Hydrologist tells Patagonia council mine discharge may be mobilizing antimony into local wells

Chris Gardner, a hydrologist with Friends of Cenodia Creek, told the Patagonia Town Council that samples from South 32’s Hermosa project discharge and nearby wells show elevated concentrations of antimony and other metals and that current monitoring and treatment plans may not adequately protect local drinking water.

"Antimony in the water is not a surprise," Gardner said in a presentation to the council, walking members through permit documents, leach test results and public sampling that he said show antimony concentrations ranging from about four to 13 parts per billion in waste-rock leach tests — and a reported 12 parts per billion in a nearby private well. Gardner cited the Arizona drinking-water guideline of six parts per billion for antimony and said those measures and sampling schedules leave gaps in understanding short-term spikes and downstream effects.

Gardner described how the mine’s permit documents and initial treatment-plant design did not list antimony among the constituents the plant was engineered to remove, and he argued that South 32’s monthly grab-sample schedule can miss short-term spikes in discharge. "They only have to prove it one day a month," he said, urging the council to ask the permitting authority to require more frequent sampling and a point-of-compliance groundwater monitoring well.

Gardner pointed to an example in Park City, Utah, where a multi‑stage drinking-water system — including a titanium dioxide absorptive medium at the end of the treatment train — was installed to reduce antimony to meet drinking-water standards. He said similar end‑of‑line treatment vessels could be added to mine treatment plants, though they add cost and require maintenance. "You can add these vessels to the end of the line to treat this," he said, describing the technology used in Park City.

Gardner asked the council to use the town’s role in any community benefits or protections agreement to require the mine operator to provide all water-level and water-quality data for wells along Harshaw Creek, to analyze the potential mobilization of legacy contamination, and to secure third‑party verification of the treatment plant’s design and operation. He said the town should request that the permitting authority reopen the discharge permit to require daily or weekly monitoring at the outfall and quarterly surface-water sampling.

Mayor noted the value of local citizen scientists and thanked Gardner for the technical briefing. The presentation did not include a response from South 32 or the state permitting agency at the meeting; Gardner said he had provided the town and federal agencies with written comments and a list of organizations and experts who reviewed the draft environmental impact materials.

What happens next: Gardner’s presentation concluded with specific monitoring and contractual asks; the council did not adopt a formal policy during the meeting but received Gardner’s recommendations and heard public comment. The council proceeded to routine business later in the agenda.