

Nechedom gave no reason in his resignation letter but spoke of directing the department during a "difficult time."

Nechedom's three-year term at the department saw a federal investigation into improper state permitting of oil-industry operations in what were supposed to be off-limits water aquifers.

State officials had no immediate comment Friday.

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This story has been corrected to show the state confirmed the resignation on Friday, not Thursday.