Skip to Content

Rosemont pit will pump deep, create lake

The following story ran in the January 8, 2010 edition of the Nogales International.  The report referenced in the story was written for the mining company and points out what opponents have been saying all along:  This is the wrong place for a mine.  it's hard to even imagine what the Santa Rita's would look like with the water table more than 2,000 feet lower than it is now!

Rosemont pit will pump deep, create lake

By Dick Kamp

A study of flow impacts from water drainage at a proposed open-pit mine in the Santa Rita Mountains predicts the formation of an 819-foot “pit lake” 100 years after the mining is completed.

According to the study commissioned by the Augusta Resource Rosemont Mine, groundwater at the bottom of the pit will have dropped 2,020 feet below current modeled groundwater levels at that site.

Open-pit miners and underground mines pump water constantly when operating so that folding does not hinder operations once a pit is below groundwater levels.

Large mines continue to “dewater” while operating and frequently continue pumping water when closed to prevent acidic heavy metals, sulfates and radioactive compounds from moving into the surrounding groundwater or surface water.

Pollution

Acidic pollution forms and travels into groundwater when compounds that have been removed from the ground contact oxygen and water combined.  That acidic water will occur if water fills a put with sulfide copper such as what would be mined at Rosemont.

Acid leaching often occurs as flows migrate toward groundwater though fractured rock that permits the oxygen to enter the water.  Rock in the Rosemont mine area is relatively fractured according to the study by the Tucson environmental consulting firm of Errol Montgomery & Associates, Inc.

Regulatory agencies frequently require a mine to keep pumping after closure to clean up or mitigate pollution.  The pumping will create what is known as a “cone of depression” in which the groundwater is sucked toward the mine.  Such pumping, while lowering surrounding groundwater, will also keep pollution in the groundwater closer to the mine.

For example, the Sierrita Mine near Green Valley, pumps to reduce the movement of sulfates and other pollutants in groundwater from moving east to nearby wells.

Once the pumping stops a pit lake may form.  Mines with put lakes, such as the Butte, Montana, Anaconda Copper Mine, can become federal “Superfund” sites.  The government pays for cleanup measures to prevent serious pollution of rivers of aquifers.

Study, analysis

The Montgomery study and a 2007 analysis conducted for Pima County by Reno-based hydrologist Tom Myers in September 2007 did not address pollution impacts.

Both focused on the groundwater and surface water flows within the pit and surrounding basins, which is information that would be required in order to then characterize potential pollution impacts as well as pit-dewatering pumping impacts.

Myers predicted about the same drop in groundwater at the pit from current levels as the Montgomery study and also raised serious concerns about water drawdown in Davidson Canyon and Cienega Creek, southeast of the mine.

Concerns raised

His study called for detailed modeling and monitoring of groundwater flows in the region to determine actual impacts, which the Montgomery study attempts to do.

The Coronado National Forest echoed concerns over the lack of data in October 2007 and the Rosemont study is designed to comply with the requirements of the National Environmental Policy Act.  The act would require the mine to characterize water impacts for their Environmental Impact Statement due to be completed in draft form in June 2010.

The Montgomery executive summary said, “Model Projections indicate groundwater level drawdown will be limited to the area in and around the Rosemont project and into Davidson Canyon with no projected drawdown at Cienega Creek.”

However, later in the report, the study suggests that springs feeding the Cienega Creek area, “depending on the degree of hydraulic connection of the perennial or intermittent springs to drawdown impacted area…flows in these springs could be adversely affected.”

Julia Fonseca, Pima County Environmental Planning Manager for the Office of Conservation Science and Environmental Policy said, “There is just a whole lot more data now than we had in 2007 on groundwater levels and potential pollution impacts of the Rosemont Mine.  We have created a Pima County team (including) Tom Myers and me to analyze the Montgomery model and to pull all the available information together.

The conclusion

“In many respects, both the Montgomery team and Myers came to the same conclusion; there’s a huge drop of groundwater at the put.  But the Montgomery study is limited because it only projects water behavior for 100 years into the future,  and not to a time when the huge cone of depression created has filled up, and the groundwater finally achieves equilibrium, which may take thousands of years.

“Pima County needs to evaluate all the studies together, and the (Forest Service) is making a decision as to whether they will give the county all the input data that went into creating the Montgomery model.  Looking already at the implications of some of the studies, we see pit-pumping impacts in Box Canyon and the Helvetia side in the Santa Cruz Basin to the west.”

Rosemont plans to post the Montgomery study, “Groundwater flow modeling conducted for simulation of proposed Rosemont put dewatering and post-closure Rosemont Project, Pima County, Ariz.” at www.rosemontcopper.com/technical.html.

(Editor’s note: Kamp is an environmental liaison for Wick Communications, which owns the Nogales International/Weekly Bulletin.)



Dr. Radut | story