By Robert Kozak 

LIMA, Peru--President Ollanta Humala has thrown his weight behind the stalled $1.4 billion Tia Maria copper project in southern Peru, sending delegations of cabinet ministers to persuade reluctant local residents of the benefits of having a mine.

For almost a month, farmers, anti-mining activists and local politicians have marched and blocked roads in the Islay province, saying they want to protect their fields in a long river valley near where Southern Copper plans to dig out 120,000 metric tons of copper a year.

The protests against Tia Maria echo other fights between anti-mining groups, farmers and mining companies over the last few years over who gets to use precious water supplies in bone dry areas of Peru.

"The people here have made the decision that the mine won't go ahead," said Pepe Julio Gutierrez, a protest leader in a group known as the Defense Front for the Tambo Valley.

Carlos Monge, Latin America Director for the Natural Resource Governance Institute, said that Southern Copper erred by first proposing to use surface and subterranean water before opting for costly desalination of ocean water. By then the opposition to the project had hardened.

"The project appears to me to be socially and politically dead," Mr. Monge said.

The government put the project on hold in 2011 after violent protests led to three deaths. The company reworked its environmental impact study, or EIA, and the government approved that study last year.

Southern Copper chief executive Oscar Gonzalez Rocha said this week that mining at Tia Maria will start in the middle of 2017, once a construction license is granted. Mexico's Grupo Mexico is the majority owner of Southern Copper.

Peru's Energy and Mines Ministry issued a statement this week saying that the company has guaranteed that it won't touch water to be used for farming, and that dust from the mining process will also be controlled.

In a meeting with the foreign press, Environment Minister Manuel Pulgar-Vidal ruled out organizing any referendum to decide the fate of the project, as some opponents have demanded.

The dispute over Tia Maria is similar to one in northern Peru over a copper and gold mine project known as Minas Conga, majority owned by Denver-based Newmont Mining Corp. Violent protests led Newmont to suspend construction work on the $5 billion Minas Conga project in 2011 while it builds some water reservoirs.

Some of the anti-mining activists who led the fight to shut down Minas Conga are now working to stop Tia Maria. Political analysts say that local politicians organizing before next year's general elections are also seeking to boost support by rallying against big mining companies.

"We won't permit violent groups to come here to stop developments," Prime Minister Pedro Cateriano said after holding meetings this week in southern Peru. The government has sent thousands of police officers to keep the order in the region.

Lower mineral prices are hitting Peru's mining industry, cutting exploration and investments, causing the government to strongly support any investments that meet government standards.

"The Humala administration, however, has little capacity to solve disputes between the firm and local communities, and therefore its ability to push for the project to move ahead is limited," Eurasia Group said in a report.

Peru is one of the world's largest producers of gold, copper, silver, zinc and other minerals.

Write to Robert Kozak at robert.kozak@wsj.com

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