Plans to rezone nearly 500 acres of land that drain to Carroll County’s massive Liberty Reservoir are raising objections from Baltimore County and Baltimore City, which rely on the reservoir for drinking water.

Carroll County commissioners are seeking the zoning changes in the watershed as part of a broader effort to attract light industry to the mostly rural area northwest of Baltimore.

But officials from Baltimore city and county — including Mayor Martin O’Malley and County Executive C.A. “Dutch” Ruppersberger — say the plan will seriously threaten the quality of drinking water for 1.8 million people in the metropolitan area.

“We will not compromise the protection of our region’s high-quality drinking water to serve the economic development interests of Carroll County,” Ruppersberger wrote in a recent e-mail to a Finksburg resident.

Residents of South Carroll who live near two of the parcels slated for development also object to the zoning changes. They fear that developing the area will increase traffic and could contaminate their groundwater wells.

But two of the three county commissioners, Donald I. Dell and Robin Bartlett Frazier, say they’re ready to rezone the property, arguing that the county needs to increase its industrial tax base. During the past decade, residential growth in Carroll County has outpaced commercial and industrial development, leading to crowded classrooms and strained police and emergency services. “We’re planning in a way that will provide industrial property that is marketable, with water, sewer and roads,” Frazier said.

At 9,200 acres in size, the Liberty Reservoir watershed reaches into almost all of the areas that Carroll County has slated for planned growth, so Dell and Frazier argue that they don’t have much choice.

Five of the eight parcels proposed for rezoning are near Finksburg or other municipalities where development already exists.

Dell and Frazier have tried unsuccessfully to change wording in the Liberty Reservoir Watershed Management Agreement so that land can be rezoned for industry. The agreement is designed to protect land around the reservoir from overdevelopment.

O’Malley said redeveloping farmland for industrial purposes in the watershed doesn’t fit his idea of “Smart Growth,” Gov. Parris Glendening’s initiative to direct development toward existing communities.

“I’m not dead-set against redevelopment near Westminster and in accordance with Smart Growth,” O'Malley said. “But I think the better place for industry, frankly, is the city of Baltimore.”

Dell and Frazier have refused to sign the watershed management agreement, which was last renewed in 1996. Their fellow commissioner and board president, Julia Walsh Gouge, is in favor of signing it.