Record Antimony Levels at Arizona Mine Discharge Raise Health and Environmental Concerns

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 APP rules requiring a 30-day study whenever an Alert Level is exceeded 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 available at https://www.sonoitacreek.org that illustrate a pattern where higher flow rates above one million gallons per day correspond with higher reported metal concentrations. While data suggests treatment adjustments decreased arsenic concentrations, antimony remains problematic, with the October 31 sample exceeding the APP Alert Level for the surface water permit.

The treatment plant's deficiencies are particularly concerning as discharge rates are anticipated to increase significantly with mine development. 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. Public requests for ADEQ to provide results of the October 31 APP sample are anticipated to take several weeks, delaying crucial information about water quality.

Potential groundwater impacts raise additional alarms 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 Town of Patagonia relies on groundwater as its sole water source with significant contribution 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. "Environmental monitoring by the public is key to better understand the issues, respond to these issues, and hold South32 and regulators accountable."

South32's long-term plans include pumping millions of gallons of groundwater daily 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 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 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 poses severe health risks at these concentrations, with the EPA warning that chronic exposure through drinking water can lead to gastrointestinal issues, respiratory problems, organ damage, and increased cancer risk. The toxic metal also impacts aquatic and terrestrial wildlife. Community advocates emphasize the need for transparency and timely information, with Lucky noting, "Water is central to the health, prosperity, and very survival of our community - transparency is essential." More information about community concerns can be found at https://www.calabasasalliance.org.