The Save Our Springs Alliance has filed a third lawsuit challenging Austin's plan to build a water treatment plant near Lake Travis.

According to the newest suit, filed late Friday, the city cannot legally carry out a key part of the project: digging a 30-foot-wide shaft on a half-acre piece of parkland in the Spicewood Springs neighborhood, then digging a six-mile transmission tunnel under a nearby nature preserve. Save Our Springs is asking a Travis County judge to stop work until a final ruling is issued.

"The city has violated state laws that protect parkland against misuse," said Spicewood Springs resident Joe Wheeler , a co-plaintiff who sold the half-acre property to the city after city codes effectively prevented him from building on it. Also joining the suit is the Spicewood Springs Road Tunnel Coalition, a collection of residents upset about the shaft project.

City spokesman Reyne Telles said the city had not seen the lawsuit. But, Telles said, "based on information Save Our Springs has released, we feel the city has fully complied" with the relevant portions of state law.

Critics say the $500 million-plus plant isn't necessary and could undermine Austin's water conservation efforts. City water officials and other proponents say that Austin will need additional treatment capacity as soon as 2014 and that a delay could cause the city to miss a chance to save on construction costs during a down economy.

The City Council gave the key approval in late 2009 to proceed. The city has cleared the land and begun design work and equipment purchases.

Lacking the votes to stop the project — the council has consistently voted 4-3 in favor of building the plant — Save Our Springs, an environmental activist group, has been the key party in three lawsuits.

One claimed the city violated the spirit of the Endangered Species Act by putting the Jollyville Plateau salamander at risk because it is likely to be deemed endangered in the next few years and the city proceeded without federal approvals required for endangered species. A federal judge dismissed that lawsuit in December.

A second suit claims the treatment plant cannot be built under the terms of a complex local and federal partnership created to manage the nearby Balcones nature preserve. That suit is pending.

The third suit, filed in state District Court, contends that Austin did not meet a key state standard necessary to drill on city parkland. That standard reads that "there is no feasible and prudent alternative." Plant opponents contend that if the plant isn't necessary, the prudent alternative to the shaft is simply not to dig it. The suit also claims the city published an unclear notice of a key public hearing and has not taken necessary steps to minimize potential damage to the environment.

City officials say they have met all state standards and that the plant is necessary. Not digging the shaft, they say, would raise the cost of the project by millions of dollars.