South32 Hermosa Mine Discharge Shows Rising Antimony Levels, Raising Water Quality Concerns in Arizona

Recent water quality monitoring at the South32 Hermosa Mine discharge near Patagonia, Arizona, has revealed the highest antimony concentration recorded to date, exceeding Arizona's Aquifer Protection Permit Alert Level and approaching the state's regulatory discharge limit. According to EPA-reported Arizona Pollutant Discharge Elimination System data, an October 31, 2025, sample measured 5.73 micrograms per liter of antimony, above the state Alert Level of 4.8 µg/L and just below the APP discharge limit of 6 µg/L. This reading triggers state requirements for a 30-day study and reflects an observable upward trend in reported antimony concentrations as discharge flows have increased over time.

Community members express concern about the 30–60 day gap in public reporting after samples are collected, suggesting current antimony concentrations may already exceed the 6 µg/L limit. Volunteer scientists have compiled publicly reported discharge monitoring results into charts showing concentrations of antimony and arsenic alongside the mine's reported discharge flow rates. The data illustrates a pattern where higher flow rates above one million gallons per day correspond with higher reported metal concentrations. While treatment adjustments appear to have decreased arsenic concentrations, antimony remains problematic, with the October 31 sample exceeding the APP Alert Level.

The public has requested the Arizona Department of Environmental Quality provide results of the October 31 APP sample, though this process is anticipated to take several weeks. Issues with the treatment plant were anticipated by community members due to its unique construction. Data suggests that even at relatively low flow rates of 1-2 million gallons per day, the treatment plant appears deficient in reducing antimony concentrations below permit standards. This deficiency is particularly concerning as discharge rates are anticipated to increase significantly as the mine develops.

Potential groundwater impacts raise additional concerns as existing dry contaminated sediments become saturated. Water levels in the aquifer along Harshaw Creek and in the Town of Patagonia aquifer are anticipated to rise due to discharge in Harshaw Creek. The Harshaw Creek watershed is recognized as a highly mineralized area with abandoned mines where Harshaw Creek flows into the Town aquifer. The Town of Patagonia relies on groundwater as its sole water source, with significant contributions from the Harshaw Creek watershed even before mine discharge. The potential for groundwater contamination has not been sufficiently analyzed and is not monitored or regulated by the State of Arizona.

"The antimony in the discharge issue is likely the first example of deficiencies by the mine and regulators to protect human health and the environment," said Chris Gardner, hydrologist and scientific advisor to local residents. "Even with assurances from South32 and regulators, will the air treatment system be as deficient as the water treatment plant? Will the Town aquifer be contaminated? Environmental monitoring by the public is key to better understand the issues, respond to these issues, and hold South32 and regulators accountable."

The South32 Hermosa Mine's long-term plans include pumping millions of gallons of groundwater per day to support mining operations, with a large portion expected to be discharged after treatment. This raises questions about potential long-term impacts on both water quantity and water quality in the Patagonia region. "Harshaw Creek is a lifeline for drinking water, irrigation and wildlife in Santa Cruz County," said Robin Lucky, President of the Calabasas Alliance. "With drought already straining our aquifers, South32's unchecked discharges risk bioaccumulation in downstream wells, lakes and the Santa Cruz Active Management Area."

Antimony is a toxic mining byproduct that poses severe health risks at these levels. The U.S. Environmental Protection Agency warns that chronic exposure through drinking water can lead to gastrointestinal issues, respiratory problems, headaches, dizziness, eye irritation, sleep disturbances and organ damage to the liver, kidneys, immune system, and thyroid. High exposures are linked to stomach cancer, cardiovascular disease, and metabolic disorders. It also impacts fish, amphibians, invertebrates, birds, mammals and other animals. For more information about environmental concerns in the region, visit https://www.sonoitacreek.org and https://www.calabasasalliance.org.

"The people of Patagonia and Lake Patagonia have lived for nearly a decade with deep uncertainty about how this project may affect our shared water resources," said Lucky. "All we are asking for is clear information, timely reporting, and an honest accounting of what the data show. Water is central to the health, prosperity, and very survival of our community - transparency is essential." Lucky added, "This is a crucial moment for trust building in our community. When reported concentrations exceed state thresholds, the citizens of Santa Cruz County deserve to know promptly and understand what steps are being taken to protect them."