Sonoma County industries produce a minuscule amount of the nearly 32 million pounds of toxic chemicals released in California in 2012, according to the U.S. Environmental Protection Agency’s latest report.

But across San Pablo Bay to the south lies a crescent of four oil refineries that makes Contra Costa County — better known for its tidy suburban communities — No. 3 among California counties as a source of toxic chemical pollution.

The four refineries — Chevron in Richmond, Phillips 66 in Rodeo and Shell Oil and Tesoro in Martinez — released 2.7 million pounds of toxics in 2012, the EPA’s Toxics Release Inventory said.

Adding the Valero refinery in nearby Benicia (Solano County) brings the San Francisco Bay Area refinery output to 3.4 million pounds, exceeding the combined release of 2.5 million pounds from the state’s 16 other refineries in Los Angeles, Kern and San Luis Obispo counties.

Put together, the 21 refineries released nearly 6 million pounds of toxics, accounting for 19 percent of California’s total industrial releases — and 42 percent of the releases into the air, the EPA reported.

The Bay Area refineries reported the following releases to the EPA in 2012: Phillips 66, 1.1 million pounds; Valero, 655,285 pounds; Chevron, 611,255 pounds; Shell Oil, 529,045 pounds and Tesoro, 507,714 pounds.

Sonoma County industries reported a total of 6,801 pounds.

Toxic chemical releases by California wineries have declined from more than 9 million pounds in 2007 to just under 6 million pounds in 2012, the most recent year reported by the EPA.

The EPA defines a release as the amount of a toxic chemical released on-site to the air, water and land, and the transfer of chemicals for off-site disposal.

Toxic release amounts alone are not sufficient to determine exposure or assess potential risks to human health and the environment, the EPA said.