An Arizona Department of Environmental Quality (ADEQ) remediation project at the former Gibson Mine, in the Pinal Mountains a few miles southwest of Miami, is on target – weather permitting – to be complete by the end of 2022. It is one of several ADEQ projects designed to improve surface water quality around historic mining locations.
In the early 1900s the Gibson Mine was one of the area’s prominent copper producers. In the spring of 1910, according to the Daily Arizona Silver Belt, it sent about 25 tons of high-grade ore to the Old Dominion smelter a week. The site was mined off and on from the 1960s through the 1990s. Since then the Gibson has been inactive, but it left a legacy of copper concentrations affecting the water in nearby Pinto Creek – and ADEQ’s project is meant to further address that legacy.
A 2001 Environmental Protection Agency study identified the Gibson Mine site as a major source of copper exceedances. Remediation projects in 2007 and 2009 brought down copper concentrations by approximately 85%, but mine runoff was still degrading the creek’s water quality. Starting in 2017, ADEQ took additional water and soil samples to help determine further measures; most of their sampling was complete by July 2022. “Our goal for the former Gibson Mine project is to reduce copper concentrations in Pinto Creek to meet the surface water quality standard established to protect wildlife,” said ADEQ Communications Director Caroline Oppleman.
In January 2021 ADEQ awarded a contract for the additional remediation work to Arcadis, a global design and consulting firm with offices in the Phoenix area. The project has involved several phases; water and soil sampling, developing a safety plan, data collection and analysis, remediation design, and obtaining needed permits and clearances.
A 2021 site characterization by ADEQ showed an average copper content of 2,146 parts per million (ppm) in the soil. Project goals include cleaning up soil hot spots to less than 900 ppm copper and soil/ sediment in the tributary to less than 750 ppm as an added protective measure, said Oppleman.
The remediation now in progress includes excavating soil with high copper levels from drainages leading to Pinto Creek and the Gibson Mine tributary, and consolidating that soil into a stockpile with the remaining waste rock and tailings. By November 8, excavation work was complete at several sites. ADEQ is also installing riprap in the channels to keep water clean during storm flows and reroute storm flow to bypass waste rock and tailings.
Additional steps will include regrading, installing Turf Reinforcement Mats and reseeding. The mats will serve as a barrier between soil and rainwater or snowmelt, providing erosion control. They will be installed in the streambed and in areas of the site that slope toward it. Re-seeding, an added erosion control measure that will also encourage vegetation growth, is scheduled for December 2022 – again, if weather allows. After project completion, further water and soil testing will be conducted to verify a successful remediation.
The project area is on 230 acres of private land, donated to the Franciscan Friars of California in 1969, and a small section of public land. The Friars have been working with ADEQ and the U.S. Forest Service since 2006 to reduce copper levels in Pinto Creek. “ADEQ appreciates the Friars’ commitment to partnering with us to conduct this important work to improve surface water quality,” said Oppleman.