Debate about the potential impact of California American Water's plans to draw source water for its desalination plant from the Salinas Valley groundwater basin has taken a new turn.

A new memo from a consultant hired by the state Public Utilities Commission contradicts conclusions about the basin in a report issued by an expert for an agricultural interest group. The report is at the core of concerns about the effect of additional pumping on the aquifer.

PUC Executive Director Paul Clanon last week asked the State Water Resources Control Board staff to evaluate the memo as part of its analysis of Cal Am's proposed Monterey Peninsula Water Supply Project.

The memo was released Feb. 6 by Geoscience Inc., a groundwater modeling consultant working with the PUC's environmental review team on its own evaluation of the water project.

The PUC asked the state water board last summer to conduct the project analysis. A draft report was released in December that suggested Cal Am could probably legally pump from the basin provided it could prove no harm to other water users and could figure out a way to return fresh water pumped from its desal feeder wells to the basin.

Cal Am's project calls for building a desal plant north of Marina that would draw feeder water from slant wells on the shoreline near the Cemex plant. The desal plant is at the center of the Peninsula water project, which is designed to provide a replacement supply to offset a state-ordered cutback in pumping from the Carmel River.

Critics of Cal Am's plans have expressed concern that pumping from the aquifer would exacerbate seawater intrusion in the already overdrafted basin, thwarting efforts to halt the inland march of saltwater. They argue the company is relying on flawed assumptions to support its plans.

In a December letter to the state water board, Salinas Valley Water Coalition president Nancy Isakson cited a report from the coalition's hydrologist, Tim Durbin, that challenged Cal Am's assumptions regarding the nature of the basin and potential impact of pumping. Isakson dubbed a Geoscience model relied on by Cal Am "seriously flawed."

The Peninsula mayors' water authority required Cal Am to address all issues raised in Durbin's report as a condition of their support for the project. The report was submitted to the PUC in December but rejected on procedural grounds with an invitation to resubmit as part of formal testimony expected this week.

However, the new Geoscience memo cited by Clanon assailed Durbin's analyses as based on "outdated information." The memo said existing groundwater modeling is being updated and new modeling is planned.

Cal Am spokeswoman Catherine Bowie said company officials are still analyzing the Geoscience memo, but it appears to validate the company's assessment that its plans wouldn't affect the basin.

Bowie said company officials support a "thorough review of the technical and scientific effects" of the proposed project, including a "robust" environmental review, analysis of test well results and development of a basin monitoring program.

"We understand this is a very sensitive and important matter for Salinas Valley growers and for the health of the Monterey County economy," she said. "The vetting of the Geoscience and (Salinas Valley Water Coalition) reports at this point will assist the parties to the CPUC proceeding to provide comments and testimony on the subject of the effects on the groundwater basin, and that is a positive development."

Isakson said Durbin is reviewing the Geoscience memo and would prepare a response for the PUC's evidentiary hearings in April.

She said the coalition is pleased new and updated groundwater modeling is planned as requested, but she said Cal Am must ultimately prove its plans won't affect existing water users.

"The bottom line is they still have to prove no harm to any water user in the basin," Isakson said, adding the state water board employs that standard. "It generally comes down to a battle among the experts, but in this instance, Cal Am must prove no harm."

State water board spokesman Tim Moran said the analysis of Cal Am's proposed water project would be released once the staff determines what additional information the PUC is requesting, perhaps in the next month.

Clanon asked for the state water board's analysis to address a number of issues, including recent changes to the project description and the potential requirement for returning fresh water to the basin. He expressed support for allowing the public to comment on the analysis and asked that a final report be made available by June 1.