Pima County fears Rosemont Mine would impact water flow

Wick News Service

A new study projects major impacts on ground and surface waters in the Santa Rita Mountains, Davidson Canyon and Cienega Creek by the proposed Rosemont Mine in the Santa Ritas near Green Valley.

The report, commissioned by Pima County’s Flood Control District, was released last week by County Administrator Chuck Huckelberry.

“Depending on the exact depth to pre-mine water levels and where the measurement occurs, the pit (1,800 to 2,900 would lower the regional water table up to 1,500 feet” mining hydrologist Dr. Tom Myers wrote in the report.

Myers predicts that the disruption of groundwater flows, exacerbated by pumping of waters to dewater the pit, “would draw from the regional groundwater similar to pumping from a large diameter well.”

Myers expressed concern that if pumping affected the “aquifer southeast of the site which drains toward Cienega Creek……groundwater would be drawn north toward the pit. This would reduce the flow to Cienega Creek by an uncertain amount….”

“The Rosemont Project pit would…intercept about 650 (acre feet per year) of flow to the Davidson Canyon,” said Myers. “This is approximately the flow that reaches Cienega Creek from Davidson Canyon. The proposed project would intercept substantial amounts of groundwater discharge to the downstream basins.”

The report is particularly sobering because the hydrological impacts discussed are related to mining disruption of Rosemont area groundwater and surface flows, and not groundwater pumping for the mine. The report presumes that the mine will utilize an external source of water. This is currently mine owner Augusta Resource’s Sahuarita Road well field in the Santa Cruz basin by the Green Valley Pecan orchards. This would provide the 7,000 acre-feet of water needed annually.

Augusta has offered to extend a Central Arizona Project line south to Green Valley and discharge the same amount of water that it has purchased near the Santa Cruz River.

“So far,” wrote Huckelberry in a cover letter to the report, “the information Augusta has provided the Forest Service about groundwater conditions is deficient for developing an EIS (Environmental Impact Assessment).”

He added that extensive modeling and water field tests are needed to estimate the impacts of the mine on water flows and to provide a basis for water quality analyses.

“Previous work based on examination of 70 EISes for modern hard rock mines in the western United States found consistent underestimation of water quality impacts,” Huckelberry said.

Augusta submitted a mine “plan of operations” to the Coronado National Forest in July in the hopes of getting the Forest Service to begin a public review and development of an EIS under the National Environmental Protection Act or NEPA. In August, Coronado Forest Supervisor Jeanine Derby invited Pima County to become a cooperating agency to assist in carrying out the review. She said that the Coronado would make a decision on whether the mine plan is ready for NEPA review by Sept. 14.

NEPA review of mines can take many months and frequently leads to subsequent litigation over the decision.

Dick Kamp is the environmental liaison for Wick Communications. Contact him at bepdick@att.net.