Davidson Canyon and Cienega Creek — linchpin issues for many who oppose the Rosemont Mine — aren’t in as big a danger from mine impacts as many people think, the U.S. Forest Service says.

The Coronado National Forest staff members generally say that the two downstream watercourses aren’t likely to be seriously harmed as a result of mine construction that would remove water from the underlying aquifer to create the copper mine’s open pit.

That’s their conclusion in the draft version of their final environmental report on the proposed mine, which would be built in the Santa Rita Mountains southeast of Tucson.

The two creeks also aren’t in as much danger as previously thought from being damaged because the mine’s waste rock piles, processing plants and other facilities will cut off some or much of the storm water that would flow from the mine site, the Forest Service says.

But in written comments on the environmental report, the U.S. Environmental Protection Agency and Pima County government officials are sharply critical of the Forest Service’s conclusions.

The EPA, in particular, says it’s simply not possible to accurately predict the mine’s impacts on downstream surface waters due to the removal of underlying groundwater, particularly as far ahead as the Forest Service tries to predict — up to 1,000 years away.

The county challenges some key assumptions that the service’s scientists and consultants made about stream flows, rainfall estimates, flood peak totals and other factors that figure into the amount of water running from the mine site into Davidson and Cienega. The U.S. Army Corps of Engineers is also critical of the Forest Service’s reports on stream impacts, but not in nearly as much detail.

Specifically, the Forest Service predicts that:

Over the next half-century, there’s no indication that Cienega Creek’s stream flow will be reduced because the groundwater aquifer will be lowered for the mine pit.

Over the next 1,000 years, impacts to Cienega Creek from groundwater level declines are possible. Actual, numerical predictions of declines are “entirely speculative.”

Davidson Canyon’s surface flow 12 miles downstream from the mine could be reduced by 4.3 to 11.5 percent due to runoff upstream being captured on the mine site. But the effects of the reduced stream flows are likely to be overestimated due to the canyon’s distance from the mine site.

Evidence suggests that Davidson Canyon isn’t hydraulically connected to the regional aquifer that would be lowered by the mine pit dewatering, meaning that the canyon’s flows wouldn’t be affected by that action.

Barrel Canyon, which flows four miles east from the mine to Davidson, would lose 30 to 40 percent of its runoff during the first 10 years of mining.

But once the mine is closed, the losses would decrease to about 17 percent. That’s because as the mine is reclaimed, a rock buttress would be built around much of the site; its surface would be revegetated; and the amount of storm water running off the site would slowly be reduced. Because of a redesign of the mine site, storm water ponds that would have held back more water have been eliminated, thus reducing the reduction in post-closure runoff from an original estimate of 34 percent.

In the 50 years after the mine closes, small amounts of groundwater level declines could occur in Empire Gulch, which like Davidson is a tributary to Cienega Creek. Predictions of how that would affect stream flow would be “speculative.” Several feet of groundwater declines could occur there in the 1,000 years after mine closure, which could affect stream flow. Again, that forecast has a “high level of uncertainty.”

At stake here are the future of two prized environmental and biological jewels. Both Davidson and Cienega are intermittent streams for the most part, through which water flows off and on during the year.

An increasingly limited area of Cienega Creek, however, still flows year-round. Both stream’s flows have been hobbled recently by the Tucson area’s decade-long drought. Both creeks are lined with riparian plants, ranging from lush cottonwood and willow groves to more common desert vegetation such as mesquite, barrel and prickly pear cacti and ocotillo.

Over the years, developers have proposed major projects along or near the creeks, provoking successful efforts by conservation groups and county officials to preserve them.

In a letter to the Forest Service last month, County Administrator Chuck Huckelberry noted that the county has invested more than $64 million in watershed and stream protection in the Cienega-Davidson area, much of it for buying and saving open space.

Portions of the two creeks lying about 11 miles or more downstream of the mine sites are classified as state-certified “Outstanding Waters,” meaning they’re not supposed to be legally degraded. The Forest Service says the mine impacts also aren’t going to do anything to threaten that Outstanding Waters status.

One of the major centers of controversy over the service’s predictions is its use of computer models to look up to thousands of years ahead to foresee the mine’s impacts. Computer-model science — a technically dense, highly complex forecasting tool — is commonly used by many university researchers, state, local and federal agencies and business scientists to predict the future.

In its report, the Forest Service says it determined that groundwater forecasting models “remain the most appropriate tool for estimating potential impacts to surface waters,” while acknowledging their limits.

But the EPA says it doesn’t believe the models are suitable for predicting impacts to most or all distant springs and riparian areas, particularly when looking ahead up to a thousand years.

The EPA quotes the service’s document, which says the computer models can’t accurately predict changes in groundwater levels of fewer than five feet over hundreds to thousands of years. It also notes that the Forest Service reports frequently refer to near and long-term predictions of groundwater decline and its effects as “speculative and highly uncertain.”

The EPA also takes aim at a Forest Service conclusion that there is no direct potential for Upper Cienega Creek’s water quality to be affected by the mine, except for indirect effects such as temperature changes caused by reduced stream flows.

Small stream flow changes can cause significant water temperature changes, especially in warm, arid environments like ours, the EPA says, and water temperature is an important measure of water quality.

Similarly, small changes in groundwater levels could have adverse effects on surface and shallow subsurface flows, the EPA says. That’s in part because many Southwestern streams, including Cienega, are shallow and extremely susceptible to drying from small groundwater level changes, the EPA says.

In its comments, the Army Corps said its concern is that a reduction in Barrel Canyon flows of close to 40 percent while mining is going on “will impact downstream areas to such a degree” that the lesser decline of 17 percent after the mine is closed won’t matter.

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.