As the initial furor over the Aug. 5 mine wastewater deluge into the Animas River has slowly settled, so have potentially toxic heavy metals — leaving the possibilities for long-range impact literally lining the riverbed.

State and federal officials have offered assurances that the river is returning to “pre-event conditions,” but uncertainty remains over the residue that still lurks beneath the surface flow.

Those remaining metals on the river bottom still could affect aquatic life, agriculture and other aspects of life along the water in ways that are difficult to predict.

“The long-term effects are the concern that every time we have some sort of a high-water event, whether a good rain in the mountains or spring runoff next year, that’s going to stir up sediments and remobilize those contaminants that are sitting at the bottom of the river right now,” said Ty Churchwell, Colorado backcountry coordinator for Trout Unlimited.

Added Dan Olson, executive director of the environmental group San Juan Citizens Alliance: “People on the ground understand that what we don’t know is what we’re worried about. And that’s the sediment issue.”

Peter Butler, co-coordinator with the Animas River Stakeholders Group, a diverse collection of entities that for 21 years has addressed metal loading in the Upper Animas Basin, remains optimistic that the worst has passed.

If sediment stirs again, he said, it will be more diluted.

“Where that ends up, whether concentrations would be higher or not, I don’t know,” he added. “I’m hopeful there won’t be a lot of long-term impacts, particularly because the short-term impacts weren’t that great.”

Leadership of the sprawling Navajo Nation also voiced fears of long-term impact as the contaminants from the Animas flowed into the San Juan River, which flows for 215 miles through Navajo land.

Three million gallons inadvertently released from the Gold King Mine into Cement Creek above Silverton during an Environmental Protection Agency excavation gushed into the Animas and turned the river a shocking shade of orange.

But even as the plume passed days later and Gov. John Hickenlooper made a show of drinking water taken directly from the river, experts and advocates alike noted that the lingering metals such as cadmium, arsenic, lead and zinc pose unknown future risks.

In addition, they stressed that the current disaster should be viewed in the context of the river’s long history with mining and particularly more recent issues that severely damaged the Animas’ fish population.

The natural reproduction of fish in the Animas has been hampered by heavy metals and sedimentation for years. Since 2000, the river has seen an almost 80 percent decline in the fish biomass — the weight of all the trout collected in a certain area, said Jim White, an aquatic biologist for Colorado Parks and Wildlife.

The river basically doesn’t have naturally reproducing trout.

To offset the lack of reproduction, the state, along with the U.S. Bureau of Reclamation, annually stocks the river with about 20,000 fingerling — months-old — rainbow trout, as well as about 2,500 of a “catchable” size of about 8-12 inches. The Southern Ute Indian tribe stocks about 80,000 more.

Focus on metals

Natural metals always have leaked into the Animas, although nobody knows what those levels were before mining in the area. But in the past 10 years, since a water treatment plant closed and other prescriptions failed, dissolved metals have increased, White added.

While heavy metals likely play a role in the deterioration of the fishery, so do naturally existing metals, drought, water temperature and sedimentation issues from various tributaries.

“I think it has to do with almost a more global water- quality issue,” White said. “What we don’t know is what the role of dissolved heavy metals plays with the health of the fish. That’s something we hope to learn more about with the Animas River disaster.”

Next week, state workers will make two passes, a day apart, and use electrical current to stun and mark fish to assess the river’s population. Ten fish were delivered to the state health department on Friday to test tissue for metals.

Earlier, the state exposed 108 rainbow trout fingerlings to the fouled river water. Only two died.

“That tells me we never got to any sort of acute level of toxicity in the river,” White said. “Honestly, we were surprised they survived that first plume.”

So was Trout Unlimited.

“Our first concern was: Six hours from now are there going to be thousands of dead fish floating on the Animas?” Churchwell said. “Thankfully, that didn’t happen. But the long-term effect, we’ll be monitoring it.”

Effect on fish

Shawn Rummel, a field and research manager for Trout Unlimited, noted that with an event like a 3 million-gallon blowout, metals might be in high concentration for a short amount of time that minimizes the effect on the fish population.

“But it could be either the concentrations weren’t high enough to kill off the fish, or the exposure time wasn’t high enough to kill off the fish,” he said. “There’s an interplay there that’s hard to nail down.”

Rummel echoed concerns about sedimentation that could affect the fish population directly as well as its food source — stream macroinvertebrates such as mayflies, stoneflies and caddisflies — as sediment coats the loose gravels on the river bed.

“With the initial plume, maybe there wasn’t as big of a die-off, but long term it may become more of a habitat availability issue and also a spawning habitat issue,” he said. “Those are pretty common concerns with abandoned mine drainage.”

Long-term effects on agriculture hinge on the same issue of what lies beneath the Animas flow, said Perry Cabot, water resources specialist with the Colorado State University extension office.

While the testing focuses on suspended particles, it’s the so-called bedload sediment that could become the source of future problems, he explained. Particulates sink in places where the flow slows — such as around irrigation gates — and that residue could accumulate any number of metals.

Those of greatest concern to agriculture are arsenic, cadmium, lead and mercury, Cabot added. Other metals, such as copper and zinc, are micronutrients that can be processed by plants, although not in massive quantities.

It’s a question of degrees of toxicity posed by the accumulation of the metals. And some plants handle that accumulation better than others — grasses tend to tolerate it better than beans, for instance.

So Cabot is recommending to area farmers that they take advantage of local labs to process their own samples. That data could help them establish the safety of their products to buyers and avoid the kind of guilt-by-association that plagued innocent melon growers in the Arkansas Valley during the listeria outbreak in 2011.

“Because perception is everything,” Cabot said. “If there’s even a hint that maybe (toxicity) is there, if I’m a producer, I want to make sure I can definitively say I’ve had my water tested, and I’m not applying anything that has these four dangerous metals.”

In addition, CSU offers an online water quality interpretation tool.

Cabot said that the agricultural impacts could have been far worse but for an unusually wet spring.

“The ‘Miracle May’ has brought a fair amount of good dryland crops that look fairly good,” he said. “We have fields we haven’t irrigated since May, and they’ve survived almost entirely on natural moisture.”

Superfund or not?

River outfitter Bill Dvorak, who holds the first outfitting permit issued in Colorado, said the long view of the debacle on the Animas could be framed through the evolution of the Arkansas River, once beset by mining residue around Leadville.

Years after the area’s designation as a Superfund site, the cleanup helped give rise to many miles of state-designated Gold Medal fishing waters on the Arkansas as well as a thriving river rafting industry.

Short-term, Dvorak said the efforts to address heavy metals and acidity have had a good impact that will return activities like rafting to normal. The Animas was reopened to recreation Friday.

But the longer-term solutions lie beyond holding ponds and other measures taken by authorities to mitigate the current problems, he said.

“My hope is that people realize this needs to be identified as a Superfund site so they can begin to clean it up,” said Dvorak, who also works with sportsmen’s groups for the National Wildlife Federation. “The immediate impact I don’t think is going to be great. But the long-range deal is we need to do something about all the mining activity in the West and clean it up and make sure these things don’t happen in the future.”

But calls for steps like Superfund designation would encounter significant political opposition. And measures like a so-called good Samaritan law, which would allow various entities to participate in cleanups without risking liability, also face hurdles.

“The truth is, it’s so complex and there are any number of players and legal issues,” said Trout Unlimited’s Churchwell. “It’s not as simple as one mine popped its head and blew its water. If you walk away with one underlying theme here, it’s that this is not an isolated incident. There are ticking time bombs all over the western U.S.”

Kevin Simpson: 303-954-1739, ksimpson@denverpost.com or twitter.com/ksimpsondp