Hydrogeologist warns antimony in Harshaw Creek discharge may threaten Patagonia drinking water

Hydrogeologist Chris Gardner, representing Friends of Sonoita Creek, told the Patagonia Town Council on Jan. 14 that preliminary field and laboratory data indicate antimony in treated discharge from the South32/Hermosa Mine has appeared in waters used for drinking near Harshaw Creek and in some cases exceeded applicable limits.

Gardner said site and permit records show antimony has long been present in historic mine tailings and that reported antimony concentrations in the mine’s discharge tracked with increases in the monthly average discharge rate. “When the discharge rate to Harshaw Creek increased from about 1.0 to 2.5 million gallons per day … concentrations of antimony exceeded permit alert levels,” Gardner said, summarizing his compiled time series. He added that a Friends of Sonoita Creek sample of a domestic well was verified by ADEQ and “showed the concentration of antimony in drinking water was twice the drinking‑water standard.”

Gardner described two related concerns: first, that recent increases in discharge rate have coincided with antimony exceeding the Aquifer Protection Permit (APP) discharge limit (the APP antimony limit cited in his materials is 6 µg/L, with an APP alert level of 4.8 µg/L); and second, that ongoing discharge could saturate previously dry, contaminated stream‑channel sediments and mobilize additional metals into groundwater and downstream surface water.

His presentation reviewed historical material characterization submitted to ADEQ, recent monitoring tables and maps showing where samples were collected, and a table of predicted influent concentrations to the mine’s second water‑treatment plant (WTP2). Gardner emphasized that antimony was not included as a primary design target in material South32 submitted when sizing WTP2, and he said that ADEQ and South32 need to reassess monitoring and treatment design.

Gardner urged concrete steps: reopening the APP to require more frequent (daily or weekly) monitoring at Outfall 002, adding point‑of‑compliance groundwater wells in stream‑channel deposits, requiring South32 to share well and level data with the Town of Patagonia, and using the Community Benefits and Protections Agreement (CBPA) as a single binding vehicle to guarantee drinking‑water protections (treatment systems, well relocation or new distribution) rather than a phased benefits‑first approach. “Use the CBPA to require third‑party verification that WTP2 is better managed, operated and improved to prevent the discharge of antimony and all other contaminates to Harshaw Creek,” Gardner said.

Gardner also cited a treatment example from Park City, Utah’s Three Kings (3Kings) water‑treatment facility, noting that antimony can be removed with specialized adsorptive media under controlled pH and operational conditions. He cautioned that some media are sensitive to other metals and that a secondary barrier for other metals may be required.

The presentation included multiple charts and time‑series graphics compiled by Friends of Sonoita Creek showing reported APP/AZPDES discharge concentrations over 2023–2025, spatial sampling maps, and tables of dissolved metals and field parameters. Gardner repeatedly characterized his findings as preliminary and based on limited publicly available data and limited peer review; he asked that ADEQ and South32 provide full datasets and QC/QA information so trends can be verified.

The council received the presentation; no formal action was recorded at the meeting to direct staff or adopt Gardner’s requests. A public commenter, Carolyn Shafer, also addressed mining concerns related to Item 8 during the call to the public.