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 regulatory requirements for 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 are concerned that current antimony concentrations may already exceed 6 µg/L due to a 30–60 day gap in public reporting after samples are collected. Volunteer scientists have compiled publicly reported discharge monitoring results that illustrate a pattern where higher flow rates above one million gallons per day correspond with higher reported metal concentrations.

The data suggests the treatment process was adjusted resulting in decreased arsenic concentrations, but antimony remains problematic. 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, with long-term plans including pumping millions of gallons of groundwater per day to support mining operations.

Public concerns extend to potential groundwater impacts 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 source of water, with significant contributions from the Harshaw Creek watershed even before mine discharge. "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.

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.

"Harshaw Creek is a lifeline for drinking water, irrigation and wildlife in Santa Cruz County. With drought already straining our aquifers, South32's unchecked discharges risk bioaccumulation in downstream wells, lakes and the Santa Cruz Active Management Area," said Robin Lucky, President of Calabasas Alliance. Community members emphasize that the potential for groundwater contamination has not been sufficiently analyzed and is not monitored or regulated by the State of Arizona, creating significant uncertainty about long-term impacts on both water quantity and quality in the Patagonia region.