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

A recent water quality sample from the South32 Hermosa Mine discharge near Patagonia, Arizona, recorded the highest antimony concentration 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, the 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 when an Alert Level is exceeded and reflects an observable upward trend in reported antimony concentrations as discharge flows have increased over time.

Community members are concerned that current antimony concentrations may already exceed 6 µg/L due to a 30–60 day gap in public reporting after sample collection. Volunteer scientists have compiled publicly reported discharge monitoring results into charts illustrating patterns 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, as demonstrated by the October 31 sample exceeding the APP Alert Level. The public has requested the ADEQ provide results of the October 31 APP sample, though this process is anticipated to take several weeks.

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. The public had anticipated issues with the unique treatment plant construction. Potential groundwater impacts are another major concern 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 solely on groundwater, with significant contributions from the Harshaw Creek watershed even before mine discharge. The potential for groundwater contamination from the discharge has not been sufficiently analyzed and is not monitored or regulated by the State of Arizona. Chris Gardner, hydrologist and scientific advisor to local residents, stated that the antimony discharge issue likely represents the first example of deficiencies by the mine and regulators to protect human health and the environment.

The South32 Hermosa Mine'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. Robin Lucky, President of the Calabasas Alliance, emphasized that Harshaw Creek serves as a lifeline for drinking water, irrigation, and wildlife in Santa Cruz County. With drought already straining aquifers, South32's discharges risk bioaccumulation in downstream wells, lakes, and the Santa Cruz Active Management Area.

Antimony is a toxic mining byproduct posing severe health risks at these concentrations. 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, while also impacting fish, amphibians, invertebrates, birds, mammals, and other animals. Lucky noted that Patagonia residents have lived with uncertainty about the project's water resource impacts for nearly a decade, emphasizing the need for clear information, timely reporting, and honest data accounting, with transparency being essential for community health and survival.