Recent water quality monitoring at the South32 Hermosa Mine discharge near Patagonia, Arizona, has 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, 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 are concerned that current antimony concentrations may already exceed 6 µg/L, given a 30–60 day gap in public reporting after sample collection. Volunteer scientists have compiled publicly reported discharge monitoring results into charts available at https://www.sonoitacreek.org to illustrate patterns where higher flow rates above one million gallons per day correspond with higher reported metal concentrations.
The 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 with mine development. The public has raised questions about the treatment plant's unique construction, with data indicating adjustments reduced arsenic concentrations but failed to address antimony effectively.
Beyond surface water concerns, the potential for groundwater contamination remains insufficiently analyzed and unmonitored by the State of Arizona. The Harshaw Creek watershed, recognized as a highly mineralized area with abandoned mines, contributes significantly to the Town of Patagonia's sole water source. As water levels in the aquifer along Harshaw Creek are anticipated to rise due to mine discharge, existing dry contaminated sediments could become saturated, introducing toxins into the drinking water supply.
"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."
The mine's long-term plans include pumping millions of gallons of groundwater daily, with a large portion expected to be discharged after treatment. This raises questions about long-term impacts on both water quantity and quality in the drought-strained region. Antimony, a toxic mining byproduct, poses 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, organ damage, and increased cancer risk, while also impacting aquatic and terrestrial wildlife.
"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." Community advocates emphasize the need for transparency, timely reporting, and honest data accounting, noting that water is central to the health, prosperity, and survival of the region. More information about community concerns can be found at https://www.calabasasalliance.org.
