Opponents critical of a proposal to build a $4 million detention basin on part of the 98-acre Montini Preserve say the public process surrounding the government plan has been less than transparent.

Neighbors say they learned about the basin plan only in December, even though the February 2013 grant application to the state outlined the proposal in hundreds of pages, suggesting enough information was available at that time to update residents.

"It's inexplicable why the information was kept from the public for so long," said neighbor Mary Nesbitt.

The first community meeting to specifically address the proposal was Jan. 29.

"The first we learned about it as residents here was fliers that landed on our front porches" in December, said Jim Nelson, another neighbor.

But officials say there have been plenty of opportunities for residents to get a sense of the Water Agency's intentions. Public discussions about the project started with a publicly noticed October 2012 meeting of the Basin Advisory Panel in Sonoma, according to a timeline of the project provided by the agency.

Following the October meeting, the Board of Supervisors directed staff to craft a grant proposal in January 2013.

Before the grant was approved by the state Department of Water Resources in September, it would have been hard to discuss it more fully because there would have been nothing concrete to discuss, officials said.

"It's difficult to hold a public meeting and say we're going to be looking for opportunities for all kinds of projects," said Sonoma County Supervisor Susan Gorin, whose district includes the property. "The timing of all of this is challenging."

Supporters also emphasize the proposal is not complete.

"We don't know what it's going to be yet, its full design," said Richard Dale, the executive director of the Sonoma Ecology Center who lives near the site.

"There's been a lot of outreach, and there's going to be a lot of outreach as we move forward," said Jay Jasperse of Sonoma, the Sonoma County Water Agency's chief engineer.

A report on the site's hydrology is due in May. Another community meeting will be scheduled, likely in a few months, "when we have a more developed design that we can discuss," Water Agency spokeswoman Ann Dubay said in an email.

The debate has intensified in recent weeks in a series of back-and-forth exchanges between the Water Agency and opponents of the project. Water Agency officials, for their part, offered to meet with opponents and answer some of the questions. But Nesbitt and her allies want a wider public forum about the proposal.

In an sharply worded email, they said: "We are confident Sonoma citizens have more than enough intelligence to understand the full range and depth of questions, and answers on this issue at a public meeting that all are welcome to attend."