A new study completed for the Army Corps of Engineers casts doubt about whether Newport News needs a huge new reservoir that would draw water from the Mattaponi River to slake its thirst in the future.

The study by Illinois-based Planning and Management Consultants Ltd. suggests that the city will have less than half of the 40 million gallon a day shortfall that it projects by the year 2040.

That’s mainly because the city’s projection of population growth is too high and the anticipated savings from conservation are too low, according to the study.

City officials immediately challenged the assumptions used in PMCL’s study.

“We do not believe our estimates of need are high, but if they are, then the supply from the reservoir will simply carry us further into the future,” said Randy Hildebrandt, Newport News assistant city manager. “However, if we follow PMCL’s lead and underestimate demands through 2040, the risk is total system failure — an unacceptable consequence for our customers. Dependability and reliability are what our citizens and our customers expect from their water utility.”

Critics, though, see the report as contributing to a “sea change” in opinion about the project, said Billy Mills, executive director the Mattaponi and Pamunkey Rivers Association.

“From our perspective, the PMCL study constitutes a credible and viable third opinion on the entire set of issues and questions about alleged need for the project, and it constitutes an independent peer review by a highly respected national group selected by the Corps for their objectivity,” Mills said. “At this point, it’s hard to find anyone in the community who will talk enthusiastically about supporting this project. It just seems to be coming unglued at the seams.”

The issue is one of a growing number of disputes around the Bay watershed dealing with water supply, as expanding metropolitan areas seek adequate water for anticipated growth.

The new study was the latest twist in the ongoing controversy over the Newport News’ Waterworks plan to build a $121 million, 1,500-acre reservoir in King William County to meet the growing water needs of the Williamsburg-Newport News-Hampton area through the year 2040.

The plan calls for a 3.8-mile-long reservoir on Cohoke Creek, a tributary of the Pamunkey River, by 2005. Up to 75 million gallons of water a day would be drawn from the neighboring Mattaponi River to fill the reservoir.

Numerous environmental groups, the adjacent Mattaponi and Pamunkey tribes, King and Queen County and others oppose the plan. They not only challenge the need for the reservoir, but say it would flood hundreds of acres of wetlands, claim it could change the salinity of the Mattaponi River, and warn that it could threaten Native American cultural resources at the site as well as rare plant species.

In December, the Virginia State Water Control Board approved a permit for the project, but added several limiting conditions. Newport News filed suit challenging the conditions and several environmental groups have filed suit challenging the issuance of the permit. Arguments over those cases were to begin in early summer.

Critics of the project say the new study strengthens their argument questioning the need for the reservoir, and say it provides grounds for the State Water Control Board to reopen the permit process.

“The permit was based on a certain set of facts, and like any other permit, it is subject to review by the issuing agency when there is new information,” said Roy Hoagland, an attorney with the Virginia office of the Chesapeake Bay Foundation. “So the question is, will the State Water Control Board do what it is supposed to do and reopen the permit, accept the report, look at that data and revise or revoke the permit?”

The report by the consultant questions whether the population growth and development in the region will create a deficit of nearly 40 million gallons a day by 2040, as Newport News has suggested.

The PMCL study suggests the shortfall will only be half that large — only 16 million to 19 million gallons a day. The difference, according to PMCL, is that Newport News is overly optimistic about future growth and pessimistic about the yield of existing supplies and the ability of conservation efforts to reduce demand.

PMCL is an outside consultant selected by the Corps after questions were raised about the Newport News estimates by opponents of the project and other independent studies.

City officials were preparing a response to the report. But they criticized the study for not using the most recent Virginia data available, and for applying national water demand figures in determining water needs rather than regional figures.

Despite the permit issued by the state, the project cannot go forward unless the Corps also issues a permit. A final decision on that will not come before the end of the year at the soonest, said Pamela Painter, who is heading the Corps’ effort to evaluate the project.

This summer, though, the Corps will likely decide whether to require a supplement to a previously published final Environmental Impact Statement on the project, Painter said. Such a supplement would summarize the findings of studies since the original statement was issued.

If the Corps decides to require a supplement, a final decision on the project would be postponed for months because the supplemental EIS would require two rounds of public comment, and the Corps would have to analyze the comments received before issuing a decision of record.