SANTA BARBARA, Calif. (AP) _ Parched by drought and pressured by population growth, California utilities are looking to seawater desalting plants as a new but expensive source of fresh water.

''While the supply of water is limited, demand for water continues to increase. So desalination certainly has received renewed attention as a local source of water,'' said John I. Baum, a Santa Barbara consultant.

Desalination facilities now recharge and protect Orange County's groundwater and provide water for San Nicolas Island residents, Chevron U.S.A.'s Gaviota oil-and-gas processing plant and several electric power stations, including the Diablo Canyon nuclear power plant.

A desalting facility is under construction on Santa Catalina Island, and on tap for Santa Barbara. It also is under consideration in Marin and Monterey counties and by utilities serving millions of Californians.

The most expensive water now sold by the Metropolitan Water District of Southern California, the region's major water wholesaler, costs $230 per acre- foot. One acre-foot equals 325,852 gallons.

Desalted water costs $500 to $5,000 per acre-foot, depending on the process used and the cost of power to run desalination plants. Most estimates range from $1,000 to $2,000 per acre-foot.

The world now has about 7,500 desalination plants that produce 3.5 billion gallons of water daily, according to experts at a recent desalination seminar sponsored by the University of California, Santa Barbara's extension division.

UCSB engineering Professor Ekkehard Marschall said 60 percent of the world's desalination plants are on the Arabian Peninsula, 11 percent in the United States and the remainder spread worldwide.

At least 180 desalting plants operate in California, according to the International Desalination Association.

Most treat slightly salty brackish groundwater rather than seawater, and supply water to industry and electric power plants rather than water agencies.

Desalination isn't a panacea. On a daily basis, Southern California alone consumes water equal to the entire output of the world's desalting plants, said Dieter Emmermann, president of DK Consulting in Los Angeles.

Los Angeles Assemblyman Richard Polanco recently proposed legislation to raise $1.75 billion in bond money to build desalination plants capable of yielding 400 million gallons of water daily from seawater and brackish groundwater.

California's largest existing desalting plant, at Arlington in Riverside County, desalinates 7 million gallons of brackish groundwater daily. The water flows down the Santa Ana River to replenish groundwater in Orange County, said Jim Van Haun of the Orange County Water District.

The district's Fountain Valley desalting plant uses treated sewage to produce 5 million gallons of clean water daily, Van Haun said. The water is injected into the ground to keep seawater from reaching groundwater reservoirs.

The Metropolitan Water District, Southern California Edison and Los Angeles' and San Diego's water agencies are considering building a $1.5 billion cogeneration plant near Tijuana, Mexico.

It would be one of the world's largest, producing 100 million gallons of water daily and 500 megawatts of electricity for Southern California and the northern Baja Peninsula.

The Metropolitan Water District also plans to build a $50 million coastal desalting plant by 1996 to produce 5 million gallons of freshwater daily. If the plant is successful, it might be expanded to yield 50 million to 100 million gallons daily.

Santa Barbara plans to obtain up to 9 million gallons of water daily from a desalting plant that should start operating in 1992.

There are two major methods of desalting seawater. Saltwater can be boiled and the escaping steam condensed to produce distilled water. Or it can be squeezed through membranes that filter out salt in a process called reverse osmosis.

Baum said desalting plants do have environmental impacts, including discharges of toxic cleaning chemicals and concentrated brine, loud noise from reverse osmosis filters, air pollution from power plants needed to run desalting plants and population growth encouraged by increasing available water.