Because of mercury and lead contamination in the sediment, officials anticipate that Pena Blanca Lake 10 miles southwest of Rio Rico will be drained in September and closed for about a year.

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The lake, stocked with rainbow trout every winter by the Arizona Department of Game and Fish, was constructed in 1958 and covers about 57 acres. Small boats are allowed on it and the shoreline is also popular for weekend picnics.

The project is a U.S. Department of Agriculture Superfund clean-up location, said Keith Graves, district ranger for the Coronado National Forest. The lake is within the national forest boundary.

Graves anticipates that after the lake is closed and drained, heavy equipment will scoop out some or all of the sediment in the bottom of the lake. People won't be allowed in the area due to the danger posed by large bulldozers and other heavy machinery.

The contaminated soil will be trucked to a facility approved by the federal government to accept it. There is also sediment that's not contaminated and it will be removed and spread on forest land or go to a recipient who agrees to take it, Graves said.

Then the lake will re-fill with rainwater with a plan to open it in September 2009.

The cost of the project will be approximately $1.1 million, said Maria McGaha, regional environmental engineer for the Department of Agriculture's Forest Service.

In addition to the health and environmental benefits, the project will improve fishing, Graves said. "The lake no longer has the original depth because of the sediment, and it has vegetation growing that wouldn't normally grow there. That means that Game and Fish has to actually go mow the lake for people to go fishing."

The lake is about 57 feet deep and the depth of the sediment is not currently known but is being studied.

When the lake is drained, trees and bushes along its banks may die. "When we drain a lake that deep and there are no lateral springs, most of those (plants) will die out. But the root structure of a lot of those are designed for drought, especially cattails, so we think most of those will come back."

The trees along the bank weren't there in 1958 but grew up over the years, Graves noted.

Cause of problem

The mercury contamination occurred over many years as water and soil from old mining areas leached down from the hills around the lake into the water and then settled at the bottom.

Mercury was used in the past in the silver and gold mines that were dug around Southern Arizona. Lead contamination developed from its use in underground supports for mine shafts.

Graves said mercury contaminates were identified in fish tissue by the Arizona Department of Environmental Quality (ADEQ) in 1995. Levels in sampled tissue were 1.44mg/Kg, compared with Food and Drug Administration action level for mercury in fish flesh of 1.0 mg/Kg.

Since 1995, fishermen have been advised not to eat any fish caught in the lake other than rainbow trout, which is stocked each winter and is safe, Graves said. Other fish found in the lake include bass, catfish, crappie and bluegill.

He said some of the planted rainbow trout "has avoided being caught" and has grown larger, up to 14 inches in size, and some of those, "I would recommend that people return to the water."

It's better if people stay out of the water although no illness has been pinpointed. Graves said, "It's a concern, about swimming. It was never identified as a real threat."

Public hearings

Public hearings will be advertised and held as part of the process. The work is identified as a CERCLA project, which stands for Comprehensive Environmental Response, Compensation and Liability Act.

The hearings will likely be in June or July in the Rio Rico area, Graves said. Even though the public will be invited to speak and ask questions, the project won't be postponed or modified.

"This has got to happen. It's not something you can take to court. It's a threat to everyone," he said.

He'll also attempt to get information to residents of Nogales, Sonora, Mexico, some of who enjoy visiting the lake.

A study is in progress to consider if an alternative might be used in which the lake isn't entirely drained so that not all the fish will be killed in the project. But the analysis will have to show that a substantial amount of mercury won't remain in the lake.

If the alternative isn't chosen, the lake will be scraped to the bedrock and then allowed to fill with the 2009 winter and monsoon rains.

The drained water will flow toward Rio Rico and "will go into Pena Blanca Wash which joins up with Walker, and creates Agua Fria Wash," he said. "Part of the study will be to analyze how much water will be leaving the national forest and onto the channel that's private and what we can do to make sure that downstream interests aren't impacted.

"There shouldn't be any damage to private property. That water should be completely contained in the channel," he said.

While the lake is empty, a drain in the dam will also be repaired. Graves said it's been plugged with sediment for 30 years and attempts to clear it out haven't been successful.

Soil trucked out

He believes the lake won't be contaminated from mercury in the future because ADEQ in 1999 had 1,344 tons of contaminated soil at certain sites removed. The sites in the watershed above the lake were the St. Patrick Mine and Mill site and the Sunset Mine, as well as several other uncharted mines and possible processing sites, he said.

"It appears, for right now, from everything we know, we have found the upstream contaminant," he said.

Most recently, in September 2007, an expanded site investigation was contracted with the international firm Weston Solutions. The purpose was to assess the threat posed to human health and the environmental and to determine the need for additional investigation and action.

Studies are continuing. "We've asked for another investigation because there were some gaps in the September one," he said. He expects it to be done by June.

For additional information, Graves can be contacted at the Coronado National Forest, Nogales Ranger District office, at (520) 281-2296.

The project has to be started this year due to funding availability, Graves said. "We know the mercury is there. The big issue is how gently do we remove it, or do we just go in and remove it. We're trying to be as environmentally conscious and save as much habitat as possible, so that's why the next assessment is being done."

(Editor's Note: Freelance writer Kathleen Vandervoet can be contacted at kathleenvan@msn.com.)