New Orleans — For decades, those leading the life-and-death struggle to keep southeast Louisiana from being swallowed by the Gulf of Mexico have had a battle cry: “Put the river back into the marsh.”

But what if pollutants in the river’s fresh water will kill the marsh before those sediments can do good? Dissenters who posed this question have been treated as outliers, if not obstructionists. But a pair of recently released reports gives that question new relevance.

A nine-year project in New England showed that fertilizer-based pollutants carried in the Mississippi River led to the collapse of salt marshes dominated by the plant species that is a signature to much of Louisiana’s southeast coast.

And a review of research on Louisiana’s freshwater diversions by experts from outside the state found no solid evidence the projects improved adjacent wetlands and even saw evidence that they might hurt wetlands. The study indicated that the state’s Coastal Master Plan, built around large sediment diversions, may be forging ahead blind to the potential dangers.

The reports provide new ammunition to diversion opponents as well as scientists pushing for more research before river water is released on the wetlands.

But Garret Graves, head of the state’s Coastal Protection and Restoration Authority, dismissed the latest findings as too little, too late to apply to the state’s Master Plan.

“Look, if we were on a sustainable posture right now, sure, maybe we could spend a few more years and spend tens or hundreds of millions of dollars trying to perfect the science, but considering where we are in Louisiana, you have to take some risks,” Graves said.

Further, Graves said, the land being built at the mouth of the Atchafalaya River with water from the Mississippi shows that diversions work.

River diversions have long been the great paradox of coastal rebuilding efforts — at once its most important, yet contentious element. Primary opposition has come from fishing interests that could see their target species pushed to the edges of the Gulf. Instead, they support “slurry pipelines,” which use suction dredges to mine sediment, mix it with water and pump it into the sinking basins.

However the coastal authority said its research showed that large diversions built land at a much cheaper cost per acre and could continue building land as long as the river flows.

Diversion foes have complained for years that the Caernarvon Freshwater Diversion was actually destroying the marsh. Fishers, trappers and others making a living off the wetlands claimed the lush blooms of freshwater plants masked a steady erosion of the land.

Those impressions received scientific weight in 2008 when LSU coastal scientist Gene Turner published research indicating the river water from Caernarvon stunted root growth of critical marsh plants and accelerated decomposition of the highly organic marsh soil.

His work echoed previous studies and was confirmed by further research in Louisiana.

The coastal authority and most of the coastal restoration establishment countered that Caernarvon was a bad analogy because it was designed to move water not sediment. Further, the authority’s research showed that the diversions would put so much sediment in the wetlands — along with the water — that they would build land and heal the ecosystem.

The new papers refocus scientific attention on that debate.

The New England study added concentrated nitrogen and phosphorous to tides flowing into an unpolluted coastal salt marsh. The primary plant in that marsh — Spartina cordgrass — dominates wetlands targeted for some river diversions south of New Orleans.

In the first few years of the project, the nutrients ignited an explosion of growth, but by the fifth year the edges of the marsh began “literally falling apart,” said team member John Fleeger, a professor emeritus at Louisiana State University. The pollutants speeded decomposition of the organic soil and stunted and weakened the roots, increasing erosion.

Graves said the research did not mimic conditions found on Louisiana’s coast. The study used nutrient loads “double to 600 percent” of those found in the Mississippi, and the tidal cycle in the New England study was more frequent and stronger than in Louisiana, Graves said.

The second report, a review of published research on the impact of the state’s freshwater diversions, was requested by the coastal authority. It looked for any conclusions that could be drawn about those diversions, including their ability to build new land, encourage plant growth and stop erosion.

The panel criticized much of the science that has become standard reference material for the state’s coastal plan as sparse, insufficient and inconclusive.

While the panel saw some evidence of small-scale land-building adjacent to outflows, it noted that some papers also showed that the freshwater diversions actually led to land loss because they compromised plant growth, soil building and wetland elevation.

The authors also were concerned the state may be forging ahead without proper science to guide its decisions. They urged the coastal authority to conduct comprehensive research on how wetlands respond to nutrient-rich river water.

Graves dismissed the report as unusable, saying the authors did not follow the state’s mandate.

“It largely didn’t answer some of the key questions the state had posed to the scientists who were putting this together,” Graves said. “Questions like informing us or guiding us on how to design, operate and maintain sediment diversions called for specifically in the Master Plan.

“This largely looked at the role of freshwater diversions that exist today, and again we don’t have any of those in our Master Plan.”

In fact, while the Master Plan 2012 lists only “Sediment Diversions,” critics point out at least four of the nine move river water into the marsh at the rate of 5,000 cubic feet per second or less, which they contend is not enough to transport large loads of sediment deep into adjoining basins.

The panel’s chairman, John Teal of Woods Hole, said Graves’ charges were so inaccurate he “could not respond to them in language fit for publication.” But he relented.

“Is he right in those charges? I can respond to his claims with one word: No!” Teal said. “They asked us to look at what had been done on freshwater diversions, and that’s what we did.”

But he also understood Graves’ sense of urgency. “Look, everyone knows the only chance you have down there is to get sediment into the wetlands,” said Teal, who has been involved in Louisiana coastal research for decades.

“I agree with him that you don’t have time left, that you probably should move forward even though the science isn’t complete, mistakes and all,” Teal said. Meanwhile, “they have to conduct research to monitor the results, so that they know what mistakes they make, and what is working.”

Graves said, “Nature is showing us how to get this done” at the mouth of the Atchafalaya River and at the West Bay Diversion south of Venice, which are building plant-filled land with the same Mississippi river water.

However, some scientists say the comparison is invalid because the soil on the newly built Wax Lake and West Bay deltas, which have never been blocked by levees, is comprised primarily of minerals and clays, which provide a firmer foundation for plant roots and are not as susceptible to fertilizer compounds.

The marshes along the leveed Mississippi, blocked from sediments for more than a century, now are highly organic and vulnerable to those chemicals.

This story was reported by The Lens, an independent, nonprofit newsroom serving New Orleans.