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Patagonia council hears South32 water‑monitoring update after single antimony exceedance; residents press for action
Summary
South32 updated the Patagonia Town Council on water treatment plant changes and monitoring after an Aquifer Protection Permit grab sample registered 6.23 ppb antimony (above the 6.0 ppb APP limit); company and an independent analysis said multi‑sample averages are below drinking‑water standards while residents urged permit re‑opening and stronger oversight.
Patagonia's Town Council on Jan. 28 heard a water‑quality update from South32/Hermosa after the company reported an Aquifer Protection Permit (APP) grab sample that registered 6.23 parts per billion (ppb) antimony, slightly above the APP discharge limit of 6.0 ppb.
Pat Risner of South32 summarized monitoring and plant changes for Water Treatment Plant No. 2, saying the site recorded an elevated antimony reading from one well in late 2024 that was taken offline Feb.–Sept. 2025 while the company planned and implemented enhanced antimony removal. South32 said a Nov. 26 compliance lab report showed an APP grab sample = 6.23 ppb and the same‑day AZPDES 8‑hour composite = 5.73 ppb. South32 ordered a retest; that retest confirmed the APP result slightly over the 6.0 ppb limit. Monthly results posted Dec. 24 were below limits (4.73 ppb and 4.57 ppb), and the company said it added a second phase of antimony treatment in Q1 2026.
South32 provided the company’s independent consultant analysis (Gradient), which the company described as concluding that average concentrations are the appropriate comparator for health standards developed for long‑term exposure. Company figures presented to the council listed an average antimony concentration of 3.2 ppb in discharge water across 22 samples in 2025 and an average of 2.5 ppb in Harshaw Creek across 6 locations and 16 samples in 2025.
ADEQ spokeswoman Alma Suarez told the council that "a permit exceedance is a regulatory trigger that requires investigation and corrective action, but it doesn't by itself quantify a health risk." Suarez said the discharge to Harshaw Creek remained significantly below applicable surface‑water quality standards intended to protect human and wildlife interactions with the creek.
At public comment, residents pushed for stronger oversight. Chris Werkhoven of Sonoita asked whether the town, as a signatory to a Cooperative Project Benefit Agreement (CPBA), "will the Town of Patagonia ... demand the stay of mine operations, the re‑opening of current permits and re‑submission of adequately amended permits?" He cited findings of antimony in groundwater and concern about an underground plume described in the draft EIS.
South32 told the council the plant uses well‑established chemical precipitation and sand‑ballasted clarifiers, that the plant design can be adjusted to increase antimony removal, and that the US Forest Service's draft EIS monitoring and modeling do not predict discharge‑driven aquifer exceedances. The company said it committed to a monitoring and mitigation plan that will be available for public review with the final EIS in March.
The council did not take any formal regulatory action at the Jan. 28 meeting. South32 posted a public contact (askhermosa@south32.net) and said it will continue monitoring and mitigation work; ADEQ's exceedance finding remains a regulatory trigger for investigation and corrective measures.
