A water quality sample from the South32 Hermosa Mine discharge near Patagonia, Arizona, collected on October 31, 2025, measured 5.73 micrograms per liter (µg/L) of antimony, exceeding Arizona’s Aquifer Protection Permit (APP) Alert Level of 4.8 µg/L and approaching the discharge limit of 6 µg/L. This is the highest antimony concentration recorded to date at the site, according to Arizona Pollutant Discharge Elimination System data reported to the EPA.
The APP rules require a 30-day study whenever an Alert Level is exceeded, and the data show an upward trend in antimony concentrations as discharge flows have increased. Community members are concerned that actual 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, illustrating that higher flow rates above one million gallons per day correspond with higher metal concentrations. While treatment adjustments have reduced arsenic levels, antimony remains problematic.
The treatment plant, unique in construction, appears deficient in reducing antimony below permit standards even at relatively low flow rates of 1–2 million gallons per day. This is concerning as discharge rates are anticipated to increase significantly as the mine develops. The public also worries about potential groundwater impacts, as discharge saturates dry contaminated sediments. Water levels in the Harshaw Creek watershed and the Town of Patagonia aquifer are expected to rise due to discharge, and the area contains abandoned mines. The Town of Patagonia relies solely on groundwater, with significant contribution from Harshaw Creek, yet the potential for groundwater contamination has not been sufficiently analyzed or monitored by the state.
“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 and hold South32 and regulators accountable.”
South32 Hermosa Mine’s long-term plans include pumping millions of gallons of groundwater per day, with much of it expected to be discharged after treatment. “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. The EPA 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, birds, mammals, and other animals.
“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 Ms. 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.”
