Groundwater Quality
Corresponding pages in the DEIS:
Summary of Impacts: pp. 278-296; Environmental Consequences: pp. 286-296
Under the proposed action, seepage is expected to occur from the dry-stack tailings facility from remnant process water. Infiltration of precipitation could cause seepage from the waste rock facility. Both these sources could impact groundwater quality; however, modeling indicates that the water quality of potential seepage from these facilities would meet all Arizona Aquifer Water Quality Standards.
Following closure of the heap leach facility, seepage is expected to continue at low flow rates for 115 years. Modeling indicates that remnant heap leach seepage would exceed numeric aquifer water quality standards for cadmium, nickel, and selenium. This seepage would be collected and treated. Conceptually, modeling shows that with treatment, heap leach seepage can meet all numeric aquifer water quality standards. Long-term discharge from the heap leach facility requires permitting under the Arizona Aquifer Protection Permit program; the specific techniques for collection and treatment of the long-term discharge would be determined by the Arizona Department of Environmental Quality. The heap leach facility is located and designed to collect all possible drainage and solution, is on top of a stable rock location, and will be encapsulated by waste rock to protect from stormwater infiltration.
As modeled, mine pit lake water quality would not exceed any Arizona Aquifer Water Quality Standards.
Groundwater Quantity
Corresponding pages in the DEIS:
Summary of Impacts: pp. 205-278; Environmental Consequences: pp. 237-278
Under the proposed action, 5,400 acre-feet per year of groundwater would be pumped from the Upper Santa Cruz Subbasin of the Tucson Active Management Area and piped to the mine site in the Davidson Canyon/Cienega Basin. This would represent a 6 to 7 percent increase in groundwater pumping from the Upper Santa Cruz Subbasin and a 2 percent increase in groundwater pumping from the entire Tucson Active Management Area. Groundwater levels would decrease up to an additional 70 feet from the pumping, declining at a rate of up to 3.5 feet per year above and beyond existing groundwater declines. The geographic extent of the drawdown would be 3 to 4 miles from the Rosemont production wells during the first 20 years of pumping; the geographic extent of impacts would continue to expand an additional 1 to 2 miles for up to 140 years after completion of pumping. An estimated 400 to 450 registered wells are located within this area of drawdown; specific impacts to these wells, if any, are not known.
In the vicinity of the mine site, the presence of the mine pit would create a permanent hydraulic sink as a result of active pumping and long-term evaporation from the lake, which would result in permanent drawdown in water levels in the regional aquifer. Groundwater modeling shows that this drawdown would be greater than 100 feet in the immediate vicinity of the mine pit and from 10 to 100 feet in the vicinity of the residences in Singing Valley and at Hilton Ranch Road; drawdown would not be greater than 5 feet at the Corona del Tucson residences, along Cienega Creek, or at the Davidson Canyon/Cienega Creek confluence. Drawdown up to 10 feet would potentially occur along Empire Gulch and Gardner Canyon. An estimated 500 to 550 registered wells are located within this area of drawdown; specific impacts to these wells, if any, are not known.
Based on median flow values, a reduction in average annual flow from 1 to 3 percent would occur along Cienega Creek from drawdown in the regional aquifer, resulting in 0.16 mile of lost perennial stream length. During periods of low flow (typically May and June), impacts could be much greater. A reduction in flow of 10 percent would occur along Davidson Canyon from reduction in ephemeral flows stored in the shallow alluvial aquifer; the impact on perennial stream length in Davidson Canyon is not known.
Mountain front recharge to the Davidson Canyon/Cienega Basin would be reduced by approximately 1 percent, and the water lost in perpetuity to evaporation from the mine pit lake would represent up to 5.3 percent of the basin water balance. Groundwater outflow from Davidson Canyon would potentially be reduced by up to 6.4 percent.
A total of 63 springs would potentially be lost either directly to surface disturbance or to impacts from declining aquifer water levels. The presence of most of these springs is based on literature and map review, and they have not been field verified.
Existing groundwater withdrawals contribute to land subsidence in the Santa Cruz Valley; an incremental additional risk of subsidence would result from mine water supply pumping.
To the extent possible, stormwater will be diverted through or around project facilities to transport runoff water to downstream watersheds. Rosemont Copper will mitigate the potential effects of mine related pumping on residential water supply wells in the Sahuarita Heights neighborhood by entering into an agreement with the Rosemont United Sahuarita Well Owners. This well protection plan addresses pump inspection, pump maintenance, pump replacement, well inspection, well maintenance, and well replacement to ensure that residential water wells in the Sahuarita area remain productive throughout the life of minerals production operations.
Rosemont Copper will implement regional groundwater mitigation measures within the Tucson Active Management Area, including recharge of available Central Arizona Project water. The location of the recharge may not be in the vicinity of the mine water supply wells.
Rosemont Copper will also annually fund the U.S. Geological Survey to operate and maintain the existing surface waterflow measurement gage at Barrel Canyon.
The Potential Harmful Effects of the Rosemont Mine on Streams and Ground Water
The proposed Rosemont Mine in the northeastern Santa Rita Mountains may have substantial harmful effects on the streams that drain the mountains, as well as on the region’s most important sources of fresh, potable water — the groundwater aquifers of the Santa Cruz River Valley and the Cienega Basin.
The northeastern Santa Rita Mountains receive approximately 18 to 22 inches of rainfall each year, almost twice that received by the adjacent valleys. This rain, fresh and uncontaminated, flows from the mine area via three streams northeastward into Cienega Creek and then into Pantano Wash, which drains westward toward Tucson. The streams serve the following purposes:
- They create riparian zones that support healthy vegetation necessary for a diverse ecology and for control of erosion of stream banks.
- They supply a number of springs and tinajas that are the source of water for countless birds and other animals, large and small, that inhabit the area.
- They recharge small groundwater basins along their courses — the sources of water for a number of domestic wells.
- They are a significant source of fresh, uncontaminated water for the Tucson groundwater basin, the source of most of Tucson’s potable water.
- They serve as treasured recreation sites for residents and visitors alike.
The Rosemont Mine Plan of Operations, which has yet to be approved by the U. S. Forest Service, shows that the mine, during its 20-year life, will create approximately 4 square miles of mine and mill waste dumps at the headwaters of these mountain streams. As rain falls throughout the year onto the surfaces of these dumps, it will infiltrate the uncovered surfaces, then percolate downward, slowly becoming acidic as it does so and probably dissolving one or more different types of possibly toxic metals and other substances. Because the bases of the dumps will not be lined with impervious materials (as the leach pads at the mine and municipal solid waste landfills are required to be), this now-contaminated water will find its way into the bedrock and bedrock fractures underlying the dumps and ultimately emerge in the springs and stream beds below the mine. The nature and amount of this contamination cannot be predicted precisely, but its effect will be to degrade, possibly very seriously, the ecology and water resources in the area. In addition, major storm events could rupture surface-containment berms and other protective structures, possibly allowing large amounts of very fine-grained mill waste from the leach pads or the waste dumps to enter the streams’ drainage systems, thus causing additional contamination.
The Plan of Operations describes how the mine will need 6,000 to 8,000 acre-feet of fresh water per year (enough to supply as many as 32,000 people per year), primarily for the operation of the mill and concentrator. Rosemont Copper plans to import this water via a pipeline from a well field located 15 miles away from the mine near the Town of Sahuarita. The company has drilled and tested two wells at this well field. There are indications from these tests that they may not be able to pump the amount of water they need without seriously affecting many nearby domestic and agricultural wells because of the drawdown, or lowering, of the water table. The nearby wells could be left without enough water to supply the owners’ needs and may have to be deepened. Rosemont Copper has not performed a hydrogeological analysis of the groundwater system at the well field and vicinity, so an acceptable solution to this problem, if there is one, has not yet been determined.
The Santa Cruz River basin provides water for many users—two existing mines, six domestic water companies, large pecan groves, and eight golf courses. To serve these users in 2006, approximately 76,500 acre-feet of water were pumped out of the basin. Of this, about 40,000 acre-feet were not replaced by natural or artificial recharge. The result of this continuing overdraft is that the basin’s water table is declining at the rate of about four feet per year, a rate that will accelerate if additional large users, such as the Rosemont Mine, are added. If the decline does continue or accelerate, it could result in the curtailment of developments in the area, or water shortages and increased water costs. If it becomes extreme, it may cause significant sinking of the land surface along the Santa Cruz River, a phenomenon already detected by the Arizona Department of Water Resources.
The Rosemont Mine could be an even larger contributor to the groundwater deficit in the Santa Cruz River basin if one crucial part of its plan does not work, as follows. To minimize the amount of water the mine needs, the Plan of Operations calls for the concentrator to be equipped with a “state-of-the-art” waste filtering and water recovery system, called “dry stacking.” Presumably, this will enable the mine to recover 89% of the water it uses. But dry stacking is so new that it has never been tested in the U.S. at mines of this nature and size or in the variable climatic conditions existing in the Santa Rita Mountains. Some engineers doubt that it will work to the degree hoped for by Rosemont. If it does not work, the mine may require considerably more water from its well field at Sahuarita. The Mission and Sierrita mines, for example, together use approximately 34,500 acre-feet annually. This seems like an intolerable amount to impose on the well owners of Sahuarita and the already stressed groundwater system of the Santa Cruz River Valley. Furthermore, drinking water is becoming too precious a commodity in Arizona to allow its use for additional and unnecessary mining purposes.
