New Tollway Environmental Report Tells Good and Bad News : Transportation: Agency officials say they would mitigate effects on wildlife. Owners of upscale homes watch warily.

Kicking off what promises to be a spirited debate over the proposed Eastern tollway, authorities unveiled a key environmental document Monday that says the $630-million highway will help ease air pollution and traffic, but will cut a swath through pristine hills and displace a rare songbird and plant.

The environmental impact report, which will be the subject of a public hearing Dec. 12 in Irvine, also says that one proposed alignment for the tollway would meander within a quarter-mile of the upscale Cowan Heights neighborhood, as well as homes in Irvine.

In addition, the toll road would uproot 23 acres of oak woodland, destroy nearly two acres of wetlands and affect 270 acres of coastal sage scrub, which hosts dozens of sensitive plants and animals.

Among those are the California gnatcatcher, a bird being considered for federal protection. The tollway, which would add 23 miles of new pavement to the county’s grid of freeways, is slated to run on or near territory occupied by 15 of the birds.

Envisioned as a four-lane road that could eventually be expanded to eight lanes, the tollway skirts the Riverside Freeway in the north and splits into two legs as it heads south into Irvine. The western leg would run under the Santa Ana Freeway before connecting with Jamboree Road. The eastern leg would funnel traffic onto the Laguna and Santa Ana freeways.

Officials of the Transportation Corridor Agencies contend that they can offset most of the highway’s effects on the environment.

They plan to set aside nearly 700 acres in a regional park, which they say would mitigate many of the environmental costs, including the destruction of gnatcatcher habitat. In addition, authorities will scoop up and replant the more than 4,000 “many-stemmed dudleya”--a rare plant indigenous to Orange County--lying in the path of the road.

Although bulldozers will cut swaths as deep as 200 feet through the hills along some rugged stretches to make room for the road, authorities say the visual impact will be lessened by carefully carving the terrain, rounding off the tops of hills to make them look more natural. Native vegetation would be replanted.

In addition, tollway officials plan to split the road along some segments to make the grading work less severe. Block walls and earthen berms will be designed to ease the noise in surrounding areas.

The study found that overall air quality would improve because the highway would free the flow of traffic on other streets and freeways, reducing volumes by upward of 30%. Carbon-monoxide standards would be exceeded in Irvine near Jamboree Road, but that pollution is still less than if the road wasn’t built, the report suggests.

Despite the various efforts to ease the road’s impacts, authorities may be hard-pressed to assuage the concerns of nearby homeowners, in particular well-heeled residents clustered in the hills overlooking the highway.

Although no homes or businesses would be taken to make way for the Eastern tollway, residents all along the route have already begun marshaling their forces against the highway.

The focus for many remains the road’s impact on air quality, noise and aesthetics, said Chris Elliott of the North Irvine Villages Assn., a coalition of 17 homeowners groups in that city.

“Whether or not those concerns have been taken care of, we don’t know yet,” said Elliott, who had not had a chance to read through the 15-inch-high stack of volumes that make up the environmental report. “I hope they have been. I hope this is a clean document.”