A proposed change in state septic system rules that would open to development 9 million acres of currently unbuildable land -- about a quarter of the state -- was decried Wednesday by southeastern Wisconsin officials as a catalyst for urban sprawl.

"Its intent is to promote growth and eat up farmland."

West Bend Mayor Mike Miller told legislators the citizens of Wisconsin and their heirs would be the losers if the rule change is implemented because it would allow homes to be built on environmentally sensitive land.

The innocuous-sounding modification to the state plumbing code regulating private wastewater treatment systems has set off a far-reaching debate over land use planning, environmental issues and preservation of the state's rapidly dwindling acres of prime farmland.

The rule change is known as Comm 83 for its place in the rule book of the Department of Commerce, which proposed it. The change is endorsed by many farm organizations, rural interests, builders and Realtors.

The change would make legal nine different designs for private on-site wastewater treatment systems, an expansion from the current four designs.

It would allow construction of new septic systems on land with six to 24 inches of native soil -- areas where now only holding tanks are permitted under state law.  Many counties and other municipalities have banned new holding tanks, making land with thin soils unbuildable.

"We think it's important to have alternatives for unsewered areas that are environmentally safe and that are working in other states," said Rick Stadelman, executive director of the Wisconsin Towns Association.

He said the proposal would help preserve farmland by allowing homes to be built on marginal soils, easing development pressure on prime agricultural land that best accommodates septic systems the state now requires.

But municipal officials argue that the rule change would set off a building free-for-all by making it possible to build homes on lands that previously could not be built upon because the soil did not meet state standards for septic system construction.

Dennis Kornwolf, town chairman of Caledonia in Racine County, said towns should be given the opportunity to opt out of the new rule if it is implemented.

"We want orderly growth, and Comm 83 won't help," he said.

The Wisconsin Alliance of Cities and some environmental groups said the septic system designs permitted under Comm 83 will not meet ground water standards and could cause the pollution of drinking water in areas where they are used.

But Commerce Department officials said new septic system technology is effective in treating waste and, if operating properly, will not harm ground water quality.

Commerce Department Secretary Brenda Blanchard told the Assembly committee that state code governing septic systems needs to be updated to keep up with changing technology.

"Wisconsin has fallen behind in the implementation of this technology," she said.