New homes within an 81 square-mile study area in the Helena Valley may require more advanced and expensive septic systems, after the Lewis and Clark County Commission on Tuesday unanimously approved amendments to the county's controversial interim zoning regulations.

Not everyone within the 81 square miles, which run roughly from the city limits into the North Hills, will be required to install premium septics. The area has been divided into 40-acre increments, and officials estimate a quarter of the lands will require Level II septics, 28 percent will need Level I-b systems and 41 percent can still have standard septics. Most existing homeowners are grandfathered in.

Around 5 percent of the land, mostly near waterways, is considered high-risk and is off limits to any kind of septic system.

Citing increased levels of nitrates in groundwater in parts of the valley, the commissioners pushed ahead with the amendments.

Commission chairman Ed Tinsley said the three commissioners are obligated to protect the "health, safety and welfare" of valley residents, and that these amendments are a step in that direction.

He noted that even within the 40-acre increments, individual property owners can challenge the type of system they're required to use, if they have a site-specific engineering study that refutes the county's broader engineering analysis.

Commissioner Andy Hunthausen said that with 10,000 to 25,000 more people expected to populate the valley in the next quarter-century, now was the time to act.

"Essentially we are flushing our waste into our drinking water, and while in some cases it might be site-specific, it is happening," he said. "We have to make responsible decisions about how we protect our groundwater ongoing."

Commissioner Mike Murray noted that nitrate levels in the area, which were at .5 parts per million in 1973 had soared to 4.0 parts per million in 2000.

"The (Department of Environmental Quality) standards in the Helena Valley are not getting the job done," he said. "This is an eight-(fold) growth in nitrate levels in our valley. Something obviously is happening, and that's what's energizing us to do something."

No public comment was taken at Tuesday's meeting, but comments at earlier meetings indicated that people who were unhappy with initial attempts at interim zoning were somewhat placated that the commission was taking a more measured approach with its amended interim regulations.

Tinsley said that in addition to interim zoning, plans for a septic maintenance district are moving through the process, and comprehensive zoning is not far behind.

Reporter John Harrington: 447-4080 or john.harrington@helenair.com